fix(backend): report ingest correctness — propagate storage failure, 413 on oversize, global body cap (closes #23)

A buffer failure on POST /api/v1/reports now returns 500 instead of a
false {"status":"ok"}, so clients can retry. Decode errors are split:
an over-limit body returns 413 (via errors.As on *http.MaxBytesError),
malformed JSON stays 400. A new MaxBodyBytes middleware caps every
route (1 MiB default; rejects an oversized Content-Length up-front and
caps the read otherwise), replacing the per-route reader so the health
check and future routes are bounded too. The raw attacker-controlled
geo blob is no longer logged — only its byte length — and client_id and
timestamp are length-bounded before logging. A decodeJSON handler helper
is added per the HTTP server conventions. Panic recovery is now a local
middleware that routes the stack through slog as structured JSON rather
than chi's plain-text stderr. Error bodies still leak nothing internal.

Chosen failure code for a storage failure: 500, since a full buffer or
write error is server-side and retryable, not the client's fault.

Model: opus-4-8

.gitignore

@@ -1,4 +1,27 @@
-node_modules/
-dist/
+# OS
 .DS_Store
+Thumbs.db
+
+# Editors
+*.swp
+*.swo
+*~
+*.bak
+.idea/
+.vscode/
+*.sublime-*
+
+# Node
+node_modules/
+
+# Environment / secrets
+.env
+.env.*
+*.pem
+*.key
+
+# Build output
+dist/
+
+# Logs
 *.log

Dockerfile.backend

@@ -1,25 +1,50 @@
+# Lint stage — fast feedback on formatting and lint issues. The
+# golangci/golangci-lint image ships Go, gofmt, make and the linter, so
+# nothing is installed here.
+# golangci/golangci-lint:v2.7.2 (2026-08-09)
+FROM golangci/golangci-lint@sha256:5d6d5c70a61f1356adfd9dd6316ce286799fefc9d743421356ff1b00842368ba AS lint
+
+WORKDIR /src
+COPY backend/go.mod backend/go.sum ./
+RUN go mod download
+COPY backend/ .
+RUN make fmt-check
+RUN make lint
+
+# Build stage
 # golang:1.25-alpine (2026-02-27)
 FROM golang:1.25-alpine@sha256:f6751d823c26342f9506c03797d2527668d095b0a15f1862cddb4d927a7a4ced AS builder
 
-RUN apk add --no-cache git make gcc musl-dev
+RUN apk add --no-cache make
 
-# golangci-lint v2.7.2 (2026-02-27)
-RUN CGO_ENABLED=0 go install github.com/golangci/golangci-lint/v2/cmd/golangci-lint@9f61b0f53f80672872fced07b6874397c3ed197b
+WORKDIR /src
+
+# Force BuildKit to run the lint stage before proceeding. BuildKit runs
+# stages in parallel by default; without this no-op copy a lint failure
+# would not gate compilation.
+COPY --from=lint /src/go.sum /dev/null
 
-WORKDIR /repo/backend
 COPY backend/go.mod backend/go.sum ./
 RUN go mod download
-COPY .git /repo/.git
 COPY backend/ .
 
-RUN make check
-RUN make build
+RUN make test
 
+# The build is driven through the Makefile so there is exactly one
+# definition of the build command; it expands to
+# CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.Version=... -X main.Buildarch=..."
+# VERSION is handed over in the environment rather than as a make
+# variable so it still reaches the build if the target is ever turned
+# into a shim around a script.
+ARG VERSION=dev
+RUN VERSION="${VERSION}" make build
+
+# Runtime stage
 # alpine:3.23 (2026-02-27)
 FROM alpine:3.23@sha256:25109184c71bdad752c8312a8623239686a9a2071e8825f20acb8f2198c3f659
 
 RUN apk add --no-cache ca-certificates
-COPY --from=builder /repo/backend/netwatch-server /usr/local/bin/netwatch-server
+COPY --from=builder /src/netwatch-server /usr/local/bin/netwatch-server
 
 EXPOSE 8080
 ENTRYPOINT ["netwatch-server"]

TODO.md

@@ -22,6 +22,35 @@ files, so merging it also closes most compliance gaps.
 
 # Completed Steps
 
+- 2026-09-21: report ingest correctness (issue #23): a storage failure now
+  returns 500 instead of a false `ok`; oversize bodies return 413 (distinguished
+  from malformed JSON, which stays 400); a `MaxBodyBytes` middleware caps every
+  route, not just the report route; the raw attacker-controlled `geo` blob is no
+  longer logged (only its length) and `client_id`/`timestamp` are length-bounded
+  before logging; a `decodeJSON` handler helper was added; and panic recovery
+  now routes the stack through slog instead of chi's plain-text stderr
+- 2026-09-21: backend HTTP hardening (issue #19): added `ReadHeaderTimeout` and
+  `IdleTimeout` to the server, a `SecurityHeaders` middleware (HSTS, tight CSP,
+  frame/sniff/referrer/permissions headers) registered before CORS, and
+  trusted-proxy client IP resolution honouring `X-Forwarded-For` / `X-Real-IP`
+  only from a `TRUSTED_PROXIES` allowlist (loopback plus RFC1918 by default)
+- 2026-08-10: every interactive control now meets the 44x44 CSS px minimum tap
+  target (`.pin-btn`, `#interval-select`, the debug-log label and, on narrow
+  viewports, `#pause-btn`). The pin button's hit area grows via matching
+  negative margins, so its layout footprint and row density are unchanged
+- 2026-08-10: per-host status line wraps below the 768px breakpoint instead of
+  forcing horizontal page scroll at 320px
+- 2026-08-09: `Dockerfile.backend` reworked to the mandated Go multistage
+  lint-stage pattern: separate `lint` stage on the hash-pinned
+  `golangci/golangci-lint` image, `COPY --from=lint` stage dependency,
+  `CGO_ENABLED=0` static build driven by `ARG VERSION`, and no more `COPY .git`
+- 2026-08-09: dotfile compliance — lifted `backend/.editorconfig` to the repo
+  root so `root = true` covers the frontend too, and replaced `.gitignore` with
+  the org model (OS, editor, node, and environment/secrets sections) plus this
+  repo's `dist/` and `*.log`. `.env`, `.env.*`, `*.pem`, and `*.key` are now
+  ignored repo-wide, not just under `backend/`. Excluding `.git` from
+  `.dockerignore` stays deferred: both images read git metadata at build time
+  (`COPY .git` in `Dockerfile.backend`, `git rev-parse` in `vite.config.js`)
 - 2026-07-07 Adopted scripts-to-rule-them-all: `script/` entrypoints, Makefile
   shims, README Entrypoints section
 - 2026-02-27: backend with buffered zstd-compressed report storage; CI workflow

