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.

Impact

An attacker who controls a provider account with a verified email of their own is signed in as a different local user, holding a session bearing that user's subject, with no interaction from the victim and no need to pre-register. Because the link also rewrites the matched account's email to the attacker's address, account recovery for the victim is redirected to the attacker and the compromise survives the provider identity being unlinked.

Workarounds

Set trust_email_verified? false on the affected strategy, which refuses the sign-in rather than linking it and closes both the register and the sign-in path.

Alternatively, key the register action's upsert_identity, or the sign-in action's read filter, on the email attribute, which restores the premise the gate assumes.

Configurations

The strategy must have trust_email_verified? enabled, and the account must be matched on something other than the email: a register action whose upsert_identity keys are not the email attribute, or, with registration_enabled? false, a sign-in action whose read filter is not the email.

trust_email_verified? defaults to false on the base oauth2 strategy, but the apple, auth0, github, google and slack strategies each set it to true in their own DSL.

Solutions

Upgrading prevents new links but does not unpick existing ones. An account already linked through this path stays linked, and a victim's email may already have been rewritten to the attacker's address.

Operators whose register action or sign-in action matched on anything other than the email should review their UserIdentity rows for links whose provider email does not match the linked account's email, and check affected accounts for a rewritten email address.