feat(GRO-642): add logo magic-bytes validation to prevent MIME confusion attacks

Defensive validation in /api/branding ensures base64-encoded logo content
matches its declared MIME type by checking image magic bytes (PNG, JPEG,
GIF, WebP). If the content doesn't match, the legacy base64 fields are
nulled out before returning to prevent MIME type confusion attacks.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/index.ts

@@ -72,6 +72,60 @@ app.route("/api/webhooks/stripe", webhooksRouter);
 // Dev/demo routes — config is always public, users endpoint is guarded internally
 app.route("/api/dev", devRouter);
 
+// Magic bytes for allowed image types
+const ALLOWED_IMAGE_TYPES: Record<string, Uint8Array> = {
+  "image/png": new Uint8Array([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a]),
+  "image/jpeg": new Uint8Array([0xff, 0xd8, 0xff]),
+  "image/gif": new Uint8Array([0x47, 0x49, 0x46, 0x38]),
+  "image/webp": new Uint8Array([0x52, 0x49, 0x46, 0x46]), // followed by size then WEBP
+};
+
+/**
+ * Validates that the given base64 content matches the declared MIME type
+ * by checking magic bytes. Returns null if valid, or the field to clear if not.
+ */
+function validateLogoMagicBytes(
+  logoBase64: string | null,
+  logoMimeType: string | null
+): "logoBase64" | "logoMimeType" | null {
+  if (!logoBase64 || !logoMimeType) return null;
+
+  const expectedMagic = ALLOWED_IMAGE_TYPES[logoMimeType];
+  if (!expectedMagic) return "logoMimeType"; // unknown MIME type — reject
+
+  try {
+    const binary = Buffer.from(logoBase64, "base64");
+    // WebP needs a special check (RIFF....WEBP at offset 0, size at offset 4)
+    if (logoMimeType === "image/webp") {
+      if (binary.length < 12) return "logoBase64";
+      const webpMagic = binary.slice(0, 4);
+      const webpSig = binary.slice(8, 12);
+      if (
+        webpMagic[0] !== 0x52 ||
+        webpMagic[1] !== 0x49 ||
+        webpMagic[2] !== 0x46 ||
+        webpMagic[3] !== 0x46 ||
+        webpSig[0] !== 0x57 ||
+        webpSig[1] !== 0x45 ||
+        webpSig[2] !== 0x42 ||
+        webpSig[3] !== 0x50
+      ) {
+        return "logoBase64";
+      }
+      return null;
+    }
+
+    // All other types: check prefix
+    if (binary.length < expectedMagic.length) return "logoBase64";
+    for (let i = 0; i < expectedMagic.length; i++) {
+      if (binary[i] !== expectedMagic[i]) return "logoBase64";
+    }
+    return null;
+  } catch {
+    return "logoBase64";
+  }
+}
+
 // Public branding endpoint — no auth required, returns business name/colors/logo
 app.get("/api/branding", async (c) => {
   const db = getDb();
@@ -87,13 +141,19 @@ app.get("/api/branding", async (c) => {
     }
   }
 
+  // Defensive: validate magic bytes to prevent MIME type confusion attacks
+  // via the legacy base64 logo fields
+  const badField = validateLogoMagicBytes(settings.logoBase64 ?? null, settings.logoMimeType ?? null);
+  const safeLogoBase64 = badField === "logoBase64" ? null : settings.logoBase64;
+  const safeLogoMimeType = badField === "logoMimeType" ? null : settings.logoMimeType;
+
   return c.json({
     businessName: settings.businessName,
     primaryColor: settings.primaryColor,
     accentColor: settings.accentColor,
     logoUrl,
-    logoBase64: settings.logoBase64,
-    logoMimeType: settings.logoMimeType,
+    logoBase64: safeLogoBase64,
+    logoMimeType: safeLogoMimeType,
   });
 });