backend/Makefile

@@ -1,25 +1,22 @@
-UNAME_S := $(shell uname -s)
-VERSION := $(shell git describe --always --dirty)
+# VERSION is overridable (the Dockerfile passes its ARG VERSION in) and
+# degrades to "dev" when git is unavailable or there is no .git — the
+# build must not depend on the repository history being in the build
+# context.
+VERSION ?= $(shell { git describe --always --dirty; } 2>/dev/null || echo dev)
 BUILDARCH := $(shell uname -m)
 BINARY := netwatch-server
 
+GOLDFLAGS += -s -w
 GOLDFLAGS += -X main.Version=$(VERSION)
 GOLDFLAGS += -X main.Buildarch=$(BUILDARCH)
 
-ifeq ($(UNAME_S),Darwin)
-	GOFLAGS := -ldflags "$(GOLDFLAGS)"
-else
-	GOFLAGS = -ldflags "-linkmode external -extldflags -static $(GOLDFLAGS)"
-endif
-
 .PHONY: all build test lint fmt fmt-check check docker hooks run clean
 
 all: build
 
-build: ./$(BINARY)
-
-./$(BINARY): $(shell find . -name '*.go' -type f) go.mod go.sum
-	go build -o $@ $(GOFLAGS) ./cmd/netwatch-server/
+build:
+	CGO_ENABLED=0 go build -trimpath -ldflags "$(GOLDFLAGS)" \
+		-o ./$(BINARY) ./cmd/netwatch-server/
 
 test:
 	timeout 30 go test ./...

backend/README.md

@@ -42,11 +42,17 @@ Internal packages in `internal/` follow standard Go project layout:
 
 ### Configuration
 
-| Variable   | Default            | Description                       |
-| ---------- | ------------------ | --------------------------------- |
-| `PORT`     | `8080`             | HTTP listen port                  |
-| `DATA_DIR` | `./data/reports`   | Directory for compressed reports  |
-| `DEBUG`    | `false`            | Enable debug logging              |
+| Variable          | Default              | Description                                                                                              |
+| ----------------- | -------------------- | -------------------------------------------------------------------------------------------------------- |
+| `PORT`            | `8080`               | HTTP listen port                                                                                         |
+| `DATA_DIR`        | `./data/reports`     | Directory for compressed reports                                                                         |
+| `DEBUG`           | `false`              | Enable debug logging                                                                                     |
+| `TRUSTED_PROXIES` | loopback + RFC1918   | Comma-separated CIDRs whose `X-Forwarded-For` / `X-Real-IP` headers are trusted for client IP resolution |
+
+`TRUSTED_PROXIES` defaults to `127.0.0.1/32,::1/128,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16`.
+The loopback entries cover the reverse proxy that shares the container; the
+RFC1918 ranges match `nginx.conf`. A request whose direct peer is outside this
+set has its forwarded headers ignored, and the direct peer is logged instead.
 
 ### Report storage
 

backend/internal/config/config.go

@@ -5,6 +5,7 @@ package config
 import (
 	"errors"
 	"log/slog"
+	"strings"
 
 	"sneak.berlin/go/netwatch/internal/globals"
 	"sneak.berlin/go/netwatch/internal/logger"
@@ -14,6 +15,14 @@ import (
 	"go.uber.org/fx"
 )
 
+// defaultTrustedProxies lists the networks whose forwarded
+// headers are honoured by default. It covers the RFC1918
+// ranges (to match nginx.conf) plus IPv4 and IPv6 loopback,
+// because the reverse proxy shares the container and reaches
+// the backend over loopback.
+const defaultTrustedProxies = "127.0.0.1/32,::1/128," +
+	"10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
+
 // Params defines the dependencies for Config.
 type Params struct {
 	fx.In
@@ -30,6 +39,7 @@ type Config struct {
 	MetricsUsername string
 	Port            int
 	SentryDSN       string
+	TrustedProxies  []string
 	log             *slog.Logger
 	params          *Params
 }
@@ -56,6 +66,7 @@ func New(
 	viper.SetDefault("SENTRY_DSN", "")
 	viper.SetDefault("METRICS_USERNAME", "")
 	viper.SetDefault("METRICS_PASSWORD", "")
+	viper.SetDefault("TRUSTED_PROXIES", defaultTrustedProxies)
 
 	err := viper.ReadInConfig()
 	if err != nil {
@@ -73,6 +84,7 @@ func New(
 		MetricsUsername: viper.GetString("METRICS_USERNAME"),
 		Port:            viper.GetInt("PORT"),
 		SentryDSN:       viper.GetString("SENTRY_DSN"),
+		TrustedProxies:  splitList(viper.GetString("TRUSTED_PROXIES")),
 		log:             log,
 		params:          &params,
 	}
@@ -84,3 +96,19 @@ func New(
 
