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Medium, CVSS 4.3

Mattermost doesn't validate the response body of proxied images

CVE-2026-4054GHSA-j76w-p754-g2w7

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (github.com/mattermost/mattermost-server 11.5.2, 11.4.4, 0.0.0-20260414103857-b21ef302025e), 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

  • Go github.com/mattermost/mattermost-server

    Affected
    >= 11.5.0, < 11.5.2>= 0.0.0-20250731163400-5b955468ea1e, < 0.0.0-20260414103857-b21ef302025e>= 11.4.0, < 11.4.4
    Fixed in
    0.0.0-20260414103857-b21ef302025e, 11.4.4, 11.5.2

Sources

  • GitHub Advisory Database and maintainer security advisories on GitHub

    GitHub-reviewed advisory GHSA-j76w-p754-g2w7 lists affected ranges: Go github.com/mattermost/mattermost-server >=11.5.0 =0.0.0-20250731163400-5b955468ea1e =11.4.0 <11.4.4

    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:N/UI:R/S:U/C:N/I:N/A:L
EPSS
  • CVE-2026-4054: 0.24% estimated exploitation activity in the next 30 days; percentile 15.50 (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 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate the response body of proxied images, which allows a remote attacker to enact client-side DoS via an SVG file served from an attacker-controlled origin under a non-SVG Content-Type header (e.g. image/png) embedded in an og:image meta tag or Markdown image link. Mattermost Advisory ID: MMSA-2026-00630.

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

    Published from ghsa record GHSA-j76w-p754-g2w7

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-2026-4054

Advisory: Mattermost doesn't validate the response body of proxied images
IDs: CVE-2026-4054, GHSA-j76w-p754-g2w7
Record: https://patchnext.com/advisories/cve-2026-4054 (revision 1, source checked 26 Oct 2026, 15:22 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 11.5.2, 11.4.4, 0.0.0-20260414103857-b21ef302025e. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/mattermost/mattermost-server: affected >= 11.5.0, < 11.5.2; >= 0.0.0-20250731163400-5b955468ea1e, < 0.0.0-20260414103857-b21ef302025e; >= 11.4.0, < 11.4.4; fixed in 0.0.0-20260414103857-b21ef302025e, 11.4.4, 11.5.2

## Suggested inspection (read-only)
- 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.
- 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-j76w-p754-g2w7 (GitHub-reviewed advisory GHSA-j76w-p754-g2w7 lists affected ranges: Go github.com/mattermost/mattermost-server >=11.5.0 =0.0.0-20250731163400-5b955468ea1e =11.4.0 <11.4.4)

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