TODO.md

@@ -89,6 +89,20 @@ exhaustion
   (IPv4-mapped forms covered); enforcement stays in the dial-time
   re-resolution so the DNS-rebinding window remains closed; documented in
   `README.md` and `config.example.yml`.
+- 2026-09-21 validate dimensions and fit mode on the encrypted-URL
+  route and the token generator (closes #62): `imgcache.ValidateDimension`
+  alone holds the `MaxDimension` bound and is used by the path parser, by
+  the new `ValidateImageRequest` (which also applies `ValidateFitMode`)
+  and by the generator; both the `/v1/image/` and `/v1/e/` routes call
+  `ValidateImageRequest`, 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 answers 400 naming the field for a `width` or `height` that is
+  not a number or fails the shared check, a `quality` that is not a number
+  from 1 to 100, a `ttl` that is not a number from 0 to the largest number
+  of seconds the expiry calculation can hold, or an unknown `fit`; an empty
+  `quality` is 85 and
+  an empty `ttl` never expires; the form's width and height inputs stop at
+  8192
 - 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

@@ -1,8 +1,12 @@
 package handlers
 
 import (
+	"bytes"
 	"crypto/subtle"
+	"errors"
+	"fmt"
 	"html/template"
+	"math"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -14,6 +18,19 @@ 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")
+
+// Bounds for the generator's quality and ttl fields. maxTTL is in seconds:
+// the expiry calculation time.Duration(ttl) * time.Second overflows above it.
+const (
+	minQuality = 1
+	maxQuality = 100
+	maxTTL     = int(math.MaxInt64 / time.Second)
+)
+
 // 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) {
@@ -101,18 +118,26 @@ 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
 		}
@@ -140,17 +165,40 @@ 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 width, height, quality or ttl, or an
+// unrecognized fit mode, is a client error naming the offending field. The
+// format field is passed through unchecked.
 func buildGeneratePayload(
 	parsed *url.URL, form url.Values,
-) (*encurl.Payload, time.Time, int) {
+) (*encurl.Payload, time.Time, int, error) {
-	width, _ := strconv.Atoi(form.Get("width"))
+	width, err := parseFormDimension(form, "width")
-	height, _ := strconv.Atoi(form.Get("height"))
+	if err != nil {
-	quality, _ := strconv.Atoi(form.Get("quality"))
+		return nil, time.Time{}, 0, err
-	ttl, _ := strconv.Atoi(form.Get("ttl"))
+	}
 
-	if quality <= 0 {
+	height, err := parseFormDimension(form, "height")
-		quality = 85
+	if err != nil {
+		return nil, time.Time{}, 0, err
+	}
+
+	quality, err := parseFormInt(form, "quality",
+		encurl.DefaultQuality, minQuality, maxQuality)
+	if err != nil {
+		return nil, time.Time{}, 0, err
+	}
+
+	ttl, err := parseFormInt(form, "ttl", 0, 0, maxTTL)
+	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 (
@@ -171,11 +219,57 @@ 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 value, or one
+// imgcache.ValidateDimension rejects, is 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 {
+		return 0, fmt.Errorf("%w %s: not a number", errInvalidFormField, field)
+	}
+
+	err = imgcache.ValidateDimension(field, value)
+	if err != nil {
+		return 0, err
+	}
+
+	return value, nil
+}
+
+// parseFormInt reads an optional integer form field, returning def when the
+// field is empty and an error naming the field when the value is non-numeric
+// or outside minValue to maxValue.
+func parseFormInt(
+	form url.Values, field string, def, minValue, maxValue int,
+) (int, error) {
+	raw := form.Get(field)
+	if raw == "" {
+		return def, nil
+	}
+
+	value, err := strconv.Atoi(raw)
+	if err != nil {
+		return 0, fmt.Errorf("%w %s: not a number", errInvalidFormField, field)
+	}
+
+	if value < minValue || value > maxValue {
+		return 0, fmt.Errorf("%w %s: must be from %d to %d",
+			errInvalidFormField, field, minValue, maxValue)
+	}
+
+	return value, nil
 }
 