 	return s, nil
 }
+
+// splitList turns a comma-separated setting into a trimmed
+// slice, dropping empty entries.
+func splitList(raw string) []string {
+	parts := strings.Split(raw, ",")
+
+	out := make([]string, 0, len(parts))
+	for _, p := range parts {
+		p = strings.TrimSpace(p)
+		if p != "" {
+			out = append(out, p)
+		}
+	}
+
+	return out
+}

backend/internal/handlers/export_test.go

@@ -0,0 +1,10 @@
+package handlers
+
+import "log/slog"
+
+// NewForTest builds a Handlers around a report sink and logger,
+// bypassing the fx graph so handler behaviour (including the
+// storage failure path) is exercisable in unit tests.
+func NewForTest(buf reportAppender, log *slog.Logger) *Handlers {
+	return &Handlers{buf: buf, log: log}
+}

backend/internal/handlers/handlers.go

@@ -18,6 +18,13 @@ import (
 
 const jsonContentType = "application/json; charset=utf-8"
 
+// reportAppender is the subset of the report buffer the handlers
+// depend on. Defining it here keeps the storage failure path
+// exercisable with a stub in tests.
+type reportAppender interface {
+	Append(v any) error
+}
+
 // Params defines the dependencies for Handlers.
 type Params struct {
 	fx.In
@@ -30,7 +37,7 @@ type Params struct {
 
 // Handlers provides HTTP handler factories for all endpoints.
 type Handlers struct {
-	buf    *reportbuf.Buffer
+	buf    reportAppender
 	hc     *healthcheck.Healthcheck
 	log    *slog.Logger
 	params *Params
@@ -72,3 +79,15 @@ func (s *Handlers) respondJSON(
 		}
 	}
 }
+
+// decodeJSON decodes the request body into v. The body is
+// expected to already be bounded by the body-size middleware, so
+// a caller can distinguish an over-limit body from malformed
+// JSON by testing the returned error for *http.MaxBytesError.
+func (s *Handlers) decodeJSON(
+	_ http.ResponseWriter,
+	r *http.Request,
+	v any,
+) error {
+	return json.NewDecoder(r.Body).Decode(v)
+}

backend/internal/handlers/report.go

@@ -2,10 +2,14 @@ package handlers
 
 import (
 	"encoding/json"
+	"errors"
 	"net/http"
 )
 
-const maxReportBodyBytes = 1 << 20 // 1 MiB
+// maxLoggedFieldBytes bounds untrusted string fields before they
+// are logged, so a caller cannot inflate log volume with an
+// oversized value.
+const maxLoggedFieldBytes = 128
 
 type reportSample struct {
 	T       int64   `json:"t"`
@@ -35,48 +39,76 @@ func (s *Handlers) HandleReport() http.HandlerFunc {
 	}
 
 	return func(w http.ResponseWriter, r *http.Request) {
-		r.Body = http.MaxBytesReader(
-			w, r.Body, maxReportBodyBytes,
-		)
-
 		var rpt report
 
-		err := json.NewDecoder(r.Body).Decode(&rpt)
+		err := s.decodeJSON(w, r, &rpt)
 		if err != nil {
-			s.log.Error("failed to decode report",
-				"error", err,
-			)
 			s.respondJSON(w, r,
 				&response{Status: "error"},
-				http.StatusBadRequest,
+				s.decodeErrorStatus(err),
 			)
 
 			return
 		}
 
-		totalSamples := 0
-		for _, h := range rpt.Hosts {
-			totalSamples += len(h.History)
-		}
+		s.logReportReceived(rpt)
 
-		s.log.Info("report received",
-			"client_id", rpt.ClientID,
-			"timestamp", rpt.Timestamp,
-			"host_count", len(rpt.Hosts),
-			"total_samples", totalSamples,
-			"geo", string(rpt.Geo),
-		)
-
-		bufErr := s.buf.Append(rpt)
-		if bufErr != nil {
-			s.log.Error("failed to buffer report",
-				"error", bufErr,
+		err = s.buf.Append(rpt)
+		if err != nil {
+			s.log.Error("failed to buffer report", "error", err)
+			s.respondJSON(w, r,
+				&response{Status: "error"},
+				http.StatusInternalServerError,
 			)
+
+			return
 		}
 
-		s.respondJSON(w, r,
-			&response{Status: "ok"},
-			http.StatusOK,
-		)
+		s.respondJSON(w, r, &response{Status: "ok"}, http.StatusOK)
 	}
 }
+
+// decodeErrorStatus logs a report decode failure and returns the
+// status to send: 413 when the body exceeded the size limit,
+// otherwise 400 for malformed JSON.
+func (s *Handlers) decodeErrorStatus(err error) int {
+	var tooLarge *http.MaxBytesError
+	if errors.As(err, &tooLarge) {
+		s.log.Warn("report body too large", "limit_bytes", tooLarge.Limit)
+
+		return http.StatusRequestEntityTooLarge
+	}
+
+	s.log.Error("failed to decode report", "error", err)
+
+	return http.StatusBadRequest
+}
+
+// logReportReceived logs an accepted report. Untrusted fields are
+// bounded (client_id, timestamp) or reduced to a length
+// (geo_bytes) so the raw attacker-controlled body never reaches
+// the log.
+func (s *Handlers) logReportReceived(rpt report) {
+	totalSamples := 0
+	for _, h := range rpt.Hosts {
+		totalSamples += len(h.History)
+	}
+
+	s.log.Info("report received",
+		"client_id", boundedForLog(rpt.ClientID),
+		"timestamp", boundedForLog(rpt.Timestamp),
+		"host_count", len(rpt.Hosts),
+		"total_samples", totalSamples,
+		"geo_bytes", len(rpt.Geo),
+	)
+}
+
+// boundedForLog truncates an untrusted string to a fixed byte
+// bound so an attacker-controlled field cannot dominate the log.
+func boundedForLog(s string) string {
+	if len(s) > maxLoggedFieldBytes {
+		return s[:maxLoggedFieldBytes]
+	}
+
+	return s
+}

backend/internal/handlers/report_test.go

@@ -0,0 +1,141 @@
+package handlers_test
+
+import (
+	"bytes"
+	"encoding/json"
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/netwatch/internal/handlers"
+)
+
+var errStorageFailed = errors.New("storage failed")
+
+// stubAppender drives the storage success/failure path without a
+// real buffer or disk.
+type stubAppender struct {
+	err error
+}
+
+func (s stubAppender) Append(any) error { return s.err }
+
+func newTestHandlers(buf stubAppender, out io.Writer) *handlers.Handlers {
+	return handlers.NewForTest(buf, slog.New(slog.NewJSONHandler(out, nil)))
+}
+
+func decodeStatus(t *testing.T, body []byte) string {
+	t.Helper()
+
+	var resp struct {
+		Status string `json:"status"`
+	}
+
+	err := json.Unmarshal(body, &resp)
+	if err != nil {
+		t.Fatalf("response body not JSON: %v (%q)", err, body)
+	}
+
+	return resp.Status
+}
+
+func TestHandleReportStorageFailureIsNon2xx(t *testing.T) {
+	t.Parallel()
+
+	h := newTestHandlers(stubAppender{err: errStorageFailed}, io.Discard)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{"clientId":"c1","hosts":[]}`),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code < 500 {
+		t.Fatalf("storage failure status = %d, want a 5xx", rec.Code)
+	}
+
+	if got := decodeStatus(t, rec.Body.Bytes()); got != "error" {
+		t.Fatalf("status field = %q, want %q", got, "error")
+	}
+}
+
+func TestHandleReportMalformedJSONIs400(t *testing.T) {
+	t.Parallel()
+
+	h := newTestHandlers(stubAppender{}, io.Discard)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{not json`),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("malformed status = %d, want %d",
+			rec.Code, http.StatusBadRequest)
+	}
+}
+
+func TestHandleReportOversizeIs413(t *testing.T) {
+	t.Parallel()
+
+	const limit = 32
+
+	h := newTestHandlers(stubAppender{}, io.Discard)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{"clientId":"`+strings.Repeat("x", 200)+`"}`),
+	)
+	// Emulate the body-size middleware capping the body so the
+	// handler observes a *http.MaxBytesError while decoding.
+	req.Body = http.MaxBytesReader(rec, req.Body, limit)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("oversize status = %d, want %d",
+			rec.Code, http.StatusRequestEntityTooLarge)
+	}
+}
+
+func TestHandleReportDoesNotLogRawGeo(t *testing.T) {
+	t.Parallel()
+
+	const sentinel = "SENSITIVE-GEO-BLOB"
+
+	var logbuf bytes.Buffer
+
+	h := newTestHandlers(stubAppender{}, &logbuf)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(
+			`{"clientId":"c1","geo":{"raw":"`+sentinel+`"},"hosts":[]}`,
+		),
+	)
+
+	h.HandleReport().ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+
+	if strings.Contains(logbuf.String(), sentinel) {
+		t.Fatal("raw geo bytes were written to the log")
+	}
+
+	if !strings.Contains(logbuf.String(), "geo_bytes") {
+		t.Fatal("expected a bounded geo_bytes field in the log")
+	}
+}

backend/internal/middleware/export_test.go

@@ -0,0 +1,28 @@
+package middleware
+
+import (
+	"log/slog"
+	"net/http"
+	"net/netip"
+)
+
+// Test-only wrappers exposing unexported helpers to the
+// external middleware_test package.
+
+// NewWithLogger builds a Middleware around a logger for tests
+// that exercise the logging paths without the fx graph.
+func NewWithLogger(log *slog.Logger) *Middleware {
+	return &Middleware{log: log}
+}
+
+func ClientIP(
+	remoteAddr string,
+	header http.Header,
+	trusted []netip.Prefix,
+) string {
+	return clientIP(remoteAddr, header, trusted)
+}
+
+func ParseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
+	return parseTrustedProxies(cidrs)
+}

backend/internal/middleware/middleware.go

@@ -3,9 +3,15 @@
 package middleware
 
 import (
+	"errors"
+	"fmt"
+	"io"
 	"log/slog"
 	"net"
 	"net/http"
+	"net/netip"
+	"runtime/debug"
+	"strings"
 	"time"
 
 	"sneak.berlin/go/netwatch/internal/config"
@@ -19,6 +25,23 @@ import (
 
 const corsMaxAgeSec = 300
 
+// jsonErrorBody is the body written for errors raised inside
+// middleware, matching the {"status":"error"} shape the handlers
+// return so clients see one error contract across the API.
+const (
+	jsonContentType = "application/json; charset=utf-8"
+	jsonErrorBody   = "{\"status\":\"error\"}\n"
+)
+
+// Security header values. The backend is a JSON API with no
+// HTML surface, so the CSP forbids every resource type and
+// framing outright.
+const (
+	hstsValue              = "max-age=31536000; includeSubDomains"
+	cspValue               = "default-src 'none'; frame-ancestors 'none'"
+	permissionsPolicyValue = "camera=(), microphone=(), geolocation=()"
+)
+
 // Params defines the dependencies for Middleware.
 type Params struct {
 	fx.In
@@ -30,8 +53,9 @@ type Params struct {
 
