Harden the JPEG EXIF scan against malformed input (closes #11)

The segment scan behind `backup-metadata --exif` now checks every
segment length against the bytes that remain and stops on lengths
under 2, so a truncated or corrupt original can neither throw nor loop.
A malformed or unparseable EXIF segment is recorded as
`imageMetadata.exifError`, and a failure to read the original as
`imageMetadataError` in the per-file JSON, instead of the field being
silently left out. Tests use short hand-built byte arrays.

Model: opus-5-5

README.md

@@ -422,13 +422,18 @@ decides how to persist sessions.
 `client.toJSON()` returns a `ClientSnapshot` (a plain serializable object with
 base64-encoded keys) that the consumer can write to disk, a database, or
 whatever else fits their use case. `Client.fromJSON(snapshot)` restores a
-working client from that snapshot without re-authenticating.
+working client from that snapshot without re-authenticating; it checks every
+field and each key's length first, and throws an error naming the bad field.
+`client.logout()` clears the token and zeroes the key buffers in place; every
+later call on that client throws.
 
 The CLI stores the snapshot at the platform-appropriate data directory via
 `env-paths`: `~/Library/Application Support/quak/session.json` on macOS,
 `$XDG_DATA_HOME/quak/session.json` on Linux. The file is written with mode
 `0600`. The key material is stored in cleartext in the JSON; treat this file as
-you would treat the password itself.
+you would treat the password itself. A missing file is reported as "not logged
+in"; a file that exists but is corrupt is reported as such, naming the bad
+field. Both exit with status 1.
 
 ### CLI surface
 

TODO.md

@@ -18,6 +18,37 @@ Tag v1.0.0.
 
 # Completed Steps
 
+- 2026-09-23: Hardened the JPEG EXIF scan behind `backup-metadata --exif` (issue
+  11). Every segment length is checked against the remaining bytes and lengths
+  under 2 stop the scan, so a truncated or corrupt original can neither throw
+  nor loop. A malformed or unparseable EXIF segment is recorded as
+  `imageMetadata.exifError`, and a failure to read the original as
+  `imageMetadataError` in the per-file JSON, instead of the field being left
+  out.
+- 2026-09-22: Stopped `make test` collecting tests from checkouts nested under
+  `.claude/` (issue 25). vitest ignores `.gitignore` when finding tests, so a
+  nested checkout ran the whole suite again; `vitest.config.ts` now adds
+  `.claude/**` to vitest's default excludes, and
+  `test/packaging/nested-checkout.test.ts` plants a nested checkout in a temp
+  directory and fails if vitest would collect it.
+- 2026-09-22: Dropped the deprecated `@types/libsodium-wrappers-sumo` stub from
+  `devDependencies` (issue 27). It shipped no declarations; the types come from
+  `libsodium-wrappers-sumo` itself. `yarn.lock` regenerated by `yarn remove`.
+- 2026-09-22: Hardened the client session lifecycle (issue 10).
+  `Client.fromJSON` checks every snapshot field and each key's decoded length
+  and names the bad field; `toJSON` reads the token through
+  `ApiClient.getAuthToken` and throws when there is none; `logout` zeroes the
+  key buffers, and `collectionsSince` re-checks for logout after its request so
+  it never decrypts with zeroed keys. The CLI reports a corrupt session file
+  separately from a missing one (`src/cli-session.ts`).
+- 2026-09-22: Sanitized file names taken from server metadata (issue 9). A new
+  `src/filename.ts` holds the one sanitizer, used by `quak get`/`get-thumb`
+  without `--out`, `downloadFile`/`downloadThumbnail` without `outPath`, and the
+  backup and metadata backup trees; it removes separators, control characters,
+  leading dots and Windows device names, and falls back to a name built from the
+  ID for an empty title. Originals-cache extensions are letters and digits only,
+  else `.bin`. A user-supplied path is used as is. `decryptFile` reads a missing
+  or non-string title as "" and rejects metadata that is not a JSON object.
 - 2026-09-22: Rewrote the README API reference (and the Getting Started / usage
   snippets) to match the shipped cache/API library on `next` (issue 53, issue
   13). Documented `Library.open` and its options, the default-read vs `fresh()`

bin/quak.ts

@@ -2,13 +2,7 @@
 
 import { input, password as passwordPrompt } from "@inquirer/prompts";
 import { stdout, stderr } from "node:process";
-import {
-    copyFileSync,
-    existsSync,
-    mkdirSync,
-    readFileSync,
-    writeFileSync,
-} from "node:fs";
+import { copyFileSync, existsSync, mkdirSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
 import { Command } from "commander";
 import envPaths from "env-paths";
@@ -22,6 +16,7 @@ import {
     thumbnailName,
 } from "../src/cli-output.js";
 import { freshCollections, freshFiles, freshFile } from "../src/cli-read.js";
+import { loadSession } from "../src/cli-session.js";
 import { runMetadataBackup } from "../src/metadata-backup.js";
 import {
     listMissingThumbnails,
@@ -31,15 +26,6 @@ import {
 const paths = envPaths("quak", { suffix: "" });
 const sessionPath = join(paths.data, "session.json");
 
-const loadSession = (): ClientSnapshot | null => {
-    if (!existsSync(sessionPath)) return null;
-    try {
-        return JSON.parse(readFileSync(sessionPath, "utf-8")) as ClientSnapshot;
-    } catch {
-        return null;
-    }
-};
-
 const saveSession = (snapshot: ClientSnapshot): void => {
     mkdirSync(paths.data, { recursive: true, mode: 0o700 });
     writeFileSync(sessionPath, JSON.stringify(snapshot, null, 2), {
@@ -48,14 +34,23 @@ const saveSession = (snapshot: ClientSnapshot): void => {
 };
 
 const requireSession = (): Client => {
-    const snapshot = loadSession();
-    if (!snapshot) {
+    let client: Client | null;
+    try {
+        client = loadSession(sessionPath);
+    } catch (err) {
+        stderr.write(
+            `${err instanceof Error ? err.message : err}\n` +
+                `Run "quak logout" and then "quak login" to replace it.\n`,
+        );
+        process.exit(1);
+    }
+    if (!client) {
         stderr.write(
             `Not logged in. Run "quak login" first.\nSession file: ${sessionPath}\n`,
         );
         process.exit(1);
     }
-    return Client.fromJSON(snapshot);
+    return client;
 };
 
 const prompt = async (message: string): Promise<string> => input({ message });
@@ -157,7 +152,8 @@ program
 program
     .command("whoami")
     .description("Print the logged-in account")
-    .action(() => {
+    .action(async () => {
+        await init();
         const client = requireSession();
         const info = client.whoami();
         stdout.write(JSON.stringify(info) + "\n");

package.json

@@ -29,7 +29,6 @@
     },
     "devDependencies": {
         "@eslint/js": "9.38.0",
-        "@types/libsodium-wrappers-sumo": "0.8.2",
         "@types/node": "22.18.13",
         "eslint": "9.38.0",
         "prettier": "3.8.1",

src/api/client.ts

@@ -139,6 +139,10 @@ export class ApiClient {
         this.token = undefined;
     }
 
+    getAuthToken(): string | undefined {
+        return this.token;
+    }
+
     // The policy this client was configured with, so that a caller wrapping a
     // whole operation in its own `withRetry` — the download layer — runs under
     // the same settings rather than under the library defaults.

src/backup.ts

@@ -40,8 +40,9 @@ import {
     symlinkSync,
     writeFileSync,
 } from "node:fs";
-import { basename, dirname, extname, join, relative } from "node:path";
+import { basename, dirname, join, relative } from "node:path";
 
+import { safeExtension, sanitizeFileName } from "./filename.js";
 import type { Collection, EnteFile } from "./model/types.js";
 
 export type ProgressCallback = (message: string) => void;
@@ -108,16 +109,11 @@ interface FailureEntry {
 
 const LEDGER_VERSION = 1;
 
