Magic link single-use tokens replayable via TOCTOU race in AshAuthentication
Published Sep 17, 2026 · Updated Sep 17, 2026
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.
Summary
What happened
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.
The record
- CVE
- CVE-2026-82761
- Published
- Sep 17, 2026
- Updated
- Sep 17, 2026
- Vendor
- Alembic
- Product
- Ash Authentication
- Classifications
- Unavailable
- Attack vector
- Unavailable
- Privileges
- Unavailable
Timeline
How it unfolded
- Sep 17, 2026CVE publishedPublication date reported by the CVE source.
- Sep 17, 2026Record updatedLatest update available in the CVE record.
Exploitability
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Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=3.9.0 <4.15.0
- Affected versionversion=5.0.0-rc.0 <5.0.0-rc.14
- Affected versionversion=cf3d227ef25912cf1b0c5fa80f20001f5c46a102 <* changes=[{"at":"18dfdb36c14aa6a61df8572bce2d5ec36b1d9840","status":"unaffected"},{"at":"9ef6864b8833d3b795427a7b8dc518a4997d41ab","status":"unaffected"}]
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Activity data is delayed. The latest report is Sep 28, 2026.
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