---
title: "CVE-2025-12421: Mattermost fails to to verify the token used during code exchange"
url: https://erroneo.us/security/cve/CVE-2025-12421/
description: "CVE-2025-12421 (github.com/mattermost/mattermost/server/v8 (Go)): Mattermost versions 11.0.x"
type: vulnerability-record
section: "Security Digest"
date_published: 2025-11-27
date_modified: 2026-02-26
publisher: erroneo.us
author: erroneo.us Security Desk
cve: "CVE-2025-12421"
kev: false
cvss: 9.9
epss: 0.00345
license: All rights reserved. Quote with attribution and a link.
---

# CVE-2025-12421: Mattermost fails to to verify the token used during code exchange

**Signal:** Critical, preconditions apply. Severe if exploited, but exploitation needs conditions that may not apply to you. (Computed from CISA KEV, EPSS and CVSS; not an editorial verdict.)

## Should you act?

- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.3% , percentile 27: FIRST’s model estimates a 0.3% chance of exploitation activity in the next 30 days, higher than 27% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Needs an account (low (any account)). Exposure depends on who can log in.
- Only applies if your dependency tree includes github.com/mattermost/mattermost/server/v8 (Go) < 8.0.0-20251022210333-acda1fb5dd46, fixed in 8.0.0-20251022210333-acda1fb5dd46 ; github.com/mattermost/mattermost-server (Go) >= 11.0.0, < 11.0.3, fixed in 11.0.3 ; github.com/mattermost/mattermost-server (Go) >= 10.12.0, < 10.12.2, fixed in 10.12.2 ; github.com/mattermost/mattermost-server (Go) >= 10.11.0, < 10.11.5, fixed in 10.11.5 . Check your lockfile, including transitive dependencies.

## What it is

Mattermost versions 11.0.x

## Affected packages

- Go `github.com/mattermost/mattermost/server/v8` < 8.0.0-20251022210333-acda1fb5dd46 → fixed: 8.0.0-20251022210333-acda1fb5dd46
- Go `github.com/mattermost/mattermost-server` >= 11.0.0, < 11.0.3 → fixed: 11.0.3
- Go `github.com/mattermost/mattermost-server` >= 10.12.0, < 10.12.2 → fixed: 10.12.2
- Go `github.com/mattermost/mattermost-server` >= 10.11.0, < 10.11.5 → fixed: 10.11.5
- Go `github.com/mattermost/mattermost-server` >= 10.5.0, < 10.5.13 → fixed: 10.5.13

## Affected products (CVE record)

- Mattermost Mattermost: 11.0.0 <= 11.0.2, 10.12.0 <= 10.12.1, 10.11.0 <= 10.11.4, 10.5.0 <= 10.5.12

## Severity

- Mattermost CVSS 3.1: 9.9 `CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H`
- GitHub CVSS 3.1: 9.9 `CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H`

## Exploitation

- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none , automatable no , technical impact total .
- FIRST EPSS (model of 2026-10-21): 0.34% probability of exploitation activity in the next 30 days; percentile 27.

## Sources

- [GitHub Advisory GHSA-mp6x-97xj-9x62](https://github.com/advisories/GHSA-mp6x-97xj-9x62)
- [CVE.org record CVE-2025-12421](https://www.cve.org/CVERecord?id=CVE-2025-12421)
- [NVD entry CVE-2025-12421](https://nvd.nist.gov/vuln/detail/CVE-2025-12421)
- [mattermost.com](https://mattermost.com/security-updates)
- [github.com](https://github.com/mattermost/mattermost/commit/5072bbf689a46b3b97b2373f26149f5891396e6b)
- [github.com](https://github.com/mattermost/mattermost/commit/acda1fb5dd46a2f46c76ae67012423c760525eaa)
- [github.com](https://github.com/mattermost/mattermost/commit/f361e7d75a7ab9df5a2106e1ceb919b94b55e41d)
- [github.com](https://github.com/mattermost/mattermost/commit/feb598ed2b7ac3cb78a253b032ad7e4628b0de00)

---
Source page: https://erroneo.us/security/cve/CVE-2025-12421/ · Security Digest: https://erroneo.us/security/ · More: https://erroneo.us/llms.txt