fix: validate dimensions and fit mode on encrypted URLs (closes #62)

The encrypted /v1/e/ route built its image request straight from the
decrypted payload, so a token could request an over-limit dimension
(reaching libvips and exhausting memory) or an unknown fit mode
(surfacing as a 500 from the processor). The token generator discarded
every strconv.Atoi error, silently turning non-numeric width, height,
quality, or ttl into 0 and applying no upper bound on dimensions.

Add a shared ValidateImageRequest in internal/imgcache enforcing the
MaxDimension bound and ValidateFitMode, and apply it on both the plain
image route and the encrypted route so both reject an over-limit size or
an unrecognized fit mode with 400. The generator now parses each numeric
field explicitly and returns 400 naming the field for non-numeric or
out-of-range input, with width and height bounds-checked so an unusable
token cannot be minted.

Model: opus-4-8

TODO.md

@@ -29,6 +29,17 @@ P1: implement blocked networks configuration to extend SSRF protection
 
 # Completed Steps
 
+- 2026-09-21 validate dimensions and fit mode on the encrypted-URL
+  route and the token generator (closes #62): added a shared
+  `ValidateImageRequest` in `internal/imgcache` enforcing the
+  `MaxDimension` bound and `ValidateFitMode`, applied by both the
+  `/v1/image/` and `/v1/e/` routes, so an over-limit size or an unknown
+  fit mode is a 400 rather than an out-of-memory or a 500 from the
+  processor; the URL generator now checks every numeric form field and
+  rejects a non-numeric or out-of-range `width`, `height`, `quality`,
+  or `ttl` with a 400 naming the field instead of coercing it to `0`,
+  and `width`/`height` are bounds-checked so an unusable token cannot be
+  minted
 - 2026-09-21 http.Server hardening (closes #92): added
   `HTTPReadHeaderTimeout` (10s, bounds the slowloris header dribble) and
   `HTTPIdleTimeout` (120s, bounds keep-alive reuse) alongside the

internal/handlers/auth.go

@@ -2,6 +2,8 @@ package handlers
 
 import (
 	"crypto/subtle"
+	"errors"
+	"fmt"
 	"html/template"
 	"net/http"
 	"net/url"
@@ -13,6 +15,11 @@ import (
 	"sneak.berlin/go/pixa/internal/templates"
 )
 
+// errInvalidFormField reports a generator form field whose value is
+// non-numeric or out of range. The offending field name is wrapped in so the
+// response can name it.
+var errInvalidFormField = errors.New("invalid")
+
 // HandleRoot serves the login page or generator page based on authentication state.
 func (s *Handlers) HandleRoot() http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
@@ -98,18 +105,26 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 		// Validate source URL
 		parsed, err := url.Parse(sourceURL)
 		if err != nil || parsed.Host == "" {
-			s.renderGeneratorWithForm(w, r, "Invalid source URL", r.Form)
+			s.renderGeneratorWithForm(w, r, "Invalid source URL", r.Form,
+				http.StatusBadRequest)
 
 			return
 		}
 
-		payload, expiresAt, ttl := buildGeneratePayload(parsed, r.Form)
+		payload, expiresAt, ttl, err := buildGeneratePayload(parsed, r.Form)
+		if err != nil {
+			s.renderGeneratorWithForm(w, r, err.Error(), r.Form,
+				http.StatusBadRequest)
+
+			return
+		}
 
 		// Generate encrypted token
 		token, err := s.encGen.Generate(payload)
 		if err != nil {
 			s.log.Error("failed to generate encrypted URL", "error", err)
-			s.renderGeneratorWithForm(w, r, "Failed to generate URL", r.Form)
+			s.renderGeneratorWithForm(w, r, "Failed to generate URL", r.Form,
+				http.StatusInternalServerError)
 
 			return
 		}
@@ -137,17 +152,38 @@ func (s *Handlers) HandleGenerateURL() http.HandlerFunc {
 }
 
 // buildGeneratePayload parses the numeric form fields and assembles the
-// encrypted URL payload. ttl=0 means never expires (ExpiresAt stays 0).
+// encrypted URL payload. ttl=0 means never expires (ExpiresAt stays 0). A
+// non-numeric or out-of-range field, or an unrecognized fit mode, is a client
+// error naming the offending field, so an unusable token is never minted.
 func buildGeneratePayload(
 	parsed *url.URL, form url.Values,
-) (*encurl.Payload, time.Time, int) {
-	width, _ := strconv.Atoi(form.Get("width"))
-	height, _ := strconv.Atoi(form.Get("height"))
-	quality, _ := strconv.Atoi(form.Get("quality"))
-	ttl, _ := strconv.Atoi(form.Get("ttl"))
+) (*encurl.Payload, time.Time, int, error) {
+	width, err := parseFormDimension(form, "width")
+	if err != nil {
+		return nil, time.Time{}, 0, err
+	}
 
