fix: ensure JWT segments have the right types (#1162)

* fix: ensure JWT header is a dict before accessing its methods

google/auth/jwt.py

@@ -133,11 +133,12 @@ def _unverified_decode(token):
         token (Union[str, bytes]): The encoded JWT.
 
     Returns:
-        Tuple[str, str, str, str]: header, payload, signed_section, and
+        Tuple[Mapping, Mapping, str, str]: header, payload, signed_section, and
             signature.
 
     Raises:
-        ValueError: if there are an incorrect amount of segments in the token.
+        ValueError: if there are an incorrect amount of segments in the token or
+        segments of the wrong type.
     """
     token = _helpers.to_bytes(token)
 
@@ -152,6 +153,16 @@ def _unverified_decode(token):
     header = _decode_jwt_segment(encoded_header)
     payload = _decode_jwt_segment(encoded_payload)
 
+    if not isinstance(header, Mapping):
+        raise ValueError(
+            "Header segment should be a JSON object: {0}".format(encoded_header)
+        )
+
+    if not isinstance(payload, Mapping):
+        raise ValueError(
+            "Payload segment should be a JSON object: {0}".format(encoded_payload)
+        )
+
     return header, payload, signed_section, signature
 
 

tests/test_jwt.py

@@ -126,6 +126,29 @@ def test_decode_valid(token_factory):
     assert payload["metadata"]["meta"] == "data"
 
 
+def test_decode_header_object(token_factory):
+    payload = token_factory()
+    # Create a malformed JWT token with a number as a header instead of a
+    # dictionary (3 == base64d(M7==))
+    payload = b"M7." + b".".join(payload.split(b".")[1:])
+
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(payload, certs=PUBLIC_CERT_BYTES)
+    assert excinfo.match(r"Header segment should be a JSON object: " + str(b"M7"))
+
+
+def test_decode_payload_object(signer):
+    # Create a malformed JWT token with a payload containing both "iat" and
+    # "exp" strings, although not as fields of a dictionary
+    payload = jwt.encode(signer, "iatexp")
+
+    with pytest.raises(ValueError) as excinfo:
+        jwt.decode(payload, certs=PUBLIC_CERT_BYTES)
+    assert excinfo.match(
+        r"Payload segment should be a JSON object: " + str(b"ImlhdGV4cCI")
+    )
+
+
 def test_decode_valid_es256(token_factory):
     payload = jwt.decode(
         token_factory(use_es256_signer=True), certs=EC_PUBLIC_CERT_BYTES