diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 9c9d083b..739bb107 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.
+
+## 2026-08-22 - [Sentinel: FastAPI `hmac.compare_digest` 비-ASCII DoS 취약점 수정]
+**취약점:** 비-ASCII 문자가 포함된 `hmac.compare_digest` 문자열 비교로 인한 서비스 거부(DoS) 취약점 (CWE-400).
+**학습:** `hmac.compare_digest()`는 비-ASCII 문자가 포함된 문자열을 비교할 때 `TypeError`를 발생시킵니다. 공격자가 HTTP 헤더(`x-api-key`) 등을 통해 악의적인 비-ASCII 문자열을 주입하면, 이를 처리하지 못하고 예외가 발생하여 서버 크래시(HTTP 500) 및 서비스 거부를 초래할 수 있습니다.
+**예방:** 사용자 입력값과 설정된 키를 `hmac.compare_digest()`로 비교하기 전에 항상 `.encode('utf-8')`을 사용하여 바이트 객체로 변환해야 합니다. 성능을 위해 반복문 내부가 아닌 외부에서 미리 인코딩을 수행하는 것이 좋습니다.
diff --git a/saas_web.py b/saas_web.py
index 63265e94..92585502 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_key_bytes = provided_key.encode("utf-8")
         if not any(
-            hmac.compare_digest(provided_key, key) for key in configured_keys
+            hmac.compare_digest(provided_key_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..f3dfedc3 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -1223,5 +1223,31 @@ def test_video_content_type_accepted_by_validator(self):
         )
 
 
+@unittest.skipUnless(
+    _HAS_FASTAPI, "fastapi not installed (optional integration dependency)"
+)
+class TestSaasWebAuth(unittest.IsolatedAsyncioTestCase):
+    async def test_require_api_key_non_ascii_dos_prevention(self):
+        from starlette.requests import Request
+        from starlette.responses import JSONResponse
+        import json
+
+        with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret-key"}):
+            scope = {
+                "type": "http",
+                "method": "POST",
+                "path": "/api/upload",
+                "headers": [(b"x-api-key", "ö".encode("utf-8"))],
+            }
+            request = Request(scope)
+
+            async def mock_call_next(req):
+                return JSONResponse(status_code=200, content={"status": "ok"})
+
+            response = await saas_web.require_api_key(request, mock_call_next)
+            self.assertEqual(response.status_code, 401)
+            self.assertEqual(json.loads(response.body), {"error": "Invalid or missing API key"})
+
+
 if __name__ == "__main__":
     unittest.main()