Refuse, before decoding, input libvips would not load as a source format

govips picks the libvips loader for the input with DetermineImageType.
The image processor now names the format from that type before decoding,
and refuses with ErrUnsupportedInputFormat, answered 415, any input it
cannot name as one of the six source formats. An SVG whose first bytes
pass the first-bytes check for AVIF therefore never reaches librsvg.
orig is the format named there. README.md and TODO.md say so.

Model: opus-5-5

README.md

@@ -190,7 +190,9 @@ path under `/v1/` answers 200, in maintenance mode too.
   the cache and the upstream fetch; 403 when the upstream host, or a host it
   redirects to, is `localhost`, ends in `.localhost` or `.local`, or has an
   address in a blocked network (see `blocked_networks`); 429 over
-  `image_requests_per_minute` (below); 502 when the upstream answered with an
+  `image_requests_per_minute` (below); 415 when the upstream answers with a
+  `Content-Type` that is not one of the source formats (below), or with an image
+  libvips would not load as one of them; 502 when the upstream answered with an
   error status, and for 5 minutes after that for the same source URL; 503 when
   pixa is busy or in maintenance mode; 500 for any other failure.
 - `GET` or `HEAD` `/v1/e/<token>/<name>` — an image through an encrypted URL
@@ -200,7 +202,7 @@ path under `/v1/` answers 200, in maintenance mode too.
   expired; 504 when the upstream has not sent its response headers within
   `upstream_fetch_timeout`, but 500 when that time runs out while the image
   itself is still arriving; 400 for an `Accept` header that is not valid, and
-  406, 403, 429, 502, 503 and 500, as for `/v1/image/`.
+  406, 403, 415, 429, 502, 503 and 500, as for `/v1/image/`.
 - `GET /robots.txt` — asks every crawler to stay away (`Disallow: /`). Needs:
   nothing. Answers: 200.
 - `GET /.well-known/healthcheck.json` — JSON with `status` (`ok`), `now`,
@@ -260,9 +262,11 @@ once, is refused with 400.
   both optional (values under Signature Specification). Both are part of what is
   cached, so each value of either is a separate cached image.
 
-The source image may be JPEG, PNG, GIF, WebP, AVIF, JPEG XL or SVG. The
+The source image may be JPEG, PNG, GIF, WebP, AVIF or JPEG XL. The upstream's
-upstream's `Content-Type` must name its format, and the image's first bytes must
+`Content-Type` must name its format, and the image's first bytes must match it.
-match it.
+An upstream answering with any other `Content-Type`, SVG included, is refused
+with 415, and so is an image that libvips would not load as one of these
+formats, such as an SVG whose first bytes pass the check for AVIF.
 
 With the format `auto`, pixa chooses the format for each request from its
 `Accept` header, in this order:

TODO.md

@@ -30,6 +30,20 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-13 SVG sources are refused, and `orig` is never saved as JPEG in place
+  of the source's own format (closes #68): the upstream fetch no longer accepts
+  `image/svg+xml`, which libvips would render. Both image routes answer 415 for
+  an upstream `Content-Type` that is not one of the source formats, SVG
+  included, where they answered 500. Before decoding, the image processor
+  refuses with `ErrUnsupportedInputFormat`, also answered 415, any input that
+  govips' `DetermineImageType`, which picks the libvips loader, does not name as
+  a source format. That catches an SVG whose first bytes pass the first-bytes
+  check for another type, as an XML comment can make them do for AVIF; an SVG
+  whose first bytes do not pass that check answers 500, as before. `orig` is
+  always the source's own format, where a source the processor could not name
+  was saved as JPEG. `README.md` no longer lists SVG. The SVG detection in
+  `internal/magic` stays: the request path does not call it, and it goes with
+  the unused code of https://git.eeqj.de/sneak/pixa/issues/168.
 - 2026-10-12 per-client limit on the image routes (closes #236): the new setting
   `image_requests_per_minute` (`PIXA_IMAGE_REQUESTS_PER_MINUTE`), default 600,
   is how many `GET` and `HEAD` requests one client may send to `/v1/image/` and

internal/handlers/image.go

@@ -247,6 +247,14 @@ func (s *Handlers) respondImageError(
 		return
 	}
 
+	if errors.Is(err, httpfetcher.ErrInvalidContentType) ||
+		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat) {
+		s.respondError(w, "unsupported source format",
+			http.StatusUnsupportedMediaType)
+
+		return
+	}
+
 	s.respondError(w, "internal error", http.StatusInternalServerError)
 }
 

