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Medium, CVSS 4.3

Mattermost fails to properly validate User-Agent header tokens

CVE-2026-25783GHSA-2v3w-6g35-5f9v

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (github.com/mattermost/mattermost/server/v8 8.0.0-20260129181235-1346cf529aef; github.com/mattermost/mattermost-server 11.3.1, 11.2.3, 10.11.11, 5.3.2-0.20260129181235-1346cf529aef), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

  • GitHub Advisory Database and maintainer security advisories on GitHub

    GitHub-reviewed advisory GHSA-2v3w-6g35-5f9v lists affected ranges: Go github.com/mattermost/mattermost/server/v8 >=0 =0 =10.11.0-rc1 =11.2.0-rc1 =11.3.0-rc1 <11.3.1

    VerifiedObserved Structured GitHub Advisory Database record (type=reviewed) from the GitHub REST API.

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
4.3 · CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
EPSS
  • CVE-2026-25783: 0.30% estimated exploitation activity in the next 30 days; percentile 23.12 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to properly validate User-Agent header tokens which allows an authenticated attacker to cause a request panic via a specially crafted User-Agent header. Mattermost Advisory ID: MMSA-2026-00586

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 1 · created ·

    Published from ghsa record GHSA-2v3w-6g35-5f9v

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2026-25783

Advisory: Mattermost fails to properly validate User-Agent header tokens
IDs: CVE-2026-25783, GHSA-2v3w-6g35-5f9v
Record: https://patchnext.com/advisories/cve-2026-25783 (revision 1, source checked 26 Oct 2026, 15:18 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/mattermost/mattermost/server/v8 8.0.0-20260129181235-1346cf529aef; github.com/mattermost/mattermost-server 11.3.1, 11.2.3, 10.11.11, 5.3.2-0.20260129181235-1346cf529aef. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/mattermost/mattermost/server/v8: affected < 8.0.0-20260129181235-1346cf529aef; fixed in 8.0.0-20260129181235-1346cf529aef
- Go github.com/mattermost/mattermost-server: affected < 5.3.2-0.20260129181235-1346cf529aef; >= 10.11.0-rc1, < 10.11.11; >= 11.2.0-rc1, < 11.2.3; >= 11.3.0-rc1, < 11.3.1; fixed in 11.3.1, 10.11.11, 5.3.2-0.20260129181235-1346cf529aef, 11.2.3

## Suggested inspection (read-only)
- Go github.com/mattermost/mattermost/server/v8: check the installed version with your package manager.
- Go github.com/mattermost/mattermost-server: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/mattermost/mattermost/server/v8, github.com/mattermost/mattermost-server.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-2v3w-6g35-5f9v (GitHub-reviewed advisory GHSA-2v3w-6g35-5f9v lists affected ranges: Go github.com/mattermost/mattermost/server/v8 >=0 =0 =10.11.0-rc1 =11.2.0-rc1 =11.3.0-rc1 <11.3.1)

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