 // Middleware holds shared state for middleware factories.
 type Middleware struct {
-	log    *slog.Logger
-	params *Params
+	log            *slog.Logger
+	params         *Params
+	trustedProxies []netip.Prefix
 }
 
 // New creates a Middleware instance.
@@ -39,13 +63,38 @@ func New(
 	_ fx.Lifecycle,
 	params Params,
 ) (*Middleware, error) {
+	trusted, err := parseTrustedProxies(params.Config.TrustedProxies)
+	if err != nil {
+		return nil, err
+	}
+
 	s := new(Middleware)
 	s.params = &params
 	s.log = params.Logger.Get()
+	s.trustedProxies = trusted
 
 	return s, nil
 }
 
+// parseTrustedProxies converts CIDR strings into prefixes,
+// failing fast on any malformed entry.
+func parseTrustedProxies(cidrs []string) ([]netip.Prefix, error) {
+	prefixes := make([]netip.Prefix, 0, len(cidrs))
+
+	for _, cidr := range cidrs {
+		prefix, err := netip.ParsePrefix(cidr)
+		if err != nil {
+			return nil, fmt.Errorf(
+				"trusted proxy %q: %w", cidr, err,
+			)
+		}
+
+		prefixes = append(prefixes, prefix.Masked())
+	}
+
+	return prefixes, nil
+}
+
 type loggingResponseWriter struct {
 	http.ResponseWriter
 
@@ -72,6 +121,70 @@ func ipFromHostPort(hostPort string) string {
 	return host
 }
 
+// clientIP resolves the caller's address. X-Forwarded-For and
+// X-Real-IP are honoured only when the direct peer is a
+// trusted proxy; otherwise the direct peer is returned so a
+// spoofed header cannot forge the logged address.
+func clientIP(
+	remoteAddr string,
+	header http.Header,
+	trusted []netip.Prefix,
+) string {
+	peer := ipFromHostPort(remoteAddr)
+
+	if !addrInAny(peer, trusted) {
+		return peer
+	}
+
+	if xff := firstForwardedFor(header.Get("X-Forwarded-For")); xff != "" {
+		return xff
+	}
+
+	if xr := strings.TrimSpace(header.Get("X-Real-IP")); validIP(xr) {
+		return xr
+	}
+
+	return peer
+}
+
+// firstForwardedFor returns the left-most valid address in an
+// X-Forwarded-For list (the original client), or "" if none.
+func firstForwardedFor(value string) string {
+	for part := range strings.SplitSeq(value, ",") {
+		candidate := strings.TrimSpace(part)
+		if validIP(candidate) {
+			return candidate
+		}
+	}
+
+	return ""
+}
+
+func validIP(s string) bool {
+	_, err := netip.ParseAddr(s)
+
+	return err == nil
+}
+
+// addrInAny reports whether s parses as an address contained
+// in any of the trusted prefixes.
+func addrInAny(s string, trusted []netip.Prefix) bool {
+	addr, err := netip.ParseAddr(s)
+	if err != nil {
+		return false
+	}
+
+	addr = addr.Unmap()
+
+	for _, prefix := range trusted {
+		if prefix.Contains(addr) {
+			return true
+		}
+	}
+
+	return false
+}
+
 // Logging returns middleware that logs each request with
 // timing, status code, and client information.
 func (s *Middleware) Logging() func(http.Handler) http.Handler {
@@ -96,7 +209,11 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 						"referer", r.Referer(),
 						"proto", r.Proto,
 						"remote_ip",
-						ipFromHostPort(r.RemoteAddr),
+						clientIP(
+							r.RemoteAddr,
+							r.Header,
+							s.trustedProxies,
+						),
 						"status", lrw.statusCode,
 						"latency_ms",
 						latency.Milliseconds(),
@@ -109,6 +226,100 @@ func (s *Middleware) Logging() func(http.Handler) http.Handler {
 	}
 }
 
+// SecurityHeaders returns middleware that sets response
+// security headers. It runs before CORS so the headers are
+// present on preflight responses the CORS handler writes.
+func (s *Middleware) SecurityHeaders() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				h := w.Header()
+				h.Set("Strict-Transport-Security", hstsValue)
+				h.Set("Content-Security-Policy", cspValue)
+				h.Set("X-Frame-Options", "DENY")
+				h.Set("X-Content-Type-Options", "nosniff")
+				h.Set("Referrer-Policy", "no-referrer")
+				h.Set("Permissions-Policy", permissionsPolicyValue)
+
+				next.ServeHTTP(w, r)
+			},
+		)
+	}
+}
+
+// writeJSONError writes the shared JSON error body with the
+// given status. Used where middleware must reject a request
+// before it reaches a handler.
+func writeJSONError(w http.ResponseWriter, status int) {
+	w.Header().Set("Content-Type", jsonContentType)
+	w.WriteHeader(status)
+	_, _ = io.WriteString(w, jsonErrorBody)
+}
+
+// MaxBodyBytes returns middleware that caps the request body at
+// limit bytes. A declared Content-Length over the limit is
+// rejected immediately with 413. Bodies without a declared
+// length (or that understate it) are capped as they are read, so
+// a handler that reads the body sees a *http.MaxBytesError it can
+// map to 413. Mount it with a different limit on a route group
+// that needs a different bound.
+func (s *Middleware) MaxBodyBytes(
+	limit int64,
+) func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				if r.ContentLength > limit {
+					writeJSONError(
+						w,
+						http.StatusRequestEntityTooLarge,
+					)
+
+					return
+				}
+
+				r.Body = http.MaxBytesReader(w, r.Body, limit)
+
+				next.ServeHTTP(w, r)
+			},
+		)
+	}
+}
+
+// Recoverer returns middleware that recovers from a panic in a
+// downstream handler, logs the panic and stack trace through
+// slog, and responds 500 with no body. http.ErrAbortHandler is
+// re-panicked so the server can abort the response as intended.
+func (s *Middleware) Recoverer() func(http.Handler) http.Handler {
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(
+			func(w http.ResponseWriter, r *http.Request) {
+				defer func() {
+					rec := recover()
+					if rec == nil {
+						return
+					}
+
+					err, ok := rec.(error)
+					if ok && errors.Is(err, http.ErrAbortHandler) {
+						panic(rec)
+					}
+
+					s.log.ErrorContext(r.Context(),
+						"panic recovered",
+						"panic", fmt.Sprintf("%v", rec),
+						"stack", string(debug.Stack()),
+					)
+
+					w.WriteHeader(http.StatusInternalServerError)
+				}()
+
+				next.ServeHTTP(w, r)
+			},
+		)
+	}
+}
+
 // CORS returns middleware that adds permissive CORS headers.
 func (s *Middleware) CORS() func(http.Handler) http.Handler {
 	return cors.Handler(cors.Options{

