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EEF-CVE-2026-82761 CVE-2026-82761 GHSA-23gr-vcp4-r27q

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

Not rated yet Hex
Severity
0.0 / 10
Unknown · CVSS 4.0
Exploit chance (EPSS)
0.56%
Chance of exploitation in the next 30 days. Higher than 46.61% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-367 Time-of-check Time-of-use (TOCTOU) Race Condition
Fix
5.0.0-rc.14
Upgrade ash_authentication to this version or later

Description

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.

Affected versions

Hex

PackageVulnerableFixed in
ash_authentication
>= 3.9.0, < 4.15.0
>= 5.0.0-rc.0, < 5.0.0-rc.14
4.15.0
5.0.0-rc.14

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

ash_authentication Hex → 5.0.0-rc.14

mix
{:ash_authentication, "~> 5.0.0-rc.14"} # then: mix deps.update ash_authentication

Severity breakdown

CVSS 4.0 0.0

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

Attack vector
Network
Attack complexity
Low
Attack requirements
Present
Privileges required
None
User interaction
None
Confidentiality (vulnerable system)
High
Integrity (vulnerable system)
High
Availability (vulnerable system)
None
Confidentiality (subsequent systems)
None
Integrity (subsequent systems)
None
Availability (subsequent systems)
None

Weakness

CWE-367: Time-of-check Time-of-use (TOCTOU) Race Condition

The product checks the state of a resource before using that resource, but the resource's state can change between the check and the use in a way that invalidates the results of the check.

How to prevent it
  • Implementation: The most basic advice for TOCTOU vulnerabilities is to not perform a check before the use. This does not resolve the underlying issue of the execution of a function on a resource whose state and identity cannot be assured, but it does help to limit the false sense of security given by the check.
  • Implementation: When the file being altered is owned by the current user and group, set the effective gid and uid to that of the current user and group when executing this statement.
  • Architecture and Design: Limit the interleaving of operations on files from multiple processes.
  • Implementation, Architecture and Design: If you cannot perform operations atomically and you must share access to the resource between multiple processes or threads, then try to limit the amount of time (CPU cycles) between the check and use of the resource. This will not fix the problem, but it could make it more difficult for an attack to succeed.
  • Implementation: Recheck the resource after the use call to verify that the action was taken appropriately.
  • Architecture and Design: Ensure that some environmental locking mechanism can be used to protect resources effectively.
  • Implementation: Ensure that locking occurs before the check, as opposed to afterwards, such that the resource, as checked, is the same as it is when in use.

References

Advisories

Fixes and patches

Packages

Other links

Timeline

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy it's related
EEF-CVE-2026-86688
Session id is not renewed on authentication in ash_authentication, allowing session fixation
Not rated yet Also affects ash_authentication
EEF-CVE-2026-76949
Remember-me sign-in guard reads a session key that is never written in ash_authentication, allowing session replacement
Not rated yet Also affects ash_authentication
EEF-CVE-2026-91039
dynamic_oidc identities are not namespaced by connection in ash_authentication, allowing cross-connection account takeover
Not rated yet Also affects ash_authentication
EEF-CVE-2026-88952
OAuth2 sign-in attached to an existing account without an email comparison in AshAuthentication
Not rated yet Also affects ash_authentication
EEF-CVE-2026-85500
`require_confirmed_with` is not enforced on the action and fails open on an unreadable attribute in AshAuthentication
Not rated yet Also affects ash_authentication
EEF-CVE-2026-86533
Revoked session accepted because the session jti is never checked in AshAuthentication and AshAuthentication Phoenix
Not rated yet Also affects ash_authentication
EEF-CVE-2026-81632
Single-use sign-in token placed in a redirect query string in AshAuthenticationPhoenix
Not rated yet Also affects ash_authentication
EEF-CVE-2026-80218
Sign-in token minted for one resource accepted by another in AshAuthentication
Not rated yet Also affects ash_authentication

Credits: James Harton, Jonatan Männchen / EEF, Peter Ullrich

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