-const sanitizePath = (name: string): string =>
-    name.replace(/[/\\:*?"<>|]/g, "_").replace(/^\.+/, "_");
-
 // The originals/ filename for a file: `<id><ext>`, the extension taken from the
 // title (or `.bin`). Matches the content cache's own naming so a present check
 // lines up with what a fetch would write.
-const originalName = (file: EnteFile): string => {
-    const ext = extname(file.metadata.title || "") || ".bin";
-    return `${file.id}${ext}`;
-};
+const originalName = (file: EnteFile): string =>
+    `${file.id}${safeExtension(file.metadata.title)}`;
 
 // A regular file with content is treated as complete. A zero-byte file is not:
 // it is the shape an aborted write leaves and must be re-fetched.
@@ -370,7 +366,7 @@ export const runBackup = async (
 
     // Then the per-collection symlink trees and JSON.
     for (const c of collections) {
-        const colDirName = sanitizePath(c.name || `collection-${c.id}`);
+        const colDirName = sanitizeFileName(c.name, `collection-${c.id}`);
         const colDir = join(collectionsDir, colDirName);
         mkdirSync(colDir, { recursive: true });
 
@@ -381,8 +377,9 @@ export const runBackup = async (
             if (!includeOriginals) continue;
             const orig = join(originalsDir, originalName(file));
             if (!isPresent(orig)) continue;
-            const linkName = sanitizePath(
-                file.metadata.title || `file-${file.id}`,
+            const linkName = sanitizeFileName(
+                file.metadata.title,
+                `file-${file.id}`,
             );
             const linkPath = join(colDir, linkName);
             try {

src/cli-output.ts

@@ -9,6 +9,7 @@
 // `metadata.title`, and issue #52 requires that output stay byte-identical, so
 // the commands shape their output from the raw `EnteFile` through here.
 
+import { sanitizeFileName } from "./filename.js";
 import type { EnteFile, FileType, Microseconds } from "./model/types.js";
 
 // One row of `quak files --json`.
@@ -32,9 +33,11 @@ export const fileListRow = (file: EnteFile): FileListRow => ({
 export const fileListLine = (file: EnteFile): string =>
     `${file.id}\t${file.metadata.fileType}\t${file.metadata.title}`;
 
-// Default output path for `quak get` when `--out` is not given.
-export const originalName = (file: EnteFile): string => file.metadata.title;
+// Default output path for `quak get` when `--out` is not given. The title comes
+// from the server, so it is sanitized; `--out` is the user's and is used as is.
+export const originalName = (file: EnteFile): string =>
+    sanitizeFileName(file.metadata.title, `file-${file.id}`);
 
 // Default output path for `quak get-thumb` when `--out` is not given.
 export const thumbnailName = (file: EnteFile): string =>
-    `thumb_${file.metadata.title}`;
+    `thumb_${originalName(file)}`;

src/cli-session.ts

@@ -0,0 +1,26 @@
+// How the CLI reads its saved session file back into a `Client`.
+//
+// A missing file means "not logged in" and returns null. A file that exists but
+// cannot be read back into a client (bad JSON, a missing field, a key of the
+// wrong length) throws an error saying the session file is corrupt, so the CLI
+// can tell the user which of the two it is. Needs `init()` first.
+
+import { existsSync, readFileSync } from "node:fs";
+import type { ApiClientOptions } from "./api/client.js";
+import { Client } from "./client.js";
+
+export const loadSession = (
+    path: string,
+    apiOptions?: ApiClientOptions,
+): Client | null => {
+    if (!existsSync(path)) return null;
+    try {
+        return Client.fromJSON(
+            JSON.parse(readFileSync(path, "utf-8")),
+            apiOptions,
+        );
+    } catch (err) {
+        const reason = err instanceof Error ? err.message : String(err);
+        throw new Error(`Session file ${path} is corrupt: ${reason}`);
+    }
+};

src/client.ts

@@ -125,18 +125,57 @@ export class Client {
         );
     }
 
-    static fromJSON(
-        snapshot: ClientSnapshot,
-        apiOptions?: ApiClientOptions,
-    ): Client {
-        const api = new ApiClient({ ...apiOptions, authToken: snapshot.token });
+    // Restore a client from a `toJSON()` snapshot. The snapshot usually comes
+    // straight from `JSON.parse` of a file on disk, so every field is checked
+    // before use; a bad one throws an error naming it. Needs `init()` first.
+    static fromJSON(snapshot: unknown, apiOptions?: ApiClientOptions): Client {
+        const invalid = (field: string, problem: string): Error =>
+            new Error(`Invalid session data: ${field} ${problem}`);
+
+        if (typeof snapshot !== "object" || snapshot === null) {
+            throw new Error("Invalid session data: not a JSON object");
+        }
+        const s = snapshot as Record<string, unknown>;
+        for (const field of ["email", "token"]) {
+            if (typeof s[field] !== "string" || s[field] === "") {
+                throw invalid(field, "must be a non-empty string");
+            }
+        }
+        if (!Number.isInteger(s.userID)) {
+            throw invalid("userID", "must be an integer");
+        }
+        const key = (field: string): Uint8Array => {
+            const value = s[field];
+            if (typeof value !== "string") {
+                throw invalid(field, "must be a base64 string");
+            }
+            let bytes: Uint8Array;
+            try {
+                bytes = fromBase64(value);
+            } catch {
+                throw invalid(field, "is not valid base64");
+            }
+            // The master key (secretbox) and the key pair (box) are all 32 bytes.
+            if (bytes.length !== 32) {
+                throw invalid(
+                    field,
+                    `must decode to 32 bytes, got ${bytes.length}`,
+                );
+            }
+            return bytes;
+        };
+
+        const api = new ApiClient({
+            ...apiOptions,
+            authToken: s.token as string,
+        });
         return new Client(
             api,
-            snapshot.email,
-            snapshot.userID,
-            fromBase64(snapshot.masterKey),
-            fromBase64(snapshot.secretKey),
-            fromBase64(snapshot.publicKey),
+            s.email as string,
+            s.userID as number,
+            key("masterKey"),
+            key("secretKey"),
+            key("publicKey"),
         );
     }
 
@@ -164,19 +203,29 @@ export class Client {
 
     toJSON(): ClientSnapshot {
         this.assertLoggedIn();
+        const token = this.api.getAuthToken();
+        if (!token) {
+            throw new Error("Cannot serialize client: it has no auth token");
+        }
         return {
             email: this.email,
             userID: this.userID,
-            token: this.api["token"]!,
+            token,
             masterKey: toBase64(this.masterKey),
             secretKey: toBase64(this.secretKey),
             publicKey: toBase64(this.publicKey),
         };
     }
 
+    // Zeroes the key buffers in place, so any copy of the reference held
+    // elsewhere is wiped too. Every method checks `assertLoggedIn` before
+    // touching the keys, so nothing decrypts with the zeroed keys.
     logout(): void {
         this.loggedOut = true;
         this.api.clearAuthToken();
+        this.masterKey.fill(0);
+        this.secretKey.fill(0);
+        this.publicKey.fill(0);
     }
 
     // Enumerate collections changed since `sinceTime`. Live collections are
@@ -192,6 +241,8 @@ export class Client {
         const { collections: raws } = await this.api.getJSON<{
             collections: RawCollection[];
         }>("/collections/v2", { sinceTime: args.sinceTime });
+        // logout() may have zeroed the keys while the request was in flight.
+        this.assertLoggedIn();
 
         const collections: Collection[] = [];
         const deleted: number[] = [];

src/download/index.ts

@@ -11,6 +11,7 @@ import {
     streamTagFinal,
 } from "../crypto/index.js";
 import { TruncatedStreamError } from "../errors.js";
+import { sanitizeFileName } from "../filename.js";
 import { withRetry } from "../retry.js";
 import type { ApiClient } from "../api/client.js";
 import type { EnteFile } from "../model/types.js";
@@ -286,7 +287,10 @@ export const downloadFile = async (
     outPath?: string,
     onProgress?: ProgressCallback,
 ): Promise<DownloadResult> => {
-    const resolvedPath = outPath ?? file.metadata.title;
+    // `outPath` is the caller's and is used as is; the title is the server's
+    // and is sanitized so it can only name a file in the current directory.
+    const resolvedPath =
+        outPath ?? sanitizeFileName(file.metadata.title, `file-${file.id}`);
     const header = fromBase64(file.file.decryptionHeader);
     const bytesWritten = await fetchAndDecrypt(
         api,
@@ -305,7 +309,9 @@ export const downloadThumbnail = async (
     outPath?: string,
     onProgress?: ProgressCallback,
 ): Promise<DownloadResult> => {
-    const resolvedPath = outPath ?? `thumb_${file.metadata.title}`;
+    const resolvedPath =
+        outPath ??
+        `thumb_${sanitizeFileName(file.metadata.title, `file-${file.id}`)}`;
     const header = fromBase64(file.thumbnail.decryptionHeader);
     const bytesWritten = await fetchAndDecrypt(
         api,

