Fix substring matches in WWW-Authenticate parsing

src/mcp/client/auth/utils.py

@@ -26,13 +26,19 @@ def extract_field_from_www_auth(response: Response, field_name: str) -> str | No
     if not www_auth_header:
         return None
 
-    # Pattern matches: field_name="value" or field_name=value (unquoted)
-    pattern = rf'{field_name}=(?:"([^"]+)"|([^\s,]+))'
-    match = re.search(pattern, www_auth_header)
-
-    if match:
-        # Return quoted value if present, otherwise unquoted value
-        return match.group(1) or match.group(2)
+    # Strip the auth scheme (e.g. "Bearer") so parsing only sees auth-params.
+    _, separator, auth_params = www_auth_header.partition(" ")
+    if not separator:
+        auth_params = www_auth_header
+
+    # Match comma-delimited auth-params while respecting quoted values.
+    pattern = re.compile(
+        r'(?:^|,\s*)(?P<name>[A-Za-z][A-Za-z0-9_-]*)=(?:"(?P<quoted>[^"]+)"|(?P<unquoted>[^,\s]+))'
+    )
+    for match in pattern.finditer(auth_params):
+        if match.group("name") == field_name:
+            # Return quoted value if present, otherwise unquoted value
+            return match.group("quoted") or match.group("unquoted")
 
     return None
 

tests/client/test_auth.py

@@ -2016,6 +2016,12 @@ class TestWWWAuthenticate:
                 "resource_metadata",
                 "https://api.example.com/auth/metadata?version=1",
             ),
+            ('Bearer error_scope="decoy", scope="read write"', "scope", "read write"),
+            (
+                'Bearer error_description="missing scope=write permission", scope="read write"',
+                "scope",
+                "read write",
+            ),
         ],
     )
     def test_extract_field_from_www_auth_valid_cases(
@@ -2047,6 +2053,12 @@ def test_extract_field_from_www_auth_valid_cases(
             # Header without requested field
             ('Bearer realm="api", error="insufficient_scope"', "scope", "no scope parameter"),
             ('Bearer realm="api", scope="read write"', "resource_metadata", "no resource_metadata parameter"),
+            ('Bearer custom_scope="leaked"', "scope", "field name appears only as a substring"),
+            (
+                'Bearer x_resource_metadata="https://decoy.example.com"',
+                "resource_metadata",
+                "field name appears only as a substring",
+            ),
             # Malformed field (empty value)
             ("Bearer scope=", "scope", "malformed scope parameter"),
             ("Bearer resource_metadata=", "resource_metadata", "malformed resource_metadata parameter"),