-	if quality <= 0 {
-		quality = 85
+	height, err := parseFormDimension(form, "height")
+	if err != nil {
+		return nil, time.Time{}, 0, err
+	}
+
+	quality, err := parseFormCount(form, "quality", encurl.DefaultQuality)
+	if err != nil {
+		return nil, time.Time{}, 0, err
+	}
+
+	ttl, err := parseFormCount(form, "ttl", 0)
+	if err != nil {
+		return nil, time.Time{}, 0, err
+	}
+
+	fitMode := imgcache.FitMode(form.Get("fit"))
+
+	err = imgcache.ValidateFitMode(fitMode)
+	if err != nil {
+		return nil, time.Time{}, 0,
+			fmt.Errorf("%w: %s", imgcache.ErrInvalidFitMode, form.Get("fit"))
 	}
 
 	var (
@@ -168,11 +204,45 @@ func buildGeneratePayload(
 		Height:      height,
 		Format:      imgcache.ImageFormat(form.Get("format")),
 		Quality:     quality,
-		FitMode:     imgcache.FitMode(form.Get("fit")),
+		FitMode:     fitMode,
 		ExpiresAt:   expiresAtUnix,
 	}
 
-	return payload, expiresAt, ttl
+	return payload, expiresAt, ttl, nil
+}
+
+// parseFormDimension reads an optional width or height form field. An empty
+// value means "original size" (0). A non-numeric, negative, or over-limit
+// value is rejected with an error naming the field.
+func parseFormDimension(form url.Values, field string) (int, error) {
+	raw := form.Get(field)
+	if raw == "" {
+		return 0, nil
+	}
+
+	value, err := strconv.Atoi(raw)
+	if err != nil || value < 0 || value > imgcache.MaxDimension {
+		return 0, fmt.Errorf("%w %s", errInvalidFormField, field)
+	}
+
+	return value, nil
+}
+
+// parseFormCount reads an optional non-negative integer form field, returning
+// def when the field is empty and an error naming the field when the value is
+// non-numeric or negative.
+func parseFormCount(form url.Values, field string, def int) (int, error) {
+	raw := form.Get(field)
+	if raw == "" {
+		return def, nil
+	}
+
+	value, err := strconv.Atoi(raw)
+	if err != nil || value < 0 {
+		return 0, fmt.Errorf("%w %s", errInvalidFormField, field)
+	}
+
+	return value, nil
 }
 
 // generatorData holds template data for the generator page.
@@ -212,6 +282,15 @@ func (s *Handlers) renderLogin(
 
 func (s *Handlers) renderGenerator(
 	w http.ResponseWriter, r *http.Request, data *generatorData,
+) {
+	s.renderGeneratorStatus(w, r, data, http.StatusOK)
+}
+
+// renderGeneratorStatus renders the generator page with an explicit HTTP
+// status. The status is written before the body so both it and the
+// Content-Type header take effect; a rejected form uses 400.
+func (s *Handlers) renderGeneratorStatus(
+	w http.ResponseWriter, r *http.Request, data *generatorData, status int,
 ) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 
@@ -221,17 +300,19 @@ func (s *Handlers) renderGenerator(
 
 	data.CSRFField = csrfField(r)
 
+	w.WriteHeader(status)
+
 	err := templates.Render(w, "generator.html", data)
 	if err != nil {
 		s.log.Error("failed to render generator template", "error", err)
-		http.Error(w, "Internal server error", http.StatusInternalServerError)
 	}
 }
 
 func (s *Handlers) renderGeneratorWithForm(
-	w http.ResponseWriter, r *http.Request, errorMsg string, form url.Values,
+	w http.ResponseWriter, r *http.Request, errorMsg string,
+	form url.Values, status int,
 ) {
-	s.renderGenerator(w, r, &generatorData{
+	s.renderGeneratorStatus(w, r, &generatorData{
 		Error:       errorMsg,
 		FormURL:     form.Get("url"),
 		FormWidth:   form.Get("width"),
@@ -240,7 +321,7 @@ func (s *Handlers) renderGeneratorWithForm(
 		FormQuality: form.Get("quality"),
 		FormFit:     form.Get("fit"),
 		FormTTL:     form.Get("ttl"),
-	})
+	}, status)
 }
 
 func (s *Handlers) buildGeneratedURL(r *http.Request, token, format string) string {