src/filename.ts

@@ -0,0 +1,37 @@
+// File names built from server-supplied metadata.
+//
+// A file's title and a collection's name are decrypted from data the server
+// hands us, and quak does not trust the server. Any name taken from them and
+// used on disk goes through here, so it can only ever name one file inside the
+// directory the caller chose: never a path, never `..`, never hidden, never a
+// Windows device name.
+//
+// A path the user typed (`--out`, `outPath`) is not passed through here: the
+// caller is trusted, the server is not.
+
+import { extname } from "node:path";
+
+// Path separators, characters Windows forbids in file names, and control
+// characters (NUL included).
+// eslint-disable-next-line no-control-regex
+const UNSAFE_CHARACTERS = /[/\\:*?"<>|\x00-\x1f\x7f]/g;
+
+// Names Windows reserves for devices, with or without an extension.
+const RESERVED_DEVICE_NAME = /^(con|prn|aux|nul|com[1-9]|lpt[1-9])(\.|$)/i;
+
+// `name` made safe to use as a single file name. Each unsafe character becomes
+// `_`, a leading run of dots becomes one `_`, and a device name gets a leading
+// `_`. A name with none of these comes back unchanged. An empty name becomes
+// `fallback`, which the caller derives from the record's ID.
+export const sanitizeFileName = (name: string, fallback: string): string => {
+    if (name === "") return fallback;
+    const cleaned = name.replace(UNSAFE_CHARACTERS, "_").replace(/^\.+/, "_");
+    return RESERVED_DEVICE_NAME.test(cleaned) ? `_${cleaned}` : cleaned;
+};
+
+// The extension of `title` (".jpg"), or ".bin" when it has none or it holds
+// anything but letters and digits.
+export const safeExtension = (title: string): string => {
+    const ext = extname(title);
+    return /^\.[A-Za-z0-9]+$/.test(ext) ? ext : ".bin";
+};

src/library/content.ts

@@ -40,6 +40,7 @@ import {
     downloadThumbnail,
     type ProgressCallback,
 } from "../download/index.js";
+import { safeExtension } from "../filename.js";
 import type { EnteFile } from "../model/types.js";
 import type { Priority, RequestPools } from "./pools.js";
 
@@ -209,10 +210,8 @@ class AbortDrop extends Error {
     }
 }
 
-const originalName = (file: EnteFile): string => {
-    const ext = extname(file.metadata.title || "") || ".bin";
-    return `${file.id}${ext}`;
-};
+const originalName = (file: EnteFile): string =>
+    `${file.id}${safeExtension(file.metadata.title)}`;
 
 // The fileID a cache filename encodes, or undefined when the name is not one
 // the cache writes (`<digits><ext>`).

src/metadata-backup.ts

@@ -4,6 +4,7 @@ import * as jpeg from "jpeg-js";
 import exifReader from "exif-reader";
 import type { Client } from "./client.js";
 import type { Library, Photo } from "./library/index.js";
+import { sanitizeFileName } from "./filename.js";
 import { fetchMLData } from "./mldata-fetch.js";
 import type { EnteFile } from "./model/types.js";
 
@@ -14,21 +15,35 @@ export interface MetadataBackupOptions {
     onProgress?: ProgressCallback;
 }
 
-const sanitizePath = (name: string): string =>
-    name.replace(/[/\\:*?"<>|]/g, "_").replace(/^\.+/, "_");
-
-// Extract the raw EXIF APP1 segment from JPEG bytes. Returns the EXIF
-// data buffer (starting after the APP1 length field, at the "Exif\0\0"
-// header) or undefined if no APP1 marker is found.
-const extractExifFromJpeg = (buf: Uint8Array): Buffer | undefined => {
-    if (buf[0] !== 0xff || buf[1] !== 0xd8) return undefined;
+// Find the raw EXIF APP1 segment in JPEG bytes. Returns `exif` (the segment
+// data, starting at the "Exif\0\0" header) when there is one, nothing when the
+// bytes are not a JPEG or carry no EXIF, and `error` when the segment layout is
+// malformed. Each segment length is checked against the bytes that remain and
+// each step moves forward by at least 4 bytes, so the scan ends on any input.
+export const extractExifFromJpeg = (
+    buf: Uint8Array,
+): { exif?: Buffer; error?: string } => {
+    if (buf[0] !== 0xff || buf[1] !== 0xd8) return {};
     let offset = 2;
-    while (offset < buf.length - 1) {
-        if (buf[offset] !== 0xff) return undefined;
+    while (offset < buf.length) {
+        if (offset + 2 > buf.length)
+            return { error: `truncated segment marker at byte ${offset}` };
+        if (buf[offset] !== 0xff)
+            return { error: `no segment marker at byte ${offset}` };
         const marker = buf[offset + 1]!;
-        if (marker === 0xda) break; // start of scan, no more markers
-        if (offset + 3 >= buf.length) break;
+        if (marker === 0xda) return {}; // start of scan, no more markers
+        if (offset + 4 > buf.length)
+            return { error: `truncated segment length at byte ${offset}` };
         const len = (buf[offset + 2]! << 8) | buf[offset + 3]!;
+        // The length counts its own two bytes, so anything under 2 is invalid.
+        if (len < 2)
+            return {
+                error: `segment length ${len} at byte ${offset} is too small`,
+            };
+        if (offset + 2 + len > buf.length)
+            return {
+                error: `segment length ${len} at byte ${offset} runs past the end of the file`,
+            };
         if (marker === 0xe1) {
             // APP1 — check for "Exif\0\0" header
             if (
@@ -37,65 +52,70 @@ const extractExifFromJpeg = (buf: Uint8Array): Buffer | undefined => {
                 buf[offset + 6] === 0x69 &&
                 buf[offset + 7] === 0x66
             ) {
-                return Buffer.from(
-                    buf.buffer,
-                    buf.byteOffset + offset + 4,
-                    len - 2,
-                );
+                return {
+                    exif: Buffer.from(
+                        buf.buffer,
+                        buf.byteOffset + offset + 4,
+                        len - 2,
+                    ),
+                };
             }
         }
         offset += 2 + len;
     }
-    return undefined;
+    return { error: "file ends before the image data" };
 };
 
