diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 9c9d083b..1b8e0b34 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -65,3 +65,7 @@
 **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.
+## 2026-08-04 - hmac.compare_digest 비-ASCII 문자열 처리 오류 수정
+**취약점:** FastAPI 미들웨어에서 `hmac.compare_digest` 사용 시, 악의적인 사용자가 API 키 헤더에 비-ASCII 문자를 포함하면 `TypeError`가 발생하여 500 서버 에러(DoS)가 유발될 수 있음.
+**학습:** 파이썬의 `hmac.compare_digest()`는 비-ASCII 문자가 포함된 문자열을 직접 비교할 수 없음.
+**예방:** 항상 두 인자를 명시적으로 바이트(`utf-8`)로 인코딩한 후 비교해야 안전하게 검증할 수 있음.
diff --git a/saas_web.py b/saas_web.py
index 63265e94..071b1419 100644
--- a/saas_web.py
+++ b/saas_web.py
@@ -114,7 +114,7 @@ async def require_api_key(request: Request, call_next):
     if configured_keys and not (request.method == "GET" and request.url.path == "/"):
         provided_key = request.headers.get("x-api-key", "")
         if not any(
-            hmac.compare_digest(provided_key, key) for key in configured_keys
+            hmac.compare_digest(provided_key.encode("utf-8"), 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..a413455c 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -186,6 +186,28 @@ def test_shrink_media_rejects_nonpositive_target_bytes(self):
             {"error": "Invalid target_bytes value. Must be greater than 0."},
         )
 
+    def test_non_ascii_key_rejected_safely(self):
+        import asyncio
+        import starlette.requests
+        from starlette.responses import JSONResponse
+
+        async def call_next(request):
+            return JSONResponse({"status": "ok"})
+
+        scope = {
+            "type": "http",
+            "method": "POST",
+            "path": "/shrink",
+            "headers": [(b"x-api-key", "invalíd".encode("utf-8"))],
+        }
+        request = starlette.requests.Request(scope)
+
+        with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret-key"}):
+            response = asyncio.run(saas_web.require_api_key(request, call_next))
+
+        self.assertEqual(response.status_code, 401)
+        self.assertEqual(json.loads(response.body), {"error": "Invalid or missing API key"})
+
     def test_shrink_media_rejects_missing_filename(self):
         response = saas_web.shrink_media(
             BackgroundTasks(),