Magic link single-use tokens replayable via TOCTOU race in AshAuthentication

Summary

Time-of-check Time-of-use (TOCTOU) Race Condition vulnerability in team-alembic AshAuthentication allows an attacker holding a leaked magic link to replay its single-use token and authenticate as the target subject. A magic link configured with single_use_token?, which is the default, is meant to be redeemable exactly once, but nothing serialises the token's validity check against its consumption, so concurrent redemptions of one token all succeed and each yields a full user token. Sign-in verifies the JWT with Jwt.verify/4 and revokes it only afterwards: AshAuthentication.Strategy.MagicLink.SignInPreparation revokes in a Query.after_action callback, and AshAuthentication.Strategy.MagicLink.SignInChange in an after_transaction hook that runs once the sign-in has already committed. AshAuthentication.TokenResource.Actions.revoke/3 writes the revocation as an upsert, so a concurrent duplicate revocation silently succeeds instead of conflicting and no request ever loses the race. This issue affects ash_authentication: from 3.9.0 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, 6:25 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: total

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

Affected versions

• ash_authentication

  From 3.9.0 · 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