stashapp · Gykes · Aug 19, 2026 · Aug 19, 2026
diff --git a/pkg/utils/image.go b/pkg/utils/image.go
@@ -8,6 +8,7 @@ import (
 	"io"
 	"net/http"
 	"regexp"
+	"strings"
 	"time"
 )
 
@@ -32,7 +33,32 @@ func ProcessImageInput(ctx context.Context, imageInput string) ([]byte, error) {
 	}
 
 	// assume input is a URL. Read it.
-	return ReadImageFromURL(ctx, imageInput)
+	d, err := ReadImageFromURL(ctx, imageInput)
+	if err != nil {
+		return nil, err
+	}
+
+	if err := validateImageData(d); err != nil {
+		return nil, err
+	}
+
+	return d, nil
+}
+
+// validateImageData rejects HTML content, which is not a valid image and would
+// execute as a document if served back to a browser. SVG (detected as XML or
+// plain text) is still accepted and sandboxed on output by ServeImage.
+func validateImageData(data []byte) error {
+	if len(data) == 0 {
+		return nil
+	}
+
+	contentType := http.DetectContentType(data)
+	if strings.HasPrefix(contentType, "text/html") {
+		return fmt.Errorf("unsupported image content type %q", contentType)
+	}
+
+	return nil
 }
 
 // ReadImageFromURL returns image data from a URL
@@ -97,6 +123,10 @@ func ProcessBase64Image(imageString string) ([]byte, error) {
 		return nil, err
 	}
 
+	if err := validateImageData(imageData); err != nil {
+		return nil, err
+	}
+
 	return imageData, nil
 }
 
@@ -112,10 +142,22 @@ func GetBase64StringFromData(data []byte) string {
 
 func ServeImage(w http.ResponseWriter, r *http.Request, image []byte) {
 	contentType := http.DetectContentType(image)
+
+	// SVG images are detected as XML or plain text; serve them as SVG so they
+	// render. The sandboxing CSP below prevents any embedded script running.
 	if contentType == "text/xml; charset=utf-8" || contentType == "text/plain; charset=utf-8" {
 		contentType = "image/svg+xml"
+	} else if strings.HasPrefix(contentType, "text/") {
+		// any other text type (e.g. HTML) is not a valid image - never render it
+		contentType = "application/octet-stream"
+		w.Header().Set("Content-Disposition", "attachment")
 	}
 
+	// sandbox every image response so a stored SVG cannot execute script or
+	// exfiltrate data; harmless for raster images.
+	w.Header().Set("Content-Security-Policy", "default-src 'none'; img-src data:; style-src 'unsafe-inline'; sandbox")
+	w.Header().Set("X-Content-Type-Options", "nosniff")
+
 	w.Header().Set("Content-Type", contentType)
 	ServeStaticContent(w, r, image)
 }
diff --git a/pkg/utils/image_test.go b/pkg/utils/image_test.go
@@ -0,0 +1,97 @@
+package utils
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+)
+
+// a minimal valid 1x1 PNG
+var pngImage = []byte{
+	0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x00, 0x00, 0x0d,
+	0x49, 0x48, 0x44, 0x52, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0x01,
+	0x08, 0x06, 0x00, 0x00, 0x00, 0x1f, 0x15, 0xc4, 0x89, 0x00, 0x00, 0x00,
+	0x0a, 0x49, 0x44, 0x41, 0x54, 0x78, 0x9c, 0x63, 0x00, 0x01, 0x00, 0x00,
+	0x05, 0x00, 0x01, 0x0d, 0x0a, 0x2d, 0xb4, 0x00, 0x00, 0x00, 0x00, 0x49,
+	0x45, 0x4e, 0x44, 0xae, 0x42, 0x60, 0x82,
+}
+
+func TestServeImage(t *testing.T) {
+	const wantCSP = "default-src 'none'; img-src data:; style-src 'unsafe-inline'; sandbox"
+
+	tests := []struct {
+		name            string
+		image           []byte
+		wantContentType string
+		wantAttachment  bool
+	}{
+		{
+			name:            "png is served as-is",
+			image:           pngImage,
+			wantContentType: "image/png",
+		},
+		{
+			name:            "svg is served as image/svg+xml so it renders",
+			image:           []byte(`<svg xmlns="http://www.w3.org/2000/svg"><circle r="1"/></svg>`),
+			wantContentType: "image/svg+xml",
+		},
+		{
+			name:            "svg with xml prolog is served as image/svg+xml",
+			image:           []byte(`<?xml version="1.0"?><svg xmlns="http://www.w3.org/2000/svg"></svg>`),
+			wantContentType: "image/svg+xml",
+		},
+		{
+			name:            "html is never rendered - forced to an attachment",
+			image:           []byte(`<!DOCTYPE html><html><script>alert(1)</script></html>`),
+			wantContentType: "application/octet-stream",
+			wantAttachment:  true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			w := httptest.NewRecorder()
+			r := httptest.NewRequest(http.MethodGet, "/image", nil)
+
+			ServeImage(w, r, tt.image)
+
+			if got := w.Header().Get("Content-Type"); got != tt.wantContentType {
+				t.Errorf("Content-Type = %q, want %q", got, tt.wantContentType)
+			}
+			if got := w.Header().Get("Content-Security-Policy"); got != wantCSP {
+				t.Errorf("Content-Security-Policy = %q, want %q", got, wantCSP)
+			}
+			if got := w.Header().Get("X-Content-Type-Options"); got != "nosniff" {
+				t.Errorf("X-Content-Type-Options = %q, want %q", got, "nosniff")
+			}
+			gotAttachment := strings.Contains(w.Header().Get("Content-Disposition"), "attachment")
+			if gotAttachment != tt.wantAttachment {
+				t.Errorf("Content-Disposition attachment = %v, want %v (got %q)", gotAttachment, tt.wantAttachment, w.Header().Get("Content-Disposition"))
+			}
+		})
+	}
+}
+
+func TestValidateImageData(t *testing.T) {
+	tests := []struct {
+		name    string
+		data    []byte
+		wantErr bool
+	}{
+		{name: "empty is allowed", data: nil, wantErr: false},
+		{name: "png is allowed", data: pngImage, wantErr: false},
+		{name: "svg is allowed", data: []byte(`<svg xmlns="http://www.w3.org/2000/svg"></svg>`), wantErr: false},
+		{name: "html is rejected", data: []byte(`<!DOCTYPE html><html></html>`), wantErr: true},
+		{name: "html without doctype is rejected", data: []byte(`<html><body>x</body></html>`), wantErr: true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := validateImageData(tt.data)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("validateImageData() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}