backend/internal/middleware/middleware_test.go

@@ -0,0 +1,231 @@
+package middleware_test
+
+import (
+	"bytes"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/netwatch/internal/middleware"
+)
+
+func mustPrefixes(t *testing.T, cidrs ...string) []netip.Prefix {
+	t.Helper()
+
+	prefixes, err := middleware.ParseTrustedProxies(cidrs)
+	if err != nil {
+		t.Fatalf("ParseTrustedProxies(%v): %v", cidrs, err)
+	}
+
+	return prefixes
+}
+
+func TestParseTrustedProxiesRejectsMalformed(t *testing.T) {
+	t.Parallel()
+
+	_, err := middleware.ParseTrustedProxies([]string{"not-a-cidr"})
+	if err == nil {
+		t.Fatal("expected error for malformed CIDR, got nil")
+	}
+}
+
+type clientIPCase struct {
+	name       string
+	remoteAddr string
+	xff        string
+	xRealIP    string
+	want       string
+}
+
+func clientIPCases() []clientIPCase {
+	return []clientIPCase{
+		{
+			name:       "trusted proxy uses forwarded-for",
+			remoteAddr: "127.0.0.1:5000",
+			xff:        "203.0.113.7",
+			want:       "203.0.113.7",
+		},
+		{
+			name:       "trusted proxy uses left-most of chain",
+			remoteAddr: "10.1.2.3:5000",
+			xff:        "203.0.113.7, 10.1.2.3",
+			want:       "203.0.113.7",
+		},
+		{
+			name:       "trusted proxy falls back to x-real-ip",
+			remoteAddr: "127.0.0.1:5000",
+			xRealIP:    "203.0.113.9",
+			want:       "203.0.113.9",
+		},
+		{
+			name:       "untrusted peer ignores forwarded-for",
+			remoteAddr: "198.51.100.4:5000",
+			xff:        "203.0.113.7",
+			want:       "198.51.100.4",
+		},
+		{
+			name:       "untrusted peer ignores x-real-ip",
+			remoteAddr: "198.51.100.4:5000",
+			xRealIP:    "203.0.113.9",
+			want:       "198.51.100.4",
+		},
+		{
+			name:       "trusted proxy with no headers uses peer",
+			remoteAddr: "10.1.2.3:5000",
+			want:       "10.1.2.3",
+		},
+		{
+			name:       "trusted proxy with garbage header uses peer",
+			remoteAddr: "127.0.0.1:5000",
+			xff:        "not-an-ip",
+			want:       "127.0.0.1",
+		},
+	}
+}
+
+func TestClientIP(t *testing.T) {
+	t.Parallel()
+
+	trusted := mustPrefixes(t, "127.0.0.1/32", "::1/128", "10.0.0.0/8")
+
+	for _, tc := range clientIPCases() {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			header := http.Header{}
+			if tc.xff != "" {
+				header.Set("X-Forwarded-For", tc.xff)
+			}
+
+			if tc.xRealIP != "" {
+				header.Set("X-Real-IP", tc.xRealIP)
+			}
+
+			got := middleware.ClientIP(tc.remoteAddr, header, trusted)
+			if got != tc.want {
+				t.Errorf("ClientIP() = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestSecurityHeaders(t *testing.T) {
+	t.Parallel()
+
+	handler := (&middleware.Middleware{}).SecurityHeaders()(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/", http.NoBody)
+	handler.ServeHTTP(rec, req)
+
+	want := map[string]string{
+		"Strict-Transport-Security": "max-age=31536000; includeSubDomains",
+		"Content-Security-Policy":   "default-src 'none'; frame-ancestors 'none'",
+		"X-Frame-Options":           "DENY",
+		"X-Content-Type-Options":    "nosniff",
+		"Referrer-Policy":           "no-referrer",
+		"Permissions-Policy":        "camera=(), microphone=(), geolocation=()",
+	}
+
+	for name, value := range want {
+		if got := rec.Header().Get(name); got != value {
+			t.Errorf("header %s = %q, want %q", name, got, value)
+		}
+	}
+}
+
+// TestMaxBodyBytesRejectsOversizeOnNonReadingRoute confirms the
+// limit is enforced even for a handler that never reads the body
+// (for example the health check), via the Content-Length check.
+func TestMaxBodyBytesRejectsOversizeOnNonReadingRoute(t *testing.T) {
+	t.Parallel()
+
+	const limit = 16
+
+	called := false
+	handler := (&middleware.Middleware{}).MaxBodyBytes(limit)(
+		http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {
+			called = true
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(
+		http.MethodPost, "/.well-known/healthcheck",
+		strings.NewReader(strings.Repeat("x", limit+1)),
+	)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusRequestEntityTooLarge {
+		t.Fatalf("status = %d, want %d",
+			rec.Code, http.StatusRequestEntityTooLarge)
+	}
+
+	if called {
+		t.Fatal("handler ran despite oversize body")
+	}
+}
+
+func TestMaxBodyBytesAllowsWithinLimit(t *testing.T) {
+	t.Parallel()
+
+	const limit = 64
+
+	handler := (&middleware.Middleware{}).MaxBodyBytes(limit)(
+		http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(http.StatusOK)
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(
+		http.MethodPost, "/api/v1/reports",
+		strings.NewReader(`{"clientId":"c1"}`),
+	)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusOK {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusOK)
+	}
+}
+
+func TestRecovererReturns500AndLogsThroughSlog(t *testing.T) {
+	t.Parallel()
+
+	var logbuf bytes.Buffer
+
+	mw := middleware.NewWithLogger(
+		slog.New(slog.NewJSONHandler(&logbuf, nil)),
+	)
+
+	handler := mw.Recoverer()(
+		http.HandlerFunc(func(_ http.ResponseWriter, _ *http.Request) {
+			panic("boom")
+		}),
+	)
+
+	rec := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/", http.NoBody)
+	handler.ServeHTTP(rec, req)
+
+	if rec.Code != http.StatusInternalServerError {
+		t.Fatalf("status = %d, want %d",
+			rec.Code, http.StatusInternalServerError)
+	}
+
+	out := logbuf.String()
+	if !strings.Contains(out, "panic recovered") {
+		t.Fatalf("panic was not logged through slog: %q", out)
+	}
+
+	if !strings.Contains(out, `"level":"ERROR"`) {
+		t.Fatalf("panic log was not structured JSON at error level: %q", out)
+	}
+}

