# CVE-2026-82761: Magic link single-use tokens replayable via TOCTOU race in AshAuthentication

> Attackers can replay leaked magic link tokens to authenticate as victims due to a race condition in token revocation logic.

- **Severity:** Critical 9.1 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 17 Sep 2026
- **Also known as:** EEF-CVE-2026-82761, GHSA-23gr-vcp4-r27q

## What is CVE-2026-82761?

CVE-2026-82761 is a critical vulnerability in team-alembic ash_authentication, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### 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.

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

## Which versions are affected by CVE-2026-82761?

1 open source package ship an affected version, including ash_authentication.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `ash_authentication` | Hex | From 3.9.0 | 4.15.0, 5.0.0-rc.14 |

## How do I fix CVE-2026-82761?

Upgrade ash_authentication to a fixed version for your release line: 4.15.0 or 5.0.0-rc.14.

## Is CVE-2026-82761 being exploited?

There is no known exploitation of CVE-2026-82761 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-82761 and fix it if it is affected.

What it is: CVE-2026-82761 is a critical vulnerability in team-alembic ash_authentication, published on 17 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-82761 in the wild and no public exploit code.

Affected packages and fixed versions:
- ash_authentication (Hex): affected from 3.9.0; fixed in 4.15.0, 5.0.0-rc.14

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-82761/ (Markdown: https://insights.sbomify.com/CVE-2026-82761/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-82761
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-82761
- OSV: https://osv.dev/vulnerability/CVE-2026-82761

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-82761/
Summaries are generated and may contain mistakes; the official description above is the record.