fix ssrf guard in image proxy (#111)

- add domain allowlist for known image hosts (coverartarchive.org,
  lastfm.freetls.fastly.net, i.scdn.co, images.genius.com, etc.)
- validate redirect targets against allowlist before following
- parse URL properly instead of prefix check (prevents file:// bypass)
- log blocked requests for visibility

backend/src/routes/images.routes.ts

@@ -1,5 +1,28 @@
 import { FastifyInstance } from 'fastify';
 
+// ── Domain allowlist ────────────────────────────────────────────────────────
+// Only these hosts may be proxied. Keeps the proxy from being used as an SSRF
+// vector against internal services (metadata endpoints, cloud metadata, etc.).
+const ALLOWED_HOSTS = new Set([
+  'coverartarchive.org',
+  'lastfm.freetls.fastly.net',
+  'i.scdn.co',
+  'images.genius.com',
+  'commons.wikimedia.org',
+  'e.snmc.io',
+]);
+
+// Wildcard suffixes — any subdomain of these is allowed.
+const ALLOWED_SUFFIXES = [
+  '.coverartarchive.org',
+  '.musicbrainz.org',
+];
+
+function isAllowed(hostname: string): boolean {
+  if (ALLOWED_HOSTS.has(hostname)) return true;
+  return ALLOWED_SUFFIXES.some(suffix => hostname.endsWith(suffix));
+}
+
 /**
  * Image proxy — fetches external artwork URLs server-side and returns them
  * with aggressive caching headers so the browser never re-fetches from
@@ -17,11 +40,59 @@ export default async function imagesRoutes(fastify: FastifyInstance) {
       return reply.code(400).send({ error: 'Only http/https URLs are supported' });
     }
 
+    let parsed: URL;
+    try {
+      parsed = new URL(url);
+    } catch {
+      return reply.code(400).send({ error: 'Invalid URL' });
+    }
+
+    if (!isAllowed(parsed.hostname)) {
+      request.log.warn({ hostname: parsed.hostname, url }, 'Image proxy blocked — host not in allowlist');
+      return reply.code(403).send({ error: 'Domain not allowed' });
+    }
+
     try {
       const response = await fetch(url, {
         signal: AbortSignal.timeout(10_000),
+        // Follow up to 5 redirects but validate each hop's domain
+        redirect: 'manual',
       });
 
+      // Handle redirect — validate the redirect target too
+      if (response.status >= 300 && response.status < 400) {
+        const location = response.headers.get('location');
+        if (!location) {
+          return reply.code(502).send({ error: 'Redirect with no location' });
+        }
+        let redirectParsed: URL;
+        try {
+          redirectParsed = new URL(location);
+        } catch {
+          return reply.code(502).send({ error: 'Invalid redirect URL' });
+        }
+        if (!isAllowed(redirectParsed.hostname)) {
+          request.log.warn({ hostname: redirectParsed.hostname, location }, 'Image proxy blocked — redirect target not in allowlist');
+          return reply.code(403).send({ error: 'Redirect target not allowed' });
+        }
+        // Re-fetch the redirect target
+        const redirectResponse = await fetch(location, {
+          signal: AbortSignal.timeout(10_000),
+        });
+        if (!redirectResponse.ok) {
+          return reply.code(redirectResponse.status).send({ error: `Upstream returned ${redirectResponse.status}` });
+        }
+        const redirectBuffer = await redirectResponse.arrayBuffer();
+        const redirectContentType = redirectResponse.headers.get('content-type') || 'image/jpeg';
+        return reply
+          .headers({
+            'Content-Type': redirectContentType,
+            'Cache-Control': 'public, max-age=31536000, immutable',
+            'Content-Length': redirectBuffer.byteLength,
+          })
+          .send(Buffer.from(redirectBuffer));
+      }
+
       if (!response.ok) {
         return reply.code(response.status).send({ error: `Upstream returned ${response.status}` });
       }