diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 9c9d083b..f7d60c89 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.
+## 2024-10-09 - Unhandled TypeError DoS via hmac.compare_digest
+**Vulnerability:** A Denial of Service (DoS) vulnerability existed because `hmac.compare_digest` throws a `TypeError` when supplied with non-ASCII characters, such as when parsing malicious HTTP header values for API keys.
+**Learning:** Python's `hmac.compare_digest` strictly requires both arguments to be of the same encoding and fails on non-ASCII string inputs. When evaluating external input (like HTTP headers), unexpected non-ASCII bytes can crash the authentication middleware instead of cleanly denying access.
+**Prevention:** Always explicitly encode strings into `utf-8` bytes (using `.encode("utf-8", "replace")`) before performing cryptographic comparisons with `hmac.compare_digest`, ensuring unhandled exceptions are not thrown due to string decoding issues.
diff --git a/saas_web.py b/saas_web.py
index 63265e94..41bbdaa9 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", "replace"), key.encode("utf-8", "replace")) 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..1528369e 100644
--- a/tests/test_saas_web.py
+++ b/tests/test_saas_web.py
@@ -770,6 +770,12 @@ def test_empty_entries_are_ignored(self):
 
         self.assertEqual(rejected.status_code, 401)
 
+    def test_dos_with_non_ascii_headers(self):
+        with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": "secret"}):
+            response = self._post_shrink(headers={b"X-API-Key": b"secret\xe9"})
+        self.assertEqual(response.status_code, 401)
+        self.assertEqual(response.json(), {"error": "Invalid or missing API key"})
+
     def test_only_empty_entries_leave_endpoints_open(self):
         with patch.dict(os.environ, {"CODEC_CARVER_API_KEYS": " , ,"}):
             response = self._post_shrink()