diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 9c9d083b..32d98e94 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -65,3 +65,8 @@
 **Vulnerability:** Path traversal in `media_shrinker.py` via unresolved `..` segments or symlink escapes before deriving conversion output paths.
 **Learning:** `Path.relative_to()` is only a lexical containment check unless both the source and root have first been resolved into canonical absolute paths. Relative paths and symlinks can otherwise bypass root-boundary assumptions.
 **Prevention:** Resolve both source and root once, reject sources outside the resolved root with a sanitized `MediaShrinkerError`, and derive `rel_source` from the resolved paths before planning outputs.
+
+## 2024-05-20 - [HIGH] DoS via Non-ASCII Header in API Key Middleware
+**Vulnerability:** The API key middleware in `saas_web.py` uses `hmac.compare_digest` to compare strings, which throws a `TypeError` and crashes the server when provided with non-ASCII characters in the `x-api-key` header.
+**Learning:** `hmac.compare_digest` strictly requires byte-like objects when dealing with non-ASCII characters. Passing raw strings directly can lead to unhandled exceptions and potential Denial of Service (DoS) attacks if users supply arbitrary non-ASCII input.
+**Prevention:** Always convert user-controlled strings (e.g., HTTP headers) and configured keys to bytes (e.g., using `.encode('utf-8')`) before passing them to `hmac.compare_digest`.
diff --git a/saas_web.py b/saas_web.py
index 63265e94..c21179ce 100644
--- a/saas_web.py
+++ b/saas_web.py
@@ -113,8 +113,9 @@ async def require_api_key(request: Request, call_next):
     configured_keys = get_configured_api_keys()
     if configured_keys and not (request.method == "GET" and request.url.path == "/"):
         provided_key = request.headers.get("x-api-key", "")
+        provided_bytes = provided_key.encode("utf-8")
         if not any(
-            hmac.compare_digest(provided_key, key) for key in configured_keys
+            hmac.compare_digest(provided_bytes, key.encode("utf-8")) for key in configured_keys
         ):
             return JSONResponse(
                 status_code=401,
diff --git a/tests/test_saas_web.py b/tests/test_saas_web.py
index 3b57e033..5ed1619a 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -707,6 +707,30 @@ def test_missing_header_rejected_when_keys_configured(self):
         self.assertEqual(response.json(), {"error": "Invalid or missing API key"})
         self.assertNotIn("secret-key", response.text)
 
+    def test_non_ascii_header_does_not_crash(self):
+        import asyncio
+        from starlette.requests import Request
+        scope = {
+            "type": "http",
+            "method": "POST",
+            "path": "/shrink",
+            "headers": [(b"x-api-key", "안녕".encode("utf-8"))],
+        }
+        request = Request(scope)
+
+        async def mock_call_next(req):
+            from fastapi.responses import JSONResponse
+            return JSONResponse({"status": "SUCCESS"})
+
+        async def run_test():
+            with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret-key"}):
+                response = await saas_web.require_api_key(request, mock_call_next)
+                self.assertEqual(response.status_code, 401)
+                import json
+                self.assertEqual(json.loads(response.body), {"error": "Invalid or missing API key"})
+
+        asyncio.run(run_test())
+
     def test_wrong_key_rejected(self):
         with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret-key"}):
             response = self._post_shrink(headers={"X-API-Key": "wrong-key"})