-const extractImageMetadata = (
+// Extract dimensions, EXIF and XMP from a file's bytes. When the EXIF segment
+// is malformed or cannot be parsed, the record carries the reason in
+// `exifError`.
+export const extractImageMetadata = (
     fileBytes: Uint8Array,
 ): Record<string, unknown> | undefined => {
+    const result: Record<string, unknown> = {};
+
+    // Try to get dimensions from JPEG decode
     try {
-        const result: Record<string, unknown> = {};
-
-        // Try to get dimensions from JPEG decode
-        try {
-            const decoded = jpeg.decode(fileBytes, {
-                useTArray: true,
-                formatAsRGBA: false,
-            });
-            result.format = "jpeg";
-            result.width = decoded.width;
-            result.height = decoded.height;
-        } catch {
-            // Not a JPEG or corrupt; still try EXIF extraction
-        }
-
-        const exifBuf = extractExifFromJpeg(fileBytes);
-        if (exifBuf) {
-            try {
-                result.exif = exifReader(exifBuf);
-            } catch {
-                result.exifRaw = exifBuf.toString("base64");
-            }
-        }
-
-        // Extract XMP (look for "http://ns.adobe.com/xap" in the bytes)
-        const xmpStart = Buffer.from(fileBytes).indexOf("<?xpacket begin");
-        if (xmpStart !== -1) {
-            const xmpEnd = Buffer.from(fileBytes).indexOf(
-                "<?xpacket end",
-                xmpStart,
-            );
-            if (xmpEnd !== -1) {
-                const end = Buffer.from(fileBytes).indexOf("?>", xmpEnd);
-                result.xmp = Buffer.from(fileBytes)
-                    .subarray(xmpStart, end !== -1 ? end + 2 : xmpEnd + 50)
-                    .toString("utf-8");
-            }
-        }
-
-        return Object.keys(result).length > 0 ? result : undefined;
+        const decoded = jpeg.decode(fileBytes, {
+            useTArray: true,
+            formatAsRGBA: false,
+        });
+        result.format = "jpeg";
+        result.width = decoded.width;
+        result.height = decoded.height;
     } catch {
-        return undefined;
+        // Not every original is a JPEG (PNG, HEIC, video), so a failed decode
+        // is expected and only means no dimensions; a malformed JPEG is still
+        // reported below through `exifError`.
     }
+
+    const { exif, error } = extractExifFromJpeg(fileBytes);
+    if (error) result.exifError = error;
+    if (exif) {
+        try {
+            result.exif = exifReader(exif);
+        } catch (err) {
+            result.exifRaw = exif.toString("base64");
+            result.exifError = err instanceof Error ? err.message : String(err);
+        }
+    }
+
+    // Extract XMP (look for "http://ns.adobe.com/xap" in the bytes)
+    const xmpStart = Buffer.from(fileBytes).indexOf("<?xpacket begin");
+    if (xmpStart !== -1) {
+        const xmpEnd = Buffer.from(fileBytes).indexOf(
+            "<?xpacket end",
+            xmpStart,
+        );
+        if (xmpEnd !== -1) {
+            const end = Buffer.from(fileBytes).indexOf("?>", xmpEnd);
+            result.xmp = Buffer.from(fileBytes)
+                .subarray(xmpStart, end !== -1 ? end + 2 : xmpEnd + 50)
+                .toString("utf-8");
+        }
+    }
+
+    return Object.keys(result).length > 0 ? result : undefined;
 };
 
 // Read a file's original bytes through the library's content cache and extract
@@ -105,13 +125,9 @@ const extractImageMetadata = (
 const extractExif = async (
     photo: Photo,
 ): Promise<Record<string, unknown> | undefined> => {
-    try {
-        const { path } = await photo.original();
-        const fileBytes = new Uint8Array(readFileSync(path));
-        return extractImageMetadata(fileBytes);
-    } catch {
-        return undefined;
-    }
+    const { path } = await photo.original();
+    const fileBytes = new Uint8Array(readFileSync(path));
+    return extractImageMetadata(fileBytes);
 };
 
 // Dump every decrypted metadata layer the account holds into a directory tree
@@ -151,7 +167,7 @@ export const runMetadataBackup = async (
         const col = lib.getCollection(album.collectionID);
         if (!col) continue;
 
-        const dirName = `${col.id}-${sanitizePath(col.name || "unnamed")}`;
+        const dirName = `${col.id}-${sanitizeFileName(col.name, "unnamed")}`;
         const colDir = join(outDir, "collections", dirName);
         mkdirSync(colDir, { recursive: true });
 
@@ -217,8 +233,13 @@ export const runMetadataBackup = async (
 
         if (wantExif && !writtenFileIDs.has(file.id)) {
             log(`[${file.metadata.title}] Extracting EXIF...`);
-            const exifData = await extractExif(photo);
-            if (exifData) fileMeta.imageMetadata = exifData;
+            try {
+                const exifData = await extractExif(photo);
+                if (exifData) fileMeta.imageMetadata = exifData;
+            } catch (err) {
+                fileMeta.imageMetadataError =
+                    err instanceof Error ? err.message : String(err);
+            }
         }
         writtenFileIDs.add(file.id);
 

src/model/decrypt.ts

@@ -98,9 +98,18 @@ export const decryptFile = (
         key,
     );
     const metadataJSON = JSON.parse(new TextDecoder().decode(metadataBytes));
+    if (
+        typeof metadataJSON !== "object" ||
+        metadataJSON === null ||
+        Array.isArray(metadataJSON)
+    ) {
+        throw new Error(`file ${raw.id}: metadata is not a JSON object`);
+    }
 
     const metadata: FileMetadata = {
-        title: metadataJSON.title ?? "",
+        // The server controls this JSON: a title that is missing or not a
+        // string becomes "", never an arbitrary value.
+        title: typeof metadataJSON.title === "string" ? metadataJSON.title : "",
         fileType: parseFileType(metadataJSON.fileType ?? -1),
         creationTime: metadataJSON.creationTime ?? 0,
         modificationTime: metadataJSON.modificationTime ?? 0,

test/cli/backup.test.ts

@@ -36,6 +36,7 @@ import {
     lstatSync,
     mkdirSync,
     mkdtempSync,
+    readdirSync,
     readFileSync,
     readlinkSync,
     rmSync,
@@ -107,6 +108,29 @@ class MockClient {
     }
 }
 
+// A server that names an album and a file so as to climb out of the backup
+// directory.
+class HostileClient extends MockClient {
+    override async collectionsSince(): Promise<CollectionsPage> {
+        const page = await super.collectionsSince();
+        return {
+            ...page,
+            collections: page.collections.length
+                ? [collection(3, "../escape")]
+                : [],
+        };
+    }
+    override async filesSince(args: {
+        collectionID: number;
+    }): Promise<FilesPage> {
+        const files =
+            args.collectionID === 3
+                ? [file(300, 3, "../../.ssh/authorized_keys")]
+                : [];
+        return { files, deleted: [], cursor: 1 };
+    }
+}
+
 // A content source that writes byte buffers of the expected length and can be
 // told to fail one fileID's original, to exercise per-file resilience.
 interface StubSource extends ContentSource {
@@ -136,9 +160,12 @@ const stubSource = (): StubSource => {
 
 let root: string;
 
-const openLibrary = (source: ContentSource): Promise<Library> =>
+const openLibrary = (
+    source: ContentSource,
+    client: MockClient = new MockClient(),
+): Promise<Library> =>
     Library.open({
-        client: new MockClient(),
+        client,
         cacheDirectory: join(root, "cache"),
         contentSource: source,
         refreshIntervalSeconds: 3600,
@@ -243,6 +270,30 @@ describe("lib.backup", () => {
         lib.close();
     });
 
+    it("keeps server-supplied album and file names inside the backup", async () => {
+        const lib = await openLibrary(stubSource(), new HostileClient());
+        const outDir = join(root, "backup");
+
+        const result = await lib.backup({ downloadDirectory: outDir });
+
+        expect(result.failed).toBe(0);
+        // The title has no usable extension, so the original is `.bin`.
+        expect(existsSync(join(outDir, "originals", "300.bin"))).toBe(true);
+        const link = join(
+            outDir,
+            "collections",
+            "__escape",
+            "__.._.ssh_authorized_keys",
+        );
+        expect(lstatSync(link).isSymbolicLink()).toBe(true);
+        expect(existsSync(join(outDir, "collections", "__escape.json"))).toBe(
+            true,
+        );
+        // Nothing landed beside or above the backup directory.
+        expect(readdirSync(root).sort()).toEqual(["backup", "cache"]);
+        lib.close();
+    });
+
     it("is an idempotent no-op when every original is already present", async () => {
         const source = stubSource();
         const lib = await openLibrary(source);

test/cli/metadata-backup.test.ts

@@ -165,14 +165,15 @@ const buildMetaMock = async (): Promise<MetaMockState> => {
         },
     };
 
-    // Collection 2: "Work" with no magic metadata
+    // Collection 2: "../Work" with no magic metadata. The server chose a name
+    // that tries to climb out of the backup directory.
     const ck2 = sodium.crypto_secretbox_keygen();
     const { ciphertext: encCK2, nonce: ck2N } = encryptSecretbox(
         ck2,
         masterKey,
     );
     const { ciphertext: encCN2, nonce: cn2N } = encryptSecretbox(
-        new TextEncoder().encode("Work"),
+        new TextEncoder().encode("../Work"),
         ck2,
     );
     const rawColl2 = {
@@ -496,7 +497,8 @@ describe("quak backup-metadata", () => {
         await runBackup(outDir);
 
         const collDirs = readdirSync(join(outDir, "collections"));
-        expect(collDirs.length).toBe(2);
+        // "../Work" is sanitized into one directory name.
+        expect(collDirs.sort()).toEqual(["10-Vacation", "20-__Work"]);
 
