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Mattermost ServerApplication · Mattermost

CVE-2026-4858

CRITICAL · 9.9 CVSS v3.1 Published 2026-05-21
Fix available
A fix is available. Upgrade to 10.11.15 / 11.4.5 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable Zero-click

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 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to check integration URL for path traversal which allows an malicious authenticated user to call an arbitrary API via system admin Mattermost auth token using via path traversal in integration action URL.. Mattermost Advisory ID: MMSA-2026-00640

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 integration URLs for path traversal, allowing authenticated users to craft malicious integration action URLs that traverse paths and execute arbitrary API calls using a system administrator's auth token.

MitigationUpgrade Mattermost to versions beyond 11.6.0, 11.5.3, 11.4.4, and 10.11.14 to obtain the patched URL validation logic that prevents path traversal in integration URLs.

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, < 10.11.15>= 11.4.0, < 11.4.5>= 11.5.0, < 11.5.4= 11.6.0

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
Low
User interaction
None
Scope
Changed
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/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
    Run `mmctl version` or look at the Mattermost about page (System Console > About) to find the exact version number installed.
    Affected if The installed version falls within any of these ranges: 10.11.0 to 10.11.14, 11.4.0 to 11.4.4, 11.5.0 to 11.5.3, or exactly 11.6.0.
  2. Verify integrations are enabled
    Navigate to System Console > Integrations > Integration Management, or check the config.json file for `ServiceSettings.EnableIncomingWebhooks` and `ServiceSettings.EnableOutgoingWebhooks` set to true.
    Affected if Any incoming or outgoing webhooks are enabled, as the vulnerability affects integration action URL handling.
  3. Review custom integration actions
    Check for custom plugin or integration configurations at System Console > Integrations > Bot Accounts, or review any stored integration action URLs in the database (Postgres/MySQL) in tables related to integrations.
    Affected if Custom integration actions or webhooks with external URLs are configured.
  4. Audit recent integration API calls
    Review Mattermost server logs for unusual paths containing `../` sequences in integration URLs, or check audit logs for API calls to `/api/v4/actions/{id}/execute` with suspicious path patterns.
    Affected if Logs show integration URLs containing path traversal sequences (such as `../`) or unexