Replayable OAuth2 CSRF state retained after a failed callback in AshAuthentication

Summary

Insufficient Session Expiration vulnerability in team-alembic AshAuthentication allows an attacker who obtains a victim's OAuth2 state value to replay the callback and sign that victim into an attacker-controlled account. AshAuthentication.Strategy.OAuth2.Plug.callback/2 clears the stored session_params through a rebinding step inside its with chain, conn <- delete_session(conn, session_key). Elixir evaluates the else block in the scope enclosing the with, so every failure path (the provider returning ?error=access_denied, an invalid code, a token-exchange error, or a registration or sign-in failure) reaches store_authentication_result/2 holding the original connection and the session entry is never removed. The value the module's own comment describes as protection against a CSRF-related attack is therefore consumed only when authentication succeeds, and survives a cancelled or failed attempt until the next request phase or session expiry. This issue affects ash_authentication: from 0.6.0 before 4.15.0 and from 5.0.0-rc.0 before 5.0.0-rc.14.

Timeline

1. Sep 17, 2026, 1:09 PM UTCCVE published
2. Sep 17, 2026, 6:26 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: no · Technical impact: partial

3. Sep 17, 2026, 6:26 PM UTCRecord last updated

Affected versions

• ash_authentication

  From 0.6.0 · fixed in 4.15.0; From 5.0.0-rc.0 · fixed in 5.0.0-rc.14

• team-alembic/ash_authentication

  Version range not reported

References 6