         // Find the Vacation collection dir (prefixed with ID)
         const vacDir = collDirs.find((d) => d.includes("Vacation"))!;
@@ -619,5 +621,18 @@ describe("quak backup-metadata", () => {
         expect(fileMeta.imageMetadata.format).toBe("jpeg");
         expect(fileMeta.imageMetadata.width).toBe(100);
         expect(fileMeta.imageMetadata.height).toBe(80);
+        expect(fileMeta.imageMetadataError).toBeUndefined();
+
+        // File 200 has no original on the mock server, so extraction fails
+        // and the reason is recorded instead of the field being left out.
+        const workDir = collDirs.find((d) => d.includes("Work"))!;
+        const failedMeta = JSON.parse(
+            readFileSync(
+                join(outDir, "collections", workDir, "200.json"),
+                "utf-8",
+            ),
+        );
+        expect(failedMeta.imageMetadata).toBeUndefined();
+        expect(failedMeta.imageMetadataError).toEqual(expect.any(String));
     });
 });

test/cli/metadata-exif.test.ts

@@ -0,0 +1,122 @@
+/**
+ * Tests for the JPEG EXIF scan behind `quak backup-metadata --exif`.
+ *
+ * The originals come from users' libraries, so a truncated or corrupt JPEG
+ * must neither hang the scan nor throw out of it, and a malformed file must be
+ * told apart from one that simply has no EXIF: the record carries the reason in
+ * `exifError`. Each input below is a short hand-built byte array.
+ */
+
+import { describe, expect, it } from "vitest";
+import {
+    extractExifFromJpeg,
+    extractImageMetadata,
+} from "../../src/metadata-backup.js";
+
+const SOI = [0xff, 0xd8]; // start of image
+const SOS = [0xff, 0xda, 0x00, 0x02]; // start of scan, where the scan stops
+const EXIF_HEADER = [0x45, 0x78, 0x69, 0x66, 0x00, 0x00]; // "Exif\0\0"
+
+// A big-endian TIFF block with one IFD entry: Orientation (0x0112), SHORT, 6.
+const TIFF_ORIENTATION_6 = [
+    0x4d, 0x4d, 0x00, 0x2a, 0x00, 0x00, 0x00, 0x08, 0x00, 0x01, 0x01, 0x12,
+    0x00, 0x03, 0x00, 0x00, 0x00, 0x01, 0x00, 0x06, 0x00, 0x00, 0x00, 0x00,
+    0x00, 0x00,
+];
+
+// An APP1 segment whose length field matches its data.
+const app1 = (data: number[]): number[] => {
+    const len = data.length + 2;
+    return [0xff, 0xe1, len >> 8, len & 0xff, ...data];
+};
+
+const bytes = (...parts: number[][]): Uint8Array =>
+    new Uint8Array(parts.flat());
+
+describe("extractExifFromJpeg", () => {
+    it("returns the EXIF segment of a valid JPEG", () => {
+        const data = [...EXIF_HEADER, ...TIFF_ORIENTATION_6];
+        const scan = extractExifFromJpeg(bytes(SOI, app1(data), SOS));
+        expect(scan.error).toBeUndefined();
+        expect([...scan.exif!]).toEqual(data);
+    });
+
+    it("returns nothing for a file that is not a JPEG", () => {
+        const png = bytes([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]);
+        expect(extractExifFromJpeg(png)).toEqual({});
+    });
+
+    it("returns nothing for a JPEG without EXIF", () => {
+        const app0 = [0xff, 0xe0, 0x00, 0x04, 0x00, 0x00];
+        expect(extractExifFromJpeg(bytes(SOI, app0, SOS))).toEqual({});
+    });
+
+    it("reports a JPEG truncated inside a segment header", () => {
+        const scan = extractExifFromJpeg(bytes(SOI, [0xff, 0xe1, 0x00]));
+        expect(scan.exif).toBeUndefined();
+        expect(scan.error).toMatch(/truncated segment length/);
+    });
+
+    it("reports a JPEG that ends before the image data", () => {
+        const app0 = [0xff, 0xe0, 0x00, 0x04, 0x00, 0x00];
+        const scan = extractExifFromJpeg(bytes(SOI, app0));
+        expect(scan.error).toMatch(/ends before the image data/);
+    });
+
+    it("stops on a zero-length segment instead of looping", () => {
+        // A length of 0 would otherwise step the scan by 2 bytes at a time
+        // through the rest of the file, reading garbage as markers.
+        const zero = [0xff, 0xe0, 0x00, 0x00];
+        const scan = extractExifFromJpeg(
+            bytes(SOI, zero, zero, zero, zero, SOS),
+        );
+        expect(scan.error).toMatch(/segment length 0 at byte 2 is too small/);
+    });
+
+    it("stops on a segment length of 1", () => {
+        const scan = extractExifFromJpeg(
+            bytes(SOI, [0xff, 0xe0, 0x00, 0x01], SOS),
+        );
+        expect(scan.error).toMatch(/segment length 1 at byte 2 is too small/);
+    });
+
+    it("reports a segment length that runs past the end of the file", () => {
+        // APP1 claims 0x4000 bytes but only the "Exif\0\0" header follows.
+        const scan = extractExifFromJpeg(
+            bytes(SOI, [0xff, 0xe1, 0x40, 0x00], EXIF_HEADER),
+        );
+        expect(scan.exif).toBeUndefined();
+        expect(scan.error).toMatch(/runs past the end of the file/);
+    });
+});
+
+describe("extractImageMetadata", () => {
+    it("parses EXIF from a valid JPEG", () => {
+        const meta = extractImageMetadata(
+            bytes(SOI, app1([...EXIF_HEADER, ...TIFF_ORIENTATION_6]), SOS),
+        );
+        expect(meta?.exifError).toBeUndefined();
+        expect(meta?.exif).toMatchObject({ Image: { Orientation: 6 } });
+    });
+
+    it("returns nothing for a file that is not a JPEG", () => {
+        const text = new TextEncoder().encode("just some text, not an image");
+        expect(extractImageMetadata(text)).toBeUndefined();
+    });
+
+    it("records the reason when the JPEG is malformed", () => {
+        const meta = extractImageMetadata(
+            bytes(SOI, [0xff, 0xe1, 0x40, 0x00], EXIF_HEADER),
+        );
+        expect(meta?.exif).toBeUndefined();
+        expect(meta?.exifError).toMatch(/runs past the end of the file/);
+    });
+
+    it("keeps the raw bytes and the reason when EXIF cannot be parsed", () => {
+        const data = [...EXIF_HEADER, 0x58, 0x58];
+        const meta = extractImageMetadata(bytes(SOI, app1(data), SOS));
+        expect(meta?.exif).toBeUndefined();
+        expect(meta?.exifRaw).toBe(Buffer.from(data).toString("base64"));
+        expect(meta?.exifError).toEqual(expect.any(String));
+    });
+});

test/cli/output.test.ts

@@ -64,6 +64,24 @@ describe("CLI file output (issue #52)", () => {
         expect(thumbnailName(renamedFile)).toBe(`thumb_${RAW_TITLE}`);
     });
 
