fix: serve sender favicons only as inert raster images

`/api/favicon` is unauthenticated and answers on the webmail origin,
and it passed the upstream Content-Type through unchecked. Should the
favicon service ever return `image/svg+xml` or `text/html`, opening
that URL on its own would run script as the webmail. Only raster image
types are served now, with `nosniff` and a sandboxing CSP; anything
else falls back to the transparent placeholder.

app/api/favicon/route.ts

@@ -455,10 +455,7 @@ export async function GET(request: NextRequest) {
   const cached = cache.get(normalizedDomain);
   if (cached && Date.now() - cached.fetchedAt < CACHE_TTL_MS) {
     return new NextResponse(cached.data, {
-      headers: {
-        'Content-Type': cached.contentType,
-        'Cache-Control': 'public, max-age=1209600', // 2 weeks
-      },
+      headers: faviconHeaders(cached.contentType),
     });
   }
 
@@ -474,7 +471,15 @@ export async function GET(request: NextRequest) {
       return new NextResponse(TRANSPARENT_PNG, { headers: MISSING_FAVICON_HEADERS });
     }
 
-    const contentType = upstream.headers.get('content-type') || 'image/x-icon';
+    // Served inline on the webmail origin, so only raster image types pass:
+    // an image/svg+xml or text/html favicon opened on its own would run
+    // script as the webmail.
+    const contentType = rasterFaviconType(upstream.headers.get('content-type'));
+    if (!contentType) {
+      evictNegativeOldest();
+      negativeCache.set(normalizedDomain, { fetchedAt: Date.now() });
+      return new NextResponse(TRANSPARENT_PNG, { headers: MISSING_FAVICON_HEADERS });
+    }
     const data = await upstream.arrayBuffer();
 
     // Don't cache empty/tiny responses (likely no real favicon)
@@ -489,10 +494,7 @@ export async function GET(request: NextRequest) {
     cache.set(normalizedDomain, { data, contentType, fetchedAt: Date.now() });
 
     return new NextResponse(data, {
-      headers: {
-        'Content-Type': contentType,
-        'Cache-Control': 'public, max-age=1209600',
-      },
+      headers: faviconHeaders(contentType),
     });
   } catch {
     return new NextResponse(null, {
@@ -502,6 +504,31 @@ export async function GET(request: NextRequest) {
   }
 }
 
+const RASTER_FAVICON_TYPES = new Set([
+  'image/x-icon',
+  'image/vnd.microsoft.icon',
+  'image/png',
+  'image/gif',
+  'image/jpeg',
+  'image/webp',
+  'image/bmp',
+]);
+
+/** The upstream type when it is a raster image, else null. */
+function rasterFaviconType(header: string | null): string | null {
+  const type = (header ?? 'image/x-icon').split(';')[0].trim().toLowerCase();
+  return RASTER_FAVICON_TYPES.has(type) ? type : null;
+}
+
+function faviconHeaders(contentType: string): Record<string, string> {
+  return {
+    'Content-Type': contentType,
+    'Cache-Control': 'public, max-age=1209600', // 2 weeks
+    'X-Content-Type-Options': 'nosniff',
+    'Content-Security-Policy': "default-src 'none'; sandbox",
+  };
+}
+
 function evictNegativeOldest() {
   if (negativeCache.size < NEGATIVE_CACHE_MAX_SIZE) return;
   let oldestKey: string | null = null;

lib/__tests__/favicon-route-types.test.ts

@@ -0,0 +1,33 @@
+// @vitest-environment node
+import { afterEach, describe, expect, it, vi } from 'vitest';
+import { NextRequest } from 'next/server';
+
+// /api/favicon answers on the webmail origin without authentication and used
+// to pass the upstream Content-Type through: an SVG or HTML "favicon" opened
+// on its own would run script as the webmail.
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+  vi.resetModules();
+});
+
+async function favicon(domain: string, upstream: Response) {
+  vi.stubGlobal('fetch', vi.fn(async () => upstream));
+  const { GET } = await import('@/app/api/favicon/route');
+  return GET(new NextRequest(`https://webmail.example/api/favicon?domain=${domain}`));
+}
+
+describe('favicon route', () => {
+  it('serves a raster favicon inert: nosniff and a sandboxing CSP', async () => {
+    const res = await favicon('example.org', new Response(new Uint8Array(64), { headers: { 'content-type': 'image/png' } }));
+    expect(res.headers.get('content-type')).toBe('image/png');
+    expect(res.headers.get('x-content-type-options')).toBe('nosniff');
+    expect(res.headers.get('content-security-policy')).toBe("default-src 'none'; sandbox");
+  });
+
+  it.each(['image/svg+xml', 'text/html; charset=utf-8'])('does not pass %s through', async (type) => {
+    const res = await favicon('example.com', new Response('<svg><script>alert(1)</script></svg>'.padEnd(64), { headers: { 'content-type': type } }));
+    expect(res.headers.get('content-type')).toBe('image/png');
+    expect(res.headers.get('x-bulwark-favicon')).toBe('missing');
+  });
+});