---
id: CVE-2026-82761
title: >-
  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
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 configur…
severity: critical
cvss: 9.1
cvssVector: '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'
cwe:
  - CWE-367
vendor: team-alembic
product: ash_authentication
affected:
  - ash_authentication >= 3.9.0 < 4.15.0
  - ash_authentication >= 5.0.0-rc.0 < 5.0.0-rc.14
  - >-
    team-alembic/ash_authentication >= cf3d227ef25912cf1b0c5fa80f20001f5c46a102
    < *
published: '2026-09-17'
updated: '2026-09-18'
sourceUpdated: '2026-09-18T18:16:18.527'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-82761'
references:
  - url: 'https://cna.erlef.org/cves/CVE-2026-82761.html'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/team-alembic/ash_authentication/commit/18dfdb36c14aa6a61df8572bce2d5ec36b1d9840
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/team-alembic/ash_authentication/commit/9ef6864b8833d3b795427a7b8dc518a4997d41ab
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/team-alembic/ash_authentication/commit/cf3d227ef25912cf1b0c5fa80f20001f5c46a102
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: >-
      https://github.com/team-alembic/ash_authentication/security/advisories/GHSA-23gr-vcp4-r27q
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
  - url: 'https://osv.dev/vulnerability/EEF-CVE-2026-82761'
    label: 6b3ad84c-e1a6-4bf7-a703-f496b71e49db
tags:
  - nvd
  - cve.org
epss: 0.0056
epssPercentile: 0.44253
ssvc:
  exploitation: none
  automatable: 'yes'
  technicalImpact: total
  timestamp: '2026-09-17T18:25:13.930894Z'
cvssSource: cna
ingestedAt: '2026-09-17T13:18:16.985Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.