+    it("sanitizes the title when naming `quak get` downloads", () => {
+        // Without `--out`, the server-supplied title names the file, so it must
+        // not be able to point outside the working directory.
+        const hostile = {
+            ...renamedFile,
+            metadata: { ...renamedFile.metadata, title: "../../.bashrc" },
+        };
+        expect(originalName(hostile)).toBe("__.._.bashrc");
+        expect(thumbnailName(hostile)).toBe("thumb___.._.bashrc");
+
+        const untitled = {
+            ...renamedFile,
+            metadata: { ...renamedFile.metadata, title: "" },
+        };
+        expect(originalName(untitled)).toBe("file-100");
+        expect(thumbnailName(untitled)).toBe("thumb_file-100");
+    });
+
     it("does not use the editedName/editedTime projection", () => {
         const record = deriveRecords([], [renamedFile]).photos.get(100);
         // The projection prefers the edits and reports milliseconds; the CLI

test/client/session.test.ts

@@ -0,0 +1,197 @@
+/**
+ * Tests for the client session lifecycle: `toJSON`, `fromJSON`, `logout`, and
+ * the CLI's `loadSession`, which reads the saved session file back into a
+ * client.
+ */
+
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import sodium from "libsodium-wrappers-sumo";
+import { afterAll, beforeAll, describe, expect, it } from "vitest";
+import { init, toBase64 } from "../../src/crypto/index.js";
+import { Client, type ClientSnapshot } from "../../src/client.js";
+import { loadSession } from "../../src/cli-session.js";
+
+const validSnapshot = (): ClientSnapshot => {
+    const kp = sodium.crypto_box_keypair();
+    return {
+        email: "user@example.com",
+        userID: 42,
+        token: "test-token",
+        masterKey: toBase64(sodium.crypto_secretbox_keygen()),
+        secretKey: toBase64(kp.privateKey),
+        publicKey: toBase64(kp.publicKey),
+    };
+};
+
+// The client's key buffers are private; the tests read them to prove that
+// logout wipes them.
+const keyBuffers = (client: Client): Uint8Array[] => [
+    client["masterKey"],
+    client["secretKey"],
+    client["publicKey"],
+];
+
+beforeAll(async () => {
+    await init();
+});
+
+describe("Client.toJSON", () => {
+    it("round-trips through fromJSON unchanged", () => {
+        const snapshot = validSnapshot();
+        expect(Client.fromJSON(snapshot).toJSON()).toEqual(snapshot);
+    });
+
+    it("throws instead of emitting a snapshot without a token", () => {
+        const client = Client.fromJSON(validSnapshot());
+        client.getApiClient().clearAuthToken();
+        expect(() => client.toJSON()).toThrow(/no auth token/);
+    });
+});
+
+describe("Client.fromJSON", () => {
+    const shortKey = toBase64(new Uint8Array(16));
+
+    it.each([
+        ["email", undefined],
+        ["email", 7],
+        ["email", ""],
+        ["token", undefined],
+        ["token", null],
+        ["token", ""],
+        ["userID", undefined],
+        ["userID", "42"],
+        ["userID", 4.2],
+        ["masterKey", undefined],
+        ["masterKey", 7],
+        ["masterKey", "not base64!"],
+        ["masterKey", shortKey],
+        ["secretKey", undefined],
+        ["secretKey", "not base64!"],
+        ["secretKey", shortKey],
+        ["publicKey", undefined],
+        ["publicKey", "not base64!"],
+        ["publicKey", shortKey],
+    ])("rejects %s = %j, naming the field", (field, value) => {
+        const snapshot: Record<string, unknown> = { ...validSnapshot() };
+        snapshot[field] = value;
+        expect(() => Client.fromJSON(snapshot)).toThrow(
+            new RegExp(`^Invalid session data: ${field} `),
+        );
+    });
+
+    it.each([null, "a string", 42])("rejects a non-object %j", (value) => {
+        expect(() => Client.fromJSON(value)).toThrow(
+            /^Invalid session data: not a JSON object/,
+        );
+    });
+});
+
+describe("Client.logout", () => {
+    it("zeroes the key buffers and clears the token", () => {
+        const client = Client.fromJSON(validSnapshot());
+        const api = client.getApiClient();
+        const keys = keyBuffers(client);
+
+        client.logout();
+
+        for (const key of keys) {
+            expect(key.length).toBe(32);
+            expect(key.every((b) => b === 0)).toBe(true);
+        }
+        expect(api.getAuthToken()).toBeUndefined();
+    });
+
+    it("makes every later operation throw", async () => {
+        const client = Client.fromJSON(validSnapshot());
+        client.logout();
+
+        expect(() => client.whoami()).toThrow(/logged out/);
+        expect(() => client.toJSON()).toThrow(/logged out/);
+        expect(() => client.getApiClient()).toThrow(/logged out/);
+        expect(() => client.contentSource()).toThrow(/logged out/);
+        await expect(client.listCollections()).rejects.toThrow(/logged out/);
+        await expect(client.collectionsSince({ sinceTime: 0 })).rejects.toThrow(
+            /logged out/,
+        );
+        await expect(
+            client.filesSince({
+                collectionID: 1,
+                collectionKey: new Uint8Array(32),
+                sinceTime: 0,
+            }),
+        ).rejects.toThrow(/logged out/);
+        await expect(
+            client.fetchMLData({ fileIDs: [1], fileKeys: new Map() }),
+        ).rejects.toThrow(/logged out/);
+    });
+
+    it("stops a listing in flight from decrypting with the zeroed keys", async () => {
+        // The server answers only after the client has logged out. If the
+        // listing went on to decrypt this row with all-zero keys it would fail
+        // with a decryption error, not the logged-out one.
+        const row = {
+            id: 1,
+            owner: { id: 42 },
+            encryptedKey: toBase64(new Uint8Array(48)),
+            keyDecryptionNonce: toBase64(new Uint8Array(24)),
+            updationTime: 1,
+        };
+        const client: Client = Client.fromJSON(validSnapshot(), {
+            fetch: async () => {
+                client.logout();
+                return new Response(JSON.stringify({ collections: [row] }), {
+                    status: 200,
+                    headers: { "content-type": "application/json" },
+                });
+            },
+        });
+
+        await expect(client.listCollections()).rejects.toThrow(/logged out/);
+    });
+});
+
+describe("loadSession", () => {
+    let dir: string;
+
+    beforeAll(() => {
+        dir = mkdtempSync(join(tmpdir(), "quak-session-test-"));
+    });
+
+    afterAll(() => {
+        rmSync(dir, { recursive: true, force: true });
+    });
+
+    it("returns null when there is no session file", () => {
+        expect(loadSession(join(dir, "missing.json"))).toBeNull();
+    });
+
+    it("restores a client from a valid session file", () => {
+        const path = join(dir, "valid.json");
+        writeFileSync(path, JSON.stringify(validSnapshot()));
+        expect(loadSession(path)!.whoami()).toEqual({
+            email: "user@example.com",
+            userID: 42,
+        });
+    });
+
+    it("says the file is corrupt when it is not JSON", () => {
+        const path = join(dir, "truncated.json");
+        writeFileSync(path, '{"email": "user@exa');
+        expect(() => loadSession(path)).toThrow(
+            `Session file ${path} is corrupt`,
+        );
+    });
+
+    it("says the file is corrupt and names the bad field", () => {
+        const path = join(dir, "bad-key.json");
+        writeFileSync(
+            path,
+            JSON.stringify({ ...validSnapshot(), secretKey: "AAAA" }),
+        );
+        expect(() => loadSession(path)).toThrow(
+            new RegExp(`^Session file ${path} is corrupt: .*secretKey`),
+        );
+    });
+});

test/download/download.test.ts

@@ -49,6 +49,7 @@
 
 import {
     existsSync,
+    mkdirSync,
     readdirSync,
     readFileSync,
     rmSync,
@@ -501,6 +502,22 @@ const entryPoints = [
     { name: "downloadThumbnail", download: downloadThumbnail },
 ];
 
