OAuth2 sign-in attached to an existing account without an email comparison in AshAuthentication

Summary

Improper Authentication vulnerability in team-alembic AshAuthentication allows an attacker to be signed in as another user by linking an OAuth2 identity to an account that is not theirs. AshAuthentication.Strategy.OAuth2.UserResolver.resolve/3 matches an existing account using the register action's upsert_identity keys, then gates linking the incoming provider identity to it on email_trusted?/2, which reads only the provider's email_verified boolean and never compares the provider's email value with the matched account's email. That gate assumes the account was matched by its email field, so under any other upsert_identity it is vacuous and an attacker presenting their own verified email is attached to, and issued a session for, an account matched on some other attribute. The same unguarded gate applies in OAuth2.SignInPreparation on the registration_enabled? false path, where the account is matched by the sign-in action's read filter instead. The upsert also rewrites the matched account's email to the attacker's address, so later account recovery reaches the attacker rather than the owner. This issue affects ash_authentication: from 4.14.0 before 4.15.0 and from 5.0.0-rc.10 before 5.0.0-rc.14.

Timeline

1. Sep 17, 2026, 2:15 PM UTCCVE published
2. Sep 17, 2026, 7:32 PM UTCCISA SSVC assessment

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

3. Sep 17, 2026, 7:32 PM UTCRecord last updated

Affected versions

• ash_authentication

  From 4.14.0 · fixed in 4.15.0; From 5.0.0-rc.10 · fixed in 5.0.0-rc.14

• team-alembic/ash_authentication

  Version range not reported

References 7