internal/handlers/auth_generate_internal_test.go

@@ -0,0 +1,66 @@
+package handlers
+
+import (
+	"maps"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
+	"strings"
+	"testing"
+)
+
+// generatePost submits the /generate form with a valid session and CSRF token
+// plus the caller's extra fields, returning the recorder.
+func generatePost(
+	t *testing.T, extra url.Values,
+) *httptest.ResponseRecorder {
+	t.Helper()
+
+	h, srv := newCSRFTestRouter(t)
+
+	sessionCookie := newSessionCookie(t, h)
+	cookies, token := csrfCredentials(t, srv, []*http.Cookie{sessionCookie})
+	cookies = append(cookies, sessionCookie)
+
+	form := url.Values{
+		sourceURLField: {testSourceURL},
+		csrfTokenField: {token},
+	}
+	maps.Copy(form, extra)
+
+	return postForm(srv, "/generate", cookies, form)
+}
+
+// TestGeneratePostRejectsNonNumericWidth verifies that a non-numeric width is
+// rejected with 400 naming the field rather than being coerced to 0 and
+// minting a 0-width token.
+func TestGeneratePostRejectsNonNumericWidth(t *testing.T) {
+	t.Parallel()
+
+	rec := generatePost(t, url.Values{"width": {"abc"}})
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+
+	if strings.Contains(rec.Body.String(), "/v1/e/") {
+		t.Error("a token was generated for non-numeric width")
+	}
+}
+
+// TestGeneratePostRejectsOverLimitWidth verifies that a width beyond
+// MaxDimension is rejected at generation time so an unusable token cannot be
+// minted.
+func TestGeneratePostRejectsOverLimitWidth(t *testing.T) {
+	t.Parallel()
+
+	rec := generatePost(t, url.Values{"width": {"100000"}})
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+
+	if strings.Contains(rec.Body.String(), "/v1/e/") {
+		t.Error("a token was generated for an over-limit width")
+	}
+}

internal/handlers/image.go

@@ -110,13 +110,6 @@ func (s *Handlers) parseImageRequest(
 
 	if fit := query.Get("fit"); fit != "" {
 		req.FitMode = imgcache.FitMode(fit)
-
-		fitErr := imgcache.ValidateFitMode(req.FitMode)
-		if fitErr != nil {
-			s.respondError(w, "invalid fit mode: "+fit, http.StatusBadRequest)
-
-			return nil, false
-		}
 	}
 
 	// Default quality if not set
@@ -129,6 +122,18 @@ func (s *Handlers) parseImageRequest(
 		req.FitMode = imgcache.FitCover
 	}
 
+	// Enforce dimension and fit-mode bounds, shared with the encrypted-URL
+	// route. Dimensions are already bounded by the path parser above; this
+	// also rejects an unrecognized fit mode with 400 instead of letting it
+	// reach the processor as a 500.
+	err = imgcache.ValidateImageRequest(req)
+	if err != nil {
+		s.respondError(w, "invalid image request: "+err.Error(),
+			http.StatusBadRequest)
+
+		return nil, false
+	}
+
 	return req, true
 }
 

internal/handlers/imageenc.go

@@ -50,6 +50,19 @@ func (s *Handlers) HandleImageEnc() http.HandlerFunc {
 		// Convert payload to ImageRequest
 		req := payload.ToImageRequest()
 
+		// Apply the same dimension and fit-mode bounds as the plain image
+		// route: a sealed payload is trusted for its origin, not for staying
+		// within limits, so an over-limit size or unknown fit mode is a 400
+		// here rather than an out-of-memory or a 500 from the processor.
+		err = imgcache.ValidateImageRequest(req)
+		if err != nil {
+			s.log.Debug("encrypted URL failed validation", "error", err)
+			s.respondError(w, "invalid encrypted URL: "+err.Error(),
+				http.StatusBadRequest)
+
+			return
+		}
+
 		// Log the request
 		s.log.Debug("encrypted image request",
 			"host", req.SourceHost,