+// With no `outPath`, the destination is named after `metadata.title`, relative
+// to the working directory. Such tests run inside a temporary directory:
+// `make check` must not create files in the repo root.
+const inDirectory = async <T>(
+    dir: string,
+    run: () => Promise<T>,
+): Promise<T> => {
+    const previous = process.cwd();
+    process.chdir(dir);
+    try {
+        return await run();
+    } finally {
+        process.chdir(previous);
+    }
+};
+
 // ---------------------------------------------------------------------------
 // Tests
 // ---------------------------------------------------------------------------
@@ -536,26 +553,59 @@ describe("downloadFile", () => {
     });
 
     it("uses metadata.title as filename when outPath is omitted", async () => {
-        // With no `outPath`, the destination is `metadata.title`, used
-        // verbatim as a path. The title here is therefore given inside the
-        // test's temporary directory: a bare relative name would resolve
-        // against the process working directory, i.e. the repo root, and
-        // `make check` must not create files in the repo — a failure between
-        // the write and any cleanup would leave one behind.
         const plaintext = new Uint8Array([1, 2, 3]);
         const key = sodium.crypto_secretstream_xchacha20poly1305_keygen();
         const { header, ciphertext } = encryptFileBody(plaintext, key);
         const thumbPush =
             sodium.crypto_secretstream_xchacha20poly1305_init_push(key);
         const file = buildMockEnteFile(key, header, thumbPush.header);
-        const titlePath = join(testDir, "fallback-name.png");
-        file.metadata.title = titlePath;
+        file.metadata.title = "fallback-name.png";
+        const dir = mkdtempSync(join(testDir, "title-"));
 
         const api = new ApiClient({ fetch: mockFetchForBody(ciphertext) });
-        const result = await downloadFile(api, file);
+        const result = await inDirectory(dir, () => downloadFile(api, file));
 
-        expect(result.path).toBe(titlePath);
-        expect(readFileSync(result.path)).toEqual(Buffer.from(plaintext));
+        expect(result.path).toBe("fallback-name.png");
+        expect(readFileSync(join(dir, "fallback-name.png"))).toEqual(
+            Buffer.from(plaintext),
+        );
+    });
+
+    it("keeps a hostile title inside the working directory", async () => {
+        // The server controls the title. `../escaped.png` must not write to
+        // the parent directory; it becomes one file name in the current one.
+        const { api, file } = fixtureFor(
+            multiChunkKey,
+            multiChunk.header,
+            multiChunk.body,
+        );
+        file.metadata.title = "../escaped.png";
+        const parent = mkdtempSync(join(testDir, "hostile-"));
+        const dir = join(parent, "cwd");
+        mkdirSync(dir);
+
+        const result = await inDirectory(dir, () => downloadFile(api, file));
+
+        expect(result.path).toBe("__escaped.png");
+        expect(readdirSync(dir)).toEqual(["__escaped.png"]);
+        expect(readdirSync(parent)).toEqual(["cwd"]);
+    });
+
+    it("uses an explicit outPath verbatim, even one with ..", async () => {
+        // The caller is trusted: its path is not sanitized.
+        const { api, file } = fixtureFor(
+            multiChunkKey,
+            multiChunk.header,
+            multiChunk.body,
+        );
+        const dir = mkdtempSync(join(testDir, "explicit-"));
+        mkdirSync(join(dir, "sub"));
+        const outPath = join(dir, "sub", "..", "explicit.bin");
+
+        const result = await downloadFile(api, file, outPath);
+
+        expect(result.path).toBe(outPath);
+        expect(existsSync(join(dir, "explicit.bin"))).toBe(true);
     });
 
     it("handles a larger single-chunk file (random binary payload)", async () => {
@@ -617,6 +667,23 @@ describe("downloadThumbnail", () => {
         expect(result).toEqual({ path: outPath, bytesWritten: 4 });
         expect(readFileSync(outPath)).toEqual(Buffer.from(plaintext));
     });
+
+    it("names the thumbnail thumb_ plus the sanitized title", async () => {
+        const { api, file } = fixtureFor(
+            multiChunkKey,
+            multiChunk.header,
+            multiChunk.body,
+        );
+        file.metadata.title = "/etc/passwd";
+        const dir = mkdtempSync(join(testDir, "thumb-title-"));
+
+        const result = await inDirectory(dir, () =>
+            downloadThumbnail(api, file),
+        );
+
+        expect(result.path).toBe("thumb__etc_passwd");
+        expect(readdirSync(dir)).toEqual(["thumb__etc_passwd"]);
+    });
 });
 
 // ---------------------------------------------------------------------------

test/filename/filename.test.ts

@@ -0,0 +1,93 @@
+// File names built from server-supplied metadata.
+//
+// quak does not trust the server. A file's title and a collection's name are
+// decrypted from data the server hands us, and a hostile server (or a
+// compromised account) can set them to anything. quak uses them to name files
+// on disk: `quak get` without `--out`, `downloadFile` without `outPath`, the
+// backup's symlink and collection directories, and the extension of every file
+// in the originals cache. Each of those goes through `sanitizeFileName` or
+// `safeExtension`, so a title can only ever name one file inside the directory
+// the caller chose.
+//
+// A path the user supplies (`--out`, `outPath`) is never sanitized: the caller
+// is trusted, the server is not.
+
+import { describe, expect, it } from "vitest";
+
+import { safeExtension, sanitizeFileName } from "../../src/filename.js";
+
+const FALLBACK = "file-42";
+
+describe("sanitizeFileName", () => {
+    it("passes a normal title through unchanged", () => {
+        expect(sanitizeFileName("IMG_0001.HEIC", FALLBACK)).toBe(
+            "IMG_0001.HEIC",
+        );
+        expect(sanitizeFileName("Holiday 2024 (1).jpg", FALLBACK)).toBe(
+            "Holiday 2024 (1).jpg",
+        );
+        expect(sanitizeFileName("café.jpg", FALLBACK)).toBe("café.jpg");
+    });
+
+    it("cannot climb out of the directory with ../", () => {
+        // Without sanitizing, this would overwrite the user's SSH keys.
+        expect(sanitizeFileName("../../.ssh/authorized_keys", FALLBACK)).toBe(
+            "__.._.ssh_authorized_keys",
+        );
+        expect(sanitizeFileName("..", FALLBACK)).toBe("_");
+        expect(sanitizeFileName("..\\..\\x", FALLBACK)).toBe("__.._x");
+    });
+
+    it("cannot name an absolute path", () => {
+        expect(sanitizeFileName("/etc/passwd", FALLBACK)).toBe("_etc_passwd");
+        expect(sanitizeFileName("C:\\Windows\\x.dll", FALLBACK)).toBe(
+            "C__Windows_x.dll",
+        );
+    });
+
+    it("replaces embedded separators, so the name stays one file", () => {
+        expect(sanitizeFileName("a/b\\c.jpg", FALLBACK)).toBe("a_b_c.jpg");
+    });
+
+    it("replaces NUL and other control characters", () => {
+        // A NUL truncates the path in C code and makes Node's fs throw.
+        expect(sanitizeFileName("evil\0.jpg", FALLBACK)).toBe("evil_.jpg");
+        expect(sanitizeFileName("line\nbreak\x7f.jpg", FALLBACK)).toBe(
+            "line_break_.jpg",
+        );
+    });
+
+    it("does not produce a hidden file", () => {
+        expect(sanitizeFileName(".bashrc", FALLBACK)).toBe("_bashrc");
+    });
+
+    it("does not produce a Windows device name", () => {
+        expect(sanitizeFileName("CON", FALLBACK)).toBe("_CON");
+        expect(sanitizeFileName("nul.txt", FALLBACK)).toBe("_nul.txt");
+        expect(sanitizeFileName("LPT1", FALLBACK)).toBe("_LPT1");
+        // Only the exact names are reserved.
+        expect(sanitizeFileName("console.jpg", FALLBACK)).toBe("console.jpg");
+    });
+
+    it("falls back to the given name for an empty title", () => {
+        expect(sanitizeFileName("", FALLBACK)).toBe(FALLBACK);
+    });
+});
+
+describe("safeExtension", () => {
+    it("keeps a normal extension", () => {
+        expect(safeExtension("IMG_0001.HEIC")).toBe(".HEIC");
+        expect(safeExtension("clip.mp4")).toBe(".mp4");
+    });
+
+    it("uses .bin when there is no extension", () => {
+        expect(safeExtension("")).toBe(".bin");
+        expect(safeExtension("README")).toBe(".bin");
+    });
+
+    it("uses .bin when the extension holds anything but letters and digits", () => {
+        expect(safeExtension("x.j\\..\\pg")).toBe(".bin");
+        expect(safeExtension("x.jp g")).toBe(".bin");
+        expect(safeExtension("x.jpg\0")).toBe(".bin");
+    });
+});

test/library/content.test.ts

@@ -226,6 +226,25 @@ describe("ContentCache.original / thumbnail", () => {
         expect(skips).toEqual(["skipped"]);
     });
 