backend/internal/server/http.go

@@ -8,20 +8,24 @@ import (
 )
 
 const (
-	readTimeout    = 10 * time.Second
-	writeTimeout   = 10 * time.Second
-	maxHeaderBytes = 1 << 20 // 1 MiB
+	readTimeout       = 10 * time.Second
+	readHeaderTimeout = 5 * time.Second
+	writeTimeout      = 10 * time.Second
+	idleTimeout       = 60 * time.Second
+	maxHeaderBytes    = 1 << 20 // 1 MiB
 )
 
 func (s *Server) serveUntilShutdown() {
 	listenAddr := fmt.Sprintf(":%d", s.params.Config.Port)
 
 	s.httpServer = &http.Server{
-		Addr:           listenAddr,
-		Handler:        s,
-		MaxHeaderBytes: maxHeaderBytes,
-		ReadTimeout:    readTimeout,
-		WriteTimeout:   writeTimeout,
+		Addr:              listenAddr,
+		Handler:           s,
+		MaxHeaderBytes:    maxHeaderBytes,
+		ReadTimeout:       readTimeout,
+		ReadHeaderTimeout: readHeaderTimeout,
+		WriteTimeout:      writeTimeout,
+		IdleTimeout:       idleTimeout,
 	}
 
 	s.SetupRoutes()

backend/internal/server/routes.go

@@ -7,17 +7,26 @@ import (
 	"github.com/go-chi/chi/v5/middleware"
 )
 
-const requestTimeout = 60 * time.Second
+const (
+	requestTimeout = 60 * time.Second
+
+	// maxRequestBodyBytes caps every request body. A route that
+	// needs a different bound mounts s.mw.MaxBodyBytes with its
+	// own value on its group.
+	maxRequestBodyBytes int64 = 1 << 20 // 1 MiB
+)
 
 // SetupRoutes configures the chi router with middleware and
 // all application routes.
 func (s *Server) SetupRoutes() {
 	s.router = chi.NewRouter()
 
-	s.router.Use(middleware.Recoverer)
+	s.router.Use(s.mw.Recoverer())
 	s.router.Use(middleware.RequestID)
 	s.router.Use(s.mw.Logging())
+	s.router.Use(s.mw.SecurityHeaders())
 	s.router.Use(s.mw.CORS())
+	s.router.Use(s.mw.MaxBodyBytes(maxRequestBodyBytes))
 	s.router.Use(middleware.Timeout(requestTimeout))
 
 	s.router.Get(

src/main.js

@@ -537,6 +537,12 @@ class SparklineRenderer {
 
 // --- UI Renderer -------------------------------------------------------------
 
+// The per-host status line must stay wrappable: its populated content is
+// wider than the host column at a 320px viewport, and `whitespace-nowrap`
+// here overflows the element and forces the whole document to scroll
+// horizontally.
+const STATUS_TEXT_CLASS = "status-text text-xs text-right col-span-2 mt-5";
+
 function hostRowHTML(host, index, showPin = true) {
     const pinColor = host.pinned
         ? "text-blue-500"
@@ -562,7 +568,7 @@ function hostRowHTML(host, index, showPin = true) {
             <span class="text-gray-500">---</span>
           </div>
           <a href="${host.url}" target="_blank" rel="noopener" class="text-xs text-gray-500 truncate block col-span-2 -mt-2">${host.url}</a>
-          <div class="status-text text-xs text-gray-500 whitespace-nowrap text-right col-span-2 mt-5" data-host="${index}">waiting...</div>
+          <div class="${STATUS_TEXT_CLASS} text-gray-500" data-host="${index}">waiting...</div>
         </div>
         <div class="flex-grow sparkline-container rounded overflow-hidden border border-gray-700/30">
           <canvas class="sparkline-canvas w-full" data-host="${index}" height="${CONFIG.canvasHeight}"></canvas>
@@ -671,7 +677,7 @@ function buildUI(state) {
         </p>
         <p class="mt-2"><a href="https://git.eeqj.de/sneak/netwatch/commit/${__COMMIT_FULL__}" target="_blank" rel="noopener" class="text-gray-600 hover:text-gray-400">${__COMMIT_HASH__}</a></p>
         <p class="mt-2">
-          <label class="cursor-pointer">
+          <label class="debug-toggle-label cursor-pointer">
             <input type="checkbox" id="debug-toggle" class="mr-1">
             <span>Debug log</span>
           </label>
@@ -688,6 +694,26 @@ function buildUI(state) {
 