 // generatorData holds template data for the generator page.
@@ -215,6 +309,15 @@ 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; a rejected form uses 400. The page is rendered into a buffer before
+// the status is written, so a template failure can still answer 500.
+func (s *Handlers) renderGeneratorStatus(
+	w http.ResponseWriter, r *http.Request, data *generatorData, status int,
 ) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 
@@ -224,17 +327,29 @@ func (s *Handlers) renderGenerator(
 
 	data.CSRFField = csrfField(r)
 
-	err := templates.Render(w, "generator.html", data)
+	var page bytes.Buffer
+
+	err := templates.Render(&page, "generator.html", data)
 	if err != nil {
 		s.log.Error("failed to render generator template", "error", err)
 		http.Error(w, "Internal server error", http.StatusInternalServerError)
+
+		return
+	}
+
+	w.WriteHeader(status)
+
+	_, err = page.WriteTo(w)
+	if err != nil {
+		s.log.Error("failed to write generator page", "error", err)
 	}
 }
 
 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"),
@@ -243,7 +358,7 @@ 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/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/imgcache/imgcache.go

@@ -59,6 +59,24 @@ func ValidateFitMode(fit FitMode) error {
 	}
 }
 
+// ValidateImageRequest checks a request's width and height with
+// ValidateDimension and its fit mode with ValidateFitMode. 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.
+func ValidateImageRequest(req *ImageRequest) error {
+	err := ValidateDimension("width", req.Size.Width)
+	if err != nil {
+		return err
+	}
+
+	err = ValidateDimension("height", req.Size.Height)
+	if err != nil {
+		return err
+	}
+
+	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/urlparser.go

@@ -23,6 +23,21 @@ var (
 // MaxDimension is the maximum allowed width or height.
 const MaxDimension = 8192
 
+// ValidateDimension checks one requested width or height; name ("width" or
+// "height") appears in the error. 0 means "original size" and is valid.
+func ValidateDimension(name string, value int) error {
+	if value < 0 {
+		return fmt.Errorf("%w: %s is negative", ErrInvalidSize, name)
+	}
+
+	if value > MaxDimension {
+		return fmt.Errorf("%w: %s is above %d",
+			ErrDimensionTooLarge, name, MaxDimension)
+	}
+
+	return nil
+}
+
 // sizeFormatRegex matches patterns like "800x600.webp", "0x0.jpeg", "orig.png"
 var sizeFormatRegex = regexp.MustCompile(`^(\d+)x(\d+)\.(\w+)$|^(orig)\.(\w+)$`)
 
@@ -225,14 +240,20 @@ func parseSizeFormat(s string) (Size, ImageFormat, error) {
 			return Size{}, "", ErrInvalidSize
 		}
 
-		if width > MaxDimension || height > MaxDimension {
-			return Size{}, "", ErrDimensionTooLarge
-		}
-
 		size = Size{Width: width, Height: height}
 		formatStr = matches[3]
 	}
 
+	err := ValidateDimension("width", size.Width)
+	if err != nil {
+		return Size{}, "", err
+	}
+
+	err = ValidateDimension("height", size.Height)
+	if err != nil {
+		return Size{}, "", err
+	}
+
 	format, err := parseFormat(formatStr)
 	if err != nil {
 		return Size{}, "", err

internal/templates/generator.html

@@ -73,7 +73,7 @@
                         id="width"
                         name="width"
                         min="0"
-                        max="10000"
+                        max="8192"
                         value="{{if .FormWidth}}{{.FormWidth}}{{else}}0{{end}}"
                         placeholder="0 = original"
                         class="w-full px-3 py-2 border border-gray-300 rounded-md shadow-sm focus:outline-none focus:ring-2 focus:ring-blue-500 focus:border-blue-500"
@@ -88,7 +88,7 @@
                         id="height"
                         name="height"
                         min="0"
-                        max="10000"
+                        max="8192"
                         value="{{if .FormHeight}}{{.FormHeight}}{{else}}0{{end}}"
                         placeholder="0 = original"
                         class="w-full px-3 py-2 border border-gray-300 rounded-md shadow-sm focus:outline-none focus:ring-2 focus:ring-blue-500 focus:border-blue-500"