internal/handlers/imageenc.go

@@ -161,6 +161,10 @@ func (s *Handlers) handleImageError(w http.ResponseWriter, err error) {
 		errors.Is(err, imageprocessor.ErrTooManyImages):
 		s.respondError(w, "server busy, try again later",
 			http.StatusServiceUnavailable)
+	case errors.Is(err, httpfetcher.ErrInvalidContentType),
+		errors.Is(err, imageprocessor.ErrUnsupportedInputFormat):
+		s.respondError(w, "unsupported source format",
+			http.StatusUnsupportedMediaType)
 	default:
 		s.log.Error("image request failed", "error", err)
 		s.respondError(w, "internal error", http.StatusInternalServerError)

internal/handlers/unsupported_source_internal_test.go

@@ -0,0 +1,54 @@
+package handlers
+
+import (
+	"fmt"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/httpfetcher"
+	"sneak.berlin/go/pixa/internal/imageprocessor"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// TestUnsupportedSourceFormatAnswers415 checks that both image routes answer
+// 415 with a clear error when the upstream answers with a content type pixa
+// does not accept, such as SVG, and when orig is asked of a source in a
+// format the processor cannot name, wrapped as the service wraps them.
+func TestUnsupportedSourceFormatAnswers415(t *testing.T) {
+	t.Parallel()
+
+	h := &Handlers{log: slog.New(slog.DiscardHandler)}
+	req := &imgcache.ImageRequest{SourceHost: "img.example.com", SourcePath: "/a.svg"}
+
+	for _, err := range []error{
+		fmt.Errorf("upstream fetch failed: %w: image/svg+xml",
+			httpfetcher.ErrInvalidContentType),
+		fmt.Errorf("image processing failed: %w: unknown",
+			imageprocessor.ErrUnsupportedInputFormat),
+	} {
+		plain := httptest.NewRecorder()
+		h.respondImageError(plain, req, err)
+
+		encrypted := httptest.NewRecorder()
+		h.handleImageError(encrypted, err)
+
+		for route, rec := range map[string]*httptest.ResponseRecorder{
+			"HandleImage": plain, "HandleImageEnc": encrypted,
+		} {
+			t.Logf("%s for %v: %d %s", route, err, rec.Code, rec.Body.String())
+
+			if rec.Code != http.StatusUnsupportedMediaType {
+				t.Errorf("%s for %v: status = %d, want %d",
+					route, err, rec.Code, http.StatusUnsupportedMediaType)
+			}
+
+			if !strings.Contains(rec.Body.String(), "unsupported source format") {
+				t.Errorf("%s for %v: body = %q, want the unsupported source format error",
+					route, err, rec.Body.String())
+			}
+		}
+	}
+}

internal/httpfetcher/httpfetcher.go

@@ -158,7 +158,6 @@ func DefaultConfig() *Config {
 			contentTypeWebP,
 			contentTypeAVIF,
 			contentTypeJXL,
-			contentTypeSVG,
 		},
 		AllowHTTP:             false,
 		MaxConnectionsPerHost: DefaultMaxConnectionsPerHost,

internal/imageprocessor/imageprocessor.go

@@ -137,6 +137,10 @@ var ErrInputDataTooLarge = errors.New("input data exceeds maximum allowed size")
 // not supported.
 var ErrUnsupportedOutputFormat = errors.New("unsupported output format")
 
+// ErrUnsupportedInputFormat is returned, before the input is decoded, when
+// libvips would not load it as one of the source formats.
+var ErrUnsupportedInputFormat = errors.New("unsupported input format")
+
 // ErrTooManyImages is returned when MaxConcurrentProcessing images are being
 // processed and none finishes within ProcessingWaitTimeout.
 var ErrTooManyImages = errors.New("too many images being processed at once")
@@ -219,6 +223,16 @@ func (p *ImageProcessor) Process(
 		return nil, ErrInputDataTooLarge
 	}
 
+	// govips picks the libvips loader with DetermineImageType. Input it
+	// would not load as a source format, such as SVG, which librsvg would
+	// render, is refused before libvips reads it.
+	inputFormat := p.detectFormat(vips.DetermineImageType(data))
+
+	sourceFormat, err := p.formatFromString(inputFormat)
+	if err != nil {
+		return nil, err
+	}
+
 	// Decode image
 	img, err := vips.NewImageFromBuffer(data)
 	if err != nil {
@@ -237,9 +251,6 @@ func (p *ImageProcessor) Process(
 	origWidth := img.Width()
 	origHeight := img.Height()
 
-	// Detect input format
-	inputFormat := p.detectFormat(img)
-
 	// Validate input dimensions to prevent DoS via decompression bombs
 	if origWidth > MaxInputDimension || origHeight > MaxInputDimension {
 		return nil, ErrInputTooLarge
@@ -259,7 +270,7 @@ func (p *ImageProcessor) Process(
 	// orig is the source's own format; encode refuses an empty format
 	outputFormat := req.Format
 	if outputFormat == FormatOriginal {
-		outputFormat = p.formatFromString(inputFormat)
+		outputFormat = sourceFormat
 	}
 
