GHSA-fg6f-75jq-6523

I am writing to you from the Security Labs team at Snyk to report a security issue affecting Authlib, which we identified during a recent research project.

We have identified a vulnerability that can result in a 1-click Account Takeover in applications that use the Authlib library. (5.7 CVSS v3: AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:N/A:N)

Description

Cache-backed state/request-token storage is not tied to the initiating user session, so CSRF is possible for any attacker that has a valid state (easily obtainable via an attacker-initiated authentication flow). When a cache is supplied to the OAuth client registry, FrameworkIntegration.set_state_data writes the entire state blob under _state_{app}_{state}, and get_state_data ignores the caller’s session altogether. [1][2]

    def _get_cache_data(self, key):
        value = self.cache.get(key)
        if not value:
            return None
        try:
            return json.loads(value)
        except (TypeError, ValueError):
            return None
[snip]
    def get_state_data(self, session, state):
        key = f"_state_{self.name}_{state}"
        if self.cache:
            value = self._get_cache_data(key)
        else:
            value = session.get(key)
        if value:
            return value.get("data")
        return None

authlib/integrations/base_client/framework_integration.py:12-41

Retrieval in authorize_access_token therefore succeeds for whichever browser presents that opaque value, and the token exchange proceeds with the attacker’s authorization code. [3]

    def authorize_access_token(self, **kwargs):
        """Fetch access token in one step.

        :return: A token dict.
        """
        params = request.args.to_dict(flat=True)
        state = params.get("oauth_token")
        if not state:
            raise OAuthError(description='Missing "oauth_token" parameter')

        data = self.framework.get_state_data(session, state)
        if not data:
            raise OAuthError(description='Missing "request_token" in temporary data')

        params["request_token"] = data["request_token"]
        params.update(kwargs)
        self.framework.clear_state_data(session, state)
        token = self.fetch_access_token(**params)
        self.token = token
        return token

authlib/integrations/flask_client/apps.py:57-76

This opens up the avenue for Login CSRF for apps that use the cache-backed storage. Depending on the dependent app’s implementation (whether it somehow links accounts in the case of a login CSRF), this could lead to account takeover.

[1] https://github.com/authlib/authlib/blob/260d04edee23d8470057ea659c16fb8a2c7b0dc2/authlib/integrations/flask_client/apps.py#L35

[2] https://github.com/authlib/authlib/blob/260d04edee23d8470057ea659c16fb8a2c7b0dc2/authlib/integrations/base_client/framework_integration.py#L33

[3] https://github.com/authlib/authlib/blob/260d04edee23d8470057ea659c16fb8a2c7b0dc2/authlib/integrations/flask_client/apps.py#L57

Proof of Concept

Let’s think of an app - AwesomeAuthlibApp. Let’s assume that the AwesomeAuthlibApp has internal logic that, when an already logged-in user performs a callback request, links the newly provided SSO identity to the already existing user that made the request.

Then, an attacker can get account takeover inside the app by performing the following actions:

1. They start an SSO OAuth flow, but stop it right before making the callback call to AwesomeAuthlibApp;
2. The attacker tricks a logged-in user (via phishing, a drive-by attack, etc.) to perform a GET request with the attacker's state value and grant code to the AwesomeAuthlibApp callback. Because Authlib doesn’t check whether the state token is linked to the session performing the callback, the callback is processed, the grant code is sent to the provider, and the account linking takes place.

After the GET request is performed, the attacker's SSO account is linked with the victim's AwesomeAuthlibApp account permanently.

Suggested Fix

Per the OAuth RFC [4], the state should be tied to the user’s session to stop exactly such scenarios. One straightforward method of mitigating this issue is to keep storing the state in the session even when caching.

Another method would be to hash the session ID (or another per-user secret from the session) into the cache key. This way, the state will be stored inside the cache, but it is still linked to the session of the user that initiated the OAuth flow.

[4] https://www.rfc-editor.org/rfc/rfc6749#section-10.12