+    it("takes only a letters-and-digits extension from the title", async () => {
+        // The title comes from the server; an extension such as `.\..\x`
+        // must not reach the cache file name, so it becomes `.bin`.
+        const { cache } = buildCache({
+            files: [file(1, "a.jpg"), file(2, "b.\\..\\x"), file(3, "")],
+        });
+        await cache.open();
+
+        expect((await cache.original(1)).path).toBe(
+            join(cacheDir, "originals", "1.jpg"),
+        );
+        expect((await cache.original(2)).path).toBe(
+            join(cacheDir, "originals", "2.bin"),
+        );
+        expect((await cache.original(3)).path).toBe(
+            join(cacheDir, "originals", "3.bin"),
+        );
+    });
+
     it("serves a file already present in the download directory without fetching", async () => {
         const downloadDirectory = join(root, "backup");
         mkdirSync(join(downloadDirectory, "originals"), { recursive: true });

test/model/decrypt.test.ts

@@ -146,10 +146,13 @@ const buildSharedRawCollection = (
 const buildRawFile = (
     collectionKey: Uint8Array,
     opts?: {
-        title?: string;
+        // Any JSON value; `undefined` leaves the title out of the metadata.
+        title?: unknown;
         fileType?: number;
         creationTime?: number;
         info?: { fileSize?: number; thumbSize?: number };
+        // Replaces the whole metadata JSON value.
+        metadata?: unknown;
     },
 ): RawEnteFile => {
     const fileKey = sodium.crypto_secretbox_keygen();
@@ -158,8 +161,8 @@ const buildRawFile = (
         collectionKey,
     );
 
-    const metadata = {
-        title: opts?.title ?? "IMG_0001.jpg",
+    const defaultMetadata = {
+        title: opts && "title" in opts ? opts.title : "IMG_0001.jpg",
         fileType: opts?.fileType ?? 0,
         creationTime: opts?.creationTime ?? 1700000000000000,
         modificationTime: 1700000000000000,
@@ -167,6 +170,8 @@ const buildRawFile = (
         longitude: 2.3522,
         hash: "abcdef1234567890",
     };
+    const metadata =
+        opts && "metadata" in opts ? opts.metadata : defaultMetadata;
     // File metadata is encrypted as a single-chunk secretstream blob
     // (not secretbox). The decryptionHeader is the secretstream init header.
     const metadataBytes = new TextEncoder().encode(JSON.stringify(metadata));
@@ -321,6 +326,31 @@ describe("model.decryptFile", () => {
         expect(file.metadata.longitude).toBeCloseTo(2.3522);
     });
 
+    it("reads a missing or non-string title as an empty string", () => {
+        // The server controls the metadata JSON. A title that is not a
+        // string must not reach code that builds file names from it.
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { collectionKey } = buildRawCollection(masterKey);
+        for (const title of [undefined, null, 42, ["a"], { x: "../y" }]) {
+            const file = decryptFile(
+                buildRawFile(collectionKey, { title }),
+                collectionKey,
+            );
+            expect(file.metadata.title).toBe("");
+        }
+    });
+
+    it("rejects metadata that is not a JSON object", () => {
+        const masterKey = sodium.crypto_secretbox_keygen();
+        const { collectionKey } = buildRawCollection(masterKey);
+        for (const metadata of [null, "IMG_0001.jpg", 7, []]) {
+            const raw = buildRawFile(collectionKey, { metadata });
+            expect(() => decryptFile(raw, collectionKey)).toThrow(
+                "file 200: metadata is not a JSON object",
+            );
+        }
+    });
+
     it("maps fileType numbers to FileType strings", () => {
         // Ente uses: 0=image, 1=video, 2=livePhoto
         const masterKey = sodium.crypto_secretbox_keygen();

test/packaging/nested-checkout.test.ts

@@ -0,0 +1,50 @@
+// A checkout nested under `.claude/` must not add its tests to this suite.
+// The test plants one in a temporary directory next to a real test file and
+// asks vitest, with this repo's config, which test files it would run.
+import { afterEach, describe, expect, it } from "vitest";
+import { execFileSync } from "node:child_process";
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const repoRoot = fileURLToPath(new URL("../../", import.meta.url));
+
+let root = "";
+
+afterEach(() => {
+    rmSync(root, { recursive: true, force: true });
+});
+
+const writeTest = (path: string): void => {
+    mkdirSync(join(root, path, ".."), { recursive: true });
+    writeFileSync(
+        join(root, path),
+        'import { it } from "vitest";\nit("runs", () => {});\n',
+    );
+};
+
+describe("vitest.config.ts", () => {
+    it("does not collect tests from a checkout nested under .claude/", () => {
+        root = mkdtempSync(join(tmpdir(), "quak-nested-checkout-"));
+        writeTest("test/real.test.ts");
+        writeTest(".claude/worktrees/other/test/real.test.ts");
+
+        const output = execFileSync(
+            process.execPath,
+            [
+                join(repoRoot, "node_modules/vitest/vitest.mjs"),
+                "list",
+                "--filesOnly",
+                "--config",
+                join(repoRoot, "vitest.config.ts"),
+                "--root",
+                root,
+            ],
+            { cwd: root, encoding: "utf-8" },
+        );
+
+        const files = output.split("\n").filter((line) => line !== "");
+        expect(files).toEqual(["test/real.test.ts"]);
+    });
+});

vitest.config.ts

@@ -0,0 +1,10 @@
+import { configDefaults, defineConfig } from "vitest/config";
+
+// vitest does not read .gitignore when looking for tests. A checkout nested
+// under .claude/ has its own test/ tree, and without this exclude the suite
+// runs once per nested checkout and still reports success.
+export default defineConfig({
+    test: {
+        exclude: [...configDefaults.exclude, ".claude/**"],
+    },
+});

yarn.lock

@@ -528,13 +528,6 @@
   resolved "https://registry.yarnpkg.com/@types/json-schema/-/json-schema-7.0.15.tgz#596a1747233694d50f6ad8a7869fcb6f56cf5841"
   integrity sha512-5+fP8P8MFNC+AyZCDxrB2pkZFPGzqQWUzpSeuuVLvm8VMcorNYavBqoFcxK8bQz4Qsbn4oUEEem4wDLfcysGHA==
 
-"@types/libsodium-wrappers-sumo@0.8.2":
-  version "0.8.2"
-  resolved "https://registry.yarnpkg.com/@types/libsodium-wrappers-sumo/-/libsodium-wrappers-sumo-0.8.2.tgz#488e8747fbb982fe901020b5afeaddfa63da6830"
-  integrity sha512-uFOBpg/r21hExVlh2ty8YpDfSR+Yy3Jn8XS4+SSjitbhTxdYq+pBz/49XRxyUFe8SzqujHf/Wu0/O4d+FUtNfQ==
-  dependencies:
-    libsodium-wrappers-sumo "*"
-
 "@types/node@22.18.13":
   version "22.18.13"
   resolved "https://registry.yarnpkg.com/@types/node/-/node-22.18.13.tgz#a037c4f474b860be660e05dbe92a9ef945472e28"
@@ -1249,7 +1242,7 @@ libsodium-sumo@^0.8.0:
   resolved "https://registry.yarnpkg.com/libsodium-sumo/-/libsodium-sumo-0.8.4.tgz#6d4687781fa0ad398af14a7df872d5c27cf8cd31"
   integrity sha512-TMtHShQfVVsaxDygyapvUC3o7YsPgXa/hRWeIgzyFz6w5k/1hirGptCxp1U7XwW3rCskaTTYKgV10v86UiGgNw==
 
-libsodium-wrappers-sumo@*, libsodium-wrappers-sumo@0.8.4:
+libsodium-wrappers-sumo@0.8.4:
   version "0.8.4"
   resolved "https://registry.yarnpkg.com/libsodium-wrappers-sumo/-/libsodium-wrappers-sumo-0.8.4.tgz#6656a3e7e0551ecce08ddee4bfb501a092eac6fa"
   integrity sha512-ql7hcgulKZ3ekfa2DGAogcCKsWU0diA/0nArz1CFzh93WQdb46/Kj18ka/Hifq6uA3Ush34Pc6vU/6HXeRwUkg==