 	// Encode to target format
@@ -403,10 +414,9 @@ func (p *ImageProcessor) acquireSlot(ctx context.Context) (func(), error) {
 	}
 }
 
-// detectFormat returns the format string from a vips image.
+// detectFormat returns the format string for a govips image type.
-func (p *ImageProcessor) detectFormat(img *vips.ImageRef) string {
+func (p *ImageProcessor) detectFormat(imageType vips.ImageType) string {
-	format := img.Format()
+	switch imageType {
-	switch format {
 	case vips.ImageTypeJPEG:
 		return "jpeg"
 	case vips.ImageTypePNG:
@@ -687,22 +697,23 @@ func jxlDistance(quality int) float64 {
 	return 53.0/3000.0*q*q - 23.0/20.0*q + 25.0
 }
 
-// formatFromString converts a format string to Format.
+// formatFromString converts a format string from detectFormat to Format. A
-func (p *ImageProcessor) formatFromString(format string) Format {
+// format detectFormat could not name is ErrUnsupportedInputFormat.
+func (p *ImageProcessor) formatFromString(format string) (Format, error) {
 	switch format {
 	case "jpeg":
-		return FormatJPEG
+		return FormatJPEG, nil
 	case "png":
-		return FormatPNG
+		return FormatPNG, nil
 	case "gif":
-		return FormatGIF
+		return FormatGIF, nil
 	case "webp":
-		return FormatWebP
+		return FormatWebP, nil
 	case string(FormatAVIF):
-		return FormatAVIF
+		return FormatAVIF, nil
 	case string(FormatJXL):
-		return FormatJXL
+		return FormatJXL, nil
 	default:
-		return FormatJPEG
+		return "", fmt.Errorf("%w: %s", ErrUnsupportedInputFormat, format)
 	}
 }

internal/imageprocessor/orig_format_internal_test.go

@@ -0,0 +1,39 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"errors"
+	"testing"
+
+	"github.com/davidbyttow/govips/v2/vips"
+)
+
+// TestImageProcessor_OrigOfUnnamedFormatRefused verifies that orig of an
+// input in a format the processor decodes but cannot name, here TIFF, is
+// an error, and not the image saved as JPEG.
+func TestImageProcessor_OrigOfUnnamedFormatRefused(t *testing.T) {
+	t.Parallel()
+
+	img, err := vips.NewImageFromBuffer(createTestPNG(t, 20, 20))
+	if err != nil {
+		t.Fatalf("failed to decode test PNG: %v", err)
+	}
+
+	t.Cleanup(img.Close)
+
+	tiff, _, err := img.ExportTiff(vips.NewTiffExportParams())
+	if err != nil {
+		t.Fatalf("failed to save test TIFF: %v", err)
+	}
+
+	result, err := New(Params{}).Process(
+		t.Context(), bytes.NewReader(tiff), &Request{Format: FormatOriginal},
+	)
+	if result != nil {
+		t.Logf("Process() returned %s", result.ContentType)
+	}
+
+	if !errors.Is(err, ErrUnsupportedInputFormat) {
+		t.Errorf("Process() error = %v, want %v", err, ErrUnsupportedInputFormat)
+	}
+}

internal/imageprocessor/source_formats_internal_test.go

@@ -0,0 +1,46 @@
+package imageprocessor
+
+import (
+	"bytes"
+	"io"
+	"testing"
+)
+
+// TestImageProcessor_LoadsEachSourceFormat verifies that an image in each of
+// the six source formats, saved by the processor itself, is loaded and saved
+// again in its own format (orig).
+func TestImageProcessor_LoadsEachSourceFormat(t *testing.T) {
+	t.Parallel()
+
+	proc := New(Params{})
+
+	for _, format := range []Format{
+		FormatJPEG, FormatPNG, FormatGIF, FormatWebP, FormatAVIF, FormatJXL,
+	} {
+		saved, err := proc.Process(t.Context(),
+			bytes.NewReader(createTestPNG(t, 20, 20)), &Request{Format: format})
+		if err != nil {
+			t.Fatalf("saving the test image as %s: %v", format, err)
+		}
+
+		source, err := io.ReadAll(saved.Content)
+		if err != nil {
+			t.Fatalf("reading the %s image: %v", format, err)
+		}
+
+		result, err := proc.Process(t.Context(), bytes.NewReader(source),
+			&Request{Format: FormatOriginal})
+		if err != nil {
+			t.Errorf("orig of the %s image: %v", format, err)
+
+			continue
+		}
+
+		t.Logf("orig of the %s image: %s", format, result.ContentType)
+
+		if result.ContentType != FormatToMIME(format) {
+			t.Errorf("orig of the %s image is %s, want %s",
+				format, result.ContentType, FormatToMIME(format))
+		}
+	}
+}