internal/handlers/imageenc_internal_test.go

@@ -0,0 +1,98 @@
+package handlers
+
+import (
+	"context"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/go-chi/chi/v5"
+
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// newEncTestServer builds a router serving the encrypted-URL route with a
+// generator seeded by the shared test signing key. The image service is left
+// nil: these tests exercise validation that rejects a token before any image
+// is fetched, so the handler must never reach the service.
+func newEncTestServer(t *testing.T) (*encurl.Generator, http.Handler) {
+	t.Helper()
+
+	encGen, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator() error = %v", err)
+	}
+
+	h := &Handlers{
+		log:    slog.New(slog.DiscardHandler),
+		encGen: encGen,
+	}
+
+	r := chi.NewRouter()
+	r.Get("/v1/e/{token}/*", h.HandleImageEnc())
+
+	return encGen, r
+}
+
+// getEncToken issues a GET for the given token and returns the recorder.
+func getEncToken(srv http.Handler, token string) *httptest.ResponseRecorder {
+	req := httptest.NewRequestWithContext(
+		context.Background(), http.MethodGet, "/v1/e/"+token+"/img.jpg", nil)
+	rec := httptest.NewRecorder()
+	srv.ServeHTTP(rec, req)
+
+	return rec
+}
+
+// TestHandleImageEnc_OverLimitDimension_Returns400 verifies that a decrypted
+// token requesting a dimension beyond MaxDimension is rejected with 400
+// instead of reaching the image processor and libvips.
+func TestHandleImageEnc_OverLimitDimension_Returns400(t *testing.T) {
+	t.Parallel()
+
+	encGen, srv := newEncTestServer(t)
+
+	token, err := encGen.Generate(&encurl.Payload{
+		SourceHost: "cdn.example.com",
+		SourcePath: "/photo.jpg",
+		Width:      100000,
+		Height:     100000,
+	})
+	if err != nil {
+		t.Fatalf("Generate() error = %v", err)
+	}
+
+	rec := getEncToken(srv, token)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+}
+
+// TestHandleImageEnc_InvalidFitMode_Returns400 verifies that a decrypted token
+// carrying an unrecognized fit mode is rejected with 400 rather than surfacing
+// as a 500 from the image processor's default branch.
+func TestHandleImageEnc_InvalidFitMode_Returns400(t *testing.T) {
+	t.Parallel()
+
+	encGen, srv := newEncTestServer(t)
+
+	token, err := encGen.Generate(&encurl.Payload{
+		SourceHost: "cdn.example.com",
+		SourcePath: "/photo.jpg",
+		Width:      800,
+		Height:     600,
+		FitMode:    imgcache.FitMode("bogus"),
+	})
+	if err != nil {
+		t.Fatalf("Generate() error = %v", err)
+	}
+
+	rec := getEncToken(srv, token)
+
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("status = %d, want %d", rec.Code, http.StatusBadRequest)
+	}
+}

internal/imgcache/imgcache.go

@@ -59,6 +59,23 @@ func ValidateFitMode(fit FitMode) error {
 	}
 }
 
+// ValidateImageRequest checks that a request's dimensions are within
+// MaxDimension and its fit mode is recognized. Both the plain /v1/image/
+// route and the encrypted /v1/e/ route validate through this function so a
+// request from either source enforces identical bounds, regardless of how it
+// was constructed. A width or height of 0 means "original size" and is valid.
+func ValidateImageRequest(req *ImageRequest) error {
+	if req.Size.Width < 0 || req.Size.Height < 0 {
+		return ErrInvalidSize
+	}
+
+	if req.Size.Width > MaxDimension || req.Size.Height > MaxDimension {
+		return ErrDimensionTooLarge
+	}
+
+	return ValidateFitMode(req.FitMode)
+}
+
 // ImageRequest represents a request for a processed image
 type ImageRequest struct {
 	// SourceHost is the origin host (e.g., "cdn.example.com")

internal/imgcache/validate_internal_test.go

@@ -0,0 +1,64 @@
+package imgcache
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestValidateImageRequest(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		req     ImageRequest
+		wantErr error
+	}{
+		{
+			name: "within bounds",
+			req:  ImageRequest{Size: Size{Width: 800, Height: 600}, FitMode: FitCover},
+		},
+		{
+			name: "original size and empty fit",
+			req:  ImageRequest{Size: Size{Width: 0, Height: 0}},
+		},
+		{
+			name:    "width over limit",
+			req:     ImageRequest{Size: Size{Width: MaxDimension + 1, Height: 600}},
+			wantErr: ErrDimensionTooLarge,
+		},
+		{
+			name:    "height over limit",
+			req:     ImageRequest{Size: Size{Width: 800, Height: MaxDimension + 1}},
+			wantErr: ErrDimensionTooLarge,
+		},
+		{
+			name:    "negative width",
+			req:     ImageRequest{Size: Size{Width: -1, Height: 600}},
+			wantErr: ErrInvalidSize,
+		},
+		{
+			name:    "invalid fit mode",
+			req:     ImageRequest{Size: Size{Width: 800, Height: 600}, FitMode: "bogus"},
+			wantErr: ErrInvalidFitMode,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			err := ValidateImageRequest(&tt.req)
+			if tt.wantErr == nil {
+				if err != nil {
+					t.Fatalf("ValidateImageRequest() error = %v, want nil", err)
+				}
+
+				return
+			}
+
+			if !errors.Is(err, tt.wantErr) {
+				t.Fatalf("ValidateImageRequest() error = %v, want %v", err, tt.wantErr)
+			}
+		})
+	}
+}