 // --- UI Updaters -------------------------------------------------------------
 
+// Renders `min 1ms / med 2ms / avg 3ms / max 4ms`. Each label, value and
+// trailing separator is one unbreakable unit, so wrapping only ever happens
+// between stats and a wrapped line never starts with a separator.
+function statusStatsHTML(stats) {
+    return stats
+        .map(([label, value], i) => {
+            const sep =
+                i < stats.length - 1
+                    ? ` <span class="text-gray-500">/</span>`
+                    : "";
+            return (
+                `<span class="whitespace-nowrap">` +
+                `<span class="text-gray-400">${label} </span>` +
+                `<span class="${latencyClass(value, "online")}">${value}ms</span>` +
+                `${sep}</span>`
+            );
+        })
+        .join(" ");
+}
+
 function updateHostRow(host, index) {
     const latencyEl = document.querySelector(
         `.latency-value[data-host="${index}"]`,
@@ -712,28 +738,22 @@ function updateHostRow(host, index) {
     const min = host.minLatency();
     const max = host.maxLatency();
     if (host.status === "online" && avg !== null) {
-        statusEl.innerHTML =
-            `<span class="text-gray-400">min </span><span class="${latencyClass(min, "online")}">${min}ms</span>` +
-            ` <span class="text-gray-500">/</span> ` +
-            `<span class="text-gray-400">med </span><span class="${latencyClass(med, "online")}">${med}ms</span>` +
-            ` <span class="text-gray-500">/</span> ` +
-            `<span class="text-gray-400">avg </span><span class="${latencyClass(avg, "online")}">${avg}ms</span>` +
-            ` <span class="text-gray-500">/</span> ` +
-            `<span class="text-gray-400">max </span><span class="${latencyClass(max, "online")}">${max}ms</span>`;
-        statusEl.className =
-            "status-text text-xs whitespace-nowrap text-right col-span-2 mt-5";
+        statusEl.innerHTML = statusStatsHTML([
+            ["min", min],
+            ["med", med],
+            ["avg", avg],
+            ["max", max],
+        ]);
+        statusEl.className = STATUS_TEXT_CLASS;
     } else if (host.status === "offline") {
         statusEl.textContent = "unreachable";
-        statusEl.className =
-            "status-text text-xs text-red-400 whitespace-nowrap text-right col-span-2 mt-5";
+        statusEl.className = `${STATUS_TEXT_CLASS} text-red-400`;
     } else if (host.status === "error") {
         statusEl.textContent = "timeout";
-        statusEl.className =
-            "status-text text-xs text-orange-400 whitespace-nowrap text-right col-span-2 mt-5";
+        statusEl.className = `${STATUS_TEXT_CLASS} text-orange-400`;
     } else {
         statusEl.textContent = "connecting...";
-        statusEl.className =
-            "status-text text-xs text-gray-500 whitespace-nowrap text-right col-span-2 mt-5";
+        statusEl.className = `${STATUS_TEXT_CLASS} text-gray-500`;
     }
 
     SparklineRenderer.draw(canvas, host.history);
@@ -1044,8 +1064,7 @@ function greyOutUI(state) {
         }
         if (statusEl) {
             statusEl.textContent = "paused";
-            statusEl.className =
-                "status-text text-xs text-gray-500 whitespace-nowrap text-right col-span-2 mt-5";
+            statusEl.className = `${STATUS_TEXT_CLASS} text-gray-500`;
         }
         // Grey out the status dot
         const row = document.querySelector(`.host-row[data-index="${i}"]`);

src/styles.css

@@ -14,6 +14,38 @@ body {
         ui-monospace, SFMono-Regular, "SF Mono", Menlo, Consolas, monospace;
 }
 
+/* ---- Minimum tap targets ----------------------------------------------
+   Every interactive control is at least 44x44 CSS px (Apple HIG, WCAG 2.2
+   SC 2.5.5). Not scoped to a breakpoint or to `pointer: coarse`: a large
+   phone in landscape is above the 768px breakpoint and still a touch
+   device. */
+
+/* The button grows to 44x44 while the negative margins keep its layout
+   footprint at the 16x16 of the icon inside it, so row height and the
+   icon's position are unchanged. */
+.pin-btn {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    width: 2.75rem;
+    height: 2.75rem;
+    margin: -0.875rem;
+}
+
+/* A select paints its own background and border, so it has to actually be
+   44 tall rather than borrow the trick above. */
+#interval-select {
+    min-height: 2.75rem;
+}
+
+/* The tappable target for #debug-toggle is the label wrapping it. */
+.debug-toggle-label {
+    display: inline-flex;
+    align-items: center;
+    justify-content: center;
+    min-height: 2.75rem;
+}
+
 .sparkline-container {
     background: linear-gradient(
         to bottom,
@@ -38,9 +70,10 @@ body {
         gap: 0.75rem;
     }
 
-    /* Pause button: smaller on mobile */
+    /* Pause button: smaller on mobile, but not below the tap-target floor */
     #pause-btn {
         padding: 0.5rem 1rem;
+        min-height: 2.75rem;
     }
 
     #pause-btn svg {