internal/server/svg_as_avif_internal_test.go

@@ -0,0 +1,71 @@
+package server
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"sneak.berlin/go/pixa/internal/encurl"
+	"sneak.berlin/go/pixa/internal/imgcache"
+)
+
+// svgServedAsAVIF is an SVG whose first bytes pass the first-bytes check for
+// AVIF: its XML comment puts "ftypavis" at bytes 4 to 11, where an AVIF file
+// has its ftyp box and brand. libvips would load it as SVG.
+const svgServedAsAVIF = `<!--ftypavis--><svg xmlns="http://www.w3.org/2000/svg" ` +
+	`width="64" height="48"><rect width="64" height="48"/></svg>`
+
+// TestImageProxyRefusesSVGServedAsAVIF requests svgServedAsAVIF, which the
+// upstream answers as image/avif, through both image routes, resized as PNG
+// and in its own format (orig). Each request is answered 415, as for an SVG
+// source, never with an image.
+func TestImageProxyRefusesSVGServedAsAVIF(t *testing.T) {
+	t.Parallel()
+
+	upstream := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.Header().Set("Content-Type", "image/avif")
+			_, _ = io.WriteString(w, svgServedAsAVIF)
+		}))
+	t.Cleanup(upstream.Close)
+
+	s, _, _ := startImageProxy(t, upstream)
+
+	generator, err := encurl.NewGenerator(testSigningKey)
+	if err != nil {
+		t.Fatalf("encurl.NewGenerator: %v", err)
+	}
+
+	encryptedURL := func(format imgcache.ImageFormat) string {
+		token, err := generator.Generate(&encurl.Payload{
+			SourceHost: upstreamHost, SourcePath: "/drawing.avif",
+			Width: 32, Height: 24, Format: format,
+		})
+		if err != nil {
+			t.Fatalf("encurl Generate: %v", err)
+		}
+
+		return "/v1/e/" + token + "/drawing." + string(format)
+	}
+
+	for _, target := range []string{
+		"/v1/image/" + upstreamHost + "/drawing.avif/32x24.png",
+		"/v1/image/" + upstreamHost + "/drawing.avif/64x48.orig",
+		encryptedURL(imgcache.FormatPNG),
+		encryptedURL(imgcache.FormatOriginal),
+	} {
+		rec := httptest.NewRecorder()
+		s.ServeHTTP(rec, httptest.NewRequestWithContext(
+			t.Context(), http.MethodGet, target, nil))
+		t.Logf("GET %s: %d, Content-Type %s, body %q",
+			target, rec.Code, rec.Header().Get("Content-Type"), rec.Body.String())
+
+		if rec.Code != http.StatusUnsupportedMediaType ||
+			!strings.Contains(rec.Body.String(), "unsupported source format") {
+			t.Errorf("GET %s: %d %q, want %d unsupported source format",
+				target, rec.Code, rec.Body.String(), http.StatusUnsupportedMediaType)
+		}
+	}
+}

internal/server/svg_internal_test.go

@@ -0,0 +1,41 @@
+package server
+
+import (
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+// TestImageProxyRefusesSVG requests an image the upstream answers as SVG,
+// in its own format (orig) and resized as PNG, through pixa's router,
+// upstream fetcher and image processor. pixa does not accept SVG as a
+// source, so each request is answered 415, never with an image.
+func TestImageProxyRefusesSVG(t *testing.T) {
+	t.Parallel()
+
+	upstream := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.Header().Set("Content-Type", "image/svg+xml")
+			_, _ = io.WriteString(w, `<svg xmlns="http://www.w3.org/2000/svg" `+
+				`width="64" height="48"><rect width="64" height="48"/></svg>`)
+		}))
+	t.Cleanup(upstream.Close)
+
+	s, _, _ := startImageProxy(t, upstream)
+
+	for _, sizeFormat := range []string{"64x48.orig", "32x24.png"} {
+		target := "/v1/image/" + upstreamHost + "/drawing.svg/" + sizeFormat
+
+		rec := httptest.NewRecorder()
+		s.ServeHTTP(rec, httptest.NewRequestWithContext(
+			t.Context(), http.MethodGet, target, nil))
+		t.Logf("GET %s: %d, Content-Type %s, body %s",
+			target, rec.Code, rec.Header().Get("Content-Type"), rec.Body.String())
+
+		if rec.Code != http.StatusUnsupportedMediaType {
+			t.Errorf("GET %s status = %d, want %d",
+				target, rec.Code, http.StatusUnsupportedMediaType)
+		}
+	}
+}