Fix: Less restrictive content-type header check for google authentica…

…tion (ignores charset) (#1382)

* fix: added function for parsing content-type headers

* fix: improved too restrictive content-type check (now ignores charset param)

* test: Added some new tests for the parse_content_type function in _helpers and its use in _metadata

* style: Linted code

---------

Co-authored-by: arithmetic1728 <58957152+arithmetic1728@users.noreply.github.com>

google/auth/_helpers.py

@@ -17,6 +17,7 @@
 import base64
 import calendar
 import datetime
+from email.message import Message
 import sys
 import urllib
 
@@ -63,6 +64,28 @@ def decorator(method):
     return decorator
 
 
+def parse_content_type(header_value):
+    """Parse a 'content-type' header value to get just the plain media-type (without parameters).
+
+    This is done using the class Message from email.message as suggested in PEP 594
+        (because the cgi is now deprecated and will be removed in python 3.13,
+        see https://peps.python.org/pep-0594/#cgi).
+
+    Args:
+        header_value (str): The value of a 'content-type' header as a string.
+
+    Returns:
+        str: A string with just the lowercase media-type from the parsed 'content-type' header.
+            If the provided content-type is not parsable, returns 'text/plain',
+            the default value for textual files.
+    """
+    m = Message()
+    m["content-type"] = header_value
+    return (
+        m.get_content_type()
+    )  # Despite the name, actually returns just the media-type
+
+
 def utcnow():
     """Returns the current UTC datetime.
 

google/auth/compute_engine/_metadata.py

@@ -218,7 +218,10 @@ def get(
 
     if response.status == http_client.OK:
         content = _helpers.from_bytes(response.data)
-        if response.headers["content-type"] == "application/json":
+        if (
+            _helpers.parse_content_type(response.headers["content-type"])
+            == "application/json"
+        ):
             try:
                 return json.loads(content)
             except ValueError as caught_exc:

tests/compute_engine/test__metadata.py

@@ -176,6 +176,24 @@ def test_get_success_json():
     assert result[key] == value
 
 
+def test_get_success_json_content_type_charset():
+    key, value = "foo", "bar"
+
+    data = json.dumps({key: value})
+    request = make_request(
+        data, headers={"content-type": "application/json; charset=UTF-8"}
+    )
+
+    result = _metadata.get(request, PATH)
+
+    request.assert_called_once_with(
+        method="GET",
+        url=_metadata._METADATA_ROOT + PATH,
+        headers=_metadata._METADATA_HEADERS,
+    )
+    assert result[key] == value
+
+
 def test_get_success_retry():
     key, value = "foo", "bar"
 

tests/test__helpers.py

@@ -51,6 +51,32 @@ def func2():  # pragma: NO COVER
         _helpers.copy_docstring(SourceClass)(func2)
 
 
+def test_parse_content_type_plain():
+    assert _helpers.parse_content_type("text/html") == "text/html"
+    assert _helpers.parse_content_type("application/xml") == "application/xml"
+    assert _helpers.parse_content_type("application/json") == "application/json"
+
+
+def test_parse_content_type_with_parameters():
+    content_type_html = "text/html; charset=UTF-8"
+    content_type_xml = "application/xml; charset=UTF-16; version=1.0"
+    content_type_json = "application/json; charset=UTF-8; indent=2"
+    assert _helpers.parse_content_type(content_type_html) == "text/html"
+    assert _helpers.parse_content_type(content_type_xml) == "application/xml"
+    assert _helpers.parse_content_type(content_type_json) == "application/json"
+
+
+def test_parse_content_type_missing_or_broken():
+    content_type_foo = None
+    content_type_bar = ""
+    content_type_baz = "1234"
+    content_type_qux = " ; charset=UTF-8"
+    assert _helpers.parse_content_type(content_type_foo) == "text/plain"
+    assert _helpers.parse_content_type(content_type_bar) == "text/plain"
+    assert _helpers.parse_content_type(content_type_baz) == "text/plain"
+    assert _helpers.parse_content_type(content_type_qux) == "text/plain"
+
+
 def test_utcnow():
     assert isinstance(_helpers.utcnow(), datetime.datetime)