Security Advisory: Cache-Backed State Storage CSRF in Authlib

The Security Labs team at Snyk has reported a security issue affecting Authlib, identified during a recent research project.

The Snyk Security Labs team has identified a vulnerability that can result in a one-click account takeover in applications that utilize the Authlib library.

Description

Cache-backed state/request-token storage is not tied to the initiating user session, making CSRF possible for any attacker that possesses a valid state value (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 disregards the caller's session entirely. [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/baseclient/frameworkintegration.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 an avenue for Login CSRF in applications that use cache-backed storage. Depending on the dependent application's implementation (e.g., whether it links accounts in the event of a login CSRF), this could lead to account takeover.

Proof of Concept

Consider a hypothetical application — AwesomeAuthlibApp. Assume that AwesomeAuthlibApp contains internal logic such that, when an already authenticated user performs a callback request, the application links the newly provided SSO identity to the existing user account associated with that request.

Under these conditions, an attacker can achieve account takeover within the application by performing the following actions:

1. The attacker initiates an SSO OAuth flow but halts the process immediately before the callback request is made to AwesomeAuthlibApp.
2. The attacker then induces a logged-in user (via phishing, a drive-by attack, or similar means) to perform a GET request containing the attacker's state value and authorization code to the AwesomeAuthlibApp callback endpoint. Because Authlib does not verify whether the state token is bound to the session performing the callback, the callback is processed, the authorization code is sent to the provider, and the account linking proceeds.

Once the GET request is executed, the attacker's SSO account becomes permanently linked to the victim's AwesomeAuthlibApp account.

Suggested Fix

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

An alternative approach would be to hash the session ID (or another per-user secret derived from the session) into the cache key. This ensures the state remains stored in the cache while still being bound to the session of the user that initiated the OAuth flow.

Resources

• [1] flask_client/apps.py#L35
• [2] baseclient/frameworkintegration.py#L33
• [3] flask_client/apps.py#L57
• [4] RFC 6749 §10.12