Vulnerability · Published Sep 17, 2026 · Assigned by EEF
CVE-2026-82761Magic link single-use tokens replayable via TOCTOU race in AshAuthentication
Critical 9.1
In plain English
A critical-severity security flaw in ash_authentication from team-alembic. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 47% of all known vulnerabilities).
Official description
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.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| team-alembic | ash_authentication | 3.9.0 – before 4.15.05.0.0-rc.0 – before 5.0.0-rc.14 |
| team-alembic | ash_authentication | cf3d227ef25912cf1b0c5fa80f20001f5c46a102 – before * |
Weakness: CWE-367 · Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
References
- https://github.com/team-alembic/ash_authentication/security/advisories/GHSA-23gr-vcp4-r27q related, vendor-advisory
- EEF CNA record for CVE-2026-82761 related
- OSV record EEF-CVE-2026-82761 related
- Introducing commit cf3d227 in team-alembic/ash_authentication related
- Fix commit 18dfdb3 in team-alembic/ash_authentication patch
- Fix commit 9ef6864 in team-alembic/ash_authentication patch