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Mattermost ServerApplication · Mattermost

CVE-2026-4054

MEDIUM · 6.5 CVSS v3.1 Published 2026-05-15
Fix available
A fix is available. Upgrade to 10.11.14 / 11.4.4 or later.
See remediation →
72/100
Remediation priority · Elevated
Remotely reachable No privileges

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Mattermost fails to validate that proxied image response bodies match their declared Content-Type headers. An attacker can serve a malicious SVG file with a non-SVG Content-Type (e.g., image/png) from an attacker-controlled origin. When Mattermost proxies this image for display in og:image meta tags or Markdown image links, the browser interprets the SVG content, causing client-side code execution and resulting in denial of service.

MitigationUpgrade to Mattermost versions 11.5.2, 10.11.14, or 11.4.4 or later per MMSA-2026-00630. Until patched, restrict or sanitize user-controlled og:image and Markdown image sources to trusted origins.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Mattermost ServerApplication
Affected:>= 10.11.0, < 10.11.14>= 11.4.0, < 11.4.4>= 11.5.0, < 11.5.2

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Check Mattermost Server version
    Access the Mattermost System Console > About > Mattermost or use the API endpoint /api/v4/system/version
    Affected if The installed version falls within >= 10.11.0 and < 10.11.14, OR >= 11.4.0 and < 11.4.4, OR >= 11.5.0 and < 11.5.2
  2. Verify if image proxy feature is enabled
    Navigate to System Console > Environment > Image Proxy, or check the ImageProxyType setting in config.json
    Affected if Image proxy is enabled and configured to proxy images from external origins
  3. Check allowed external image domains
    Review the AllowedUntrustedInternalConnections list in config.json, or check System Console > Environment > Developer for whitelisted domains used in og:image or Markdown images
    Affected if Untrusted or attacker-controlled domains can be added as image sources for og:image meta tags or Markdown image links
  4. Confirm presence of user-controllable image sources
    Inspect posts, custom emoji, or system settings where users can specify external image URLs that get proxied by Mattermost
    Affected if Users can supply arbitrary image URLs that Mattermost will proxy without validating the response body matches the declared Content-Type

You are affected if your Mattermost Server version is within the affected ranges AND the image proxy feature is enabled allowing external image sources from untrusted domains.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 10.11.14 / 11.4.4 / 11.5.2 or later
Fixed in 10.11.1411.4.411.5.2
Interim mitigation

Upgrade to Mattermost versions 11.5.2, 10.11.14, or 11.4.4 or later per MMSA-2026-00630. Until patched, restrict or sanitize user-controlled og:image and Markdown image sources to trusted origins.

Recommended fix High confidence

10.11.14+ / 11.4.4+ / 11.5.2+ (depending on your major version branch)

  1. 1. Identify the currently running Mattermost Server version using the system console or mmctl command
  2. 2. For Mattermost 10.11.x: Upgrade to version 10.11.14 or later
  3. 3. For Mattermost 11.4.x: Upgrade to version 11.4.4 or later
  4. 4. For Mattermost 11.5.x: Upgrade to version 11.5.2 or later
  5. 5. Review upgrade notes in Mattermost documentation for any migration or compatibility considerations
  6. 6. Perform upgrade during a maintenance window following standard backup procedures
  7. 7. Verify the upgrade was successful by checking the version in the system console
Caveat Review Mattermost upgrade notes for your version branch for any migration steps or breaking changes between versions

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing3.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,210
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,936.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-4054 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data