CVE-2026-115910: Python Social Auth: OpenID Connect nonce is not bound to the browser session, so a stolen authorization code logs the attacker in as the victim

The generic OpenID Connect backend (OpenIdConnectAuth and every backend built on it) stores the nonce in the shared Association table, not in the user’s session. When the ID token comes back, validate_claims only checks that the nonce exists in that table and is not expired. It does not check that the nonce was issued to the browser that is finishing the login.

So the nonce does not do its job. OIDC Core (section 15.5.2) and RFC 9700 (section 4.5) use the nonce to stop authorization code injection: an attacker who gets hold of a victim’s unused authorization code should not be able to redeem it in the attacker’s own browser. With social-auth-core, the attacker can. They start their own login to get a valid state for their session, then hit the callback with the victim’s code and their own state. state passes because it belongs to the attacker’s session, and the nonce in the victim’s ID token passes because it is in the global table. The attacker’s browser is now logged in as the victim.

PKCE would block this, but OpenIdConnectAuth sets DEFAULT_USE_PKCE = False (the OAuth2 base class defaults to True), so the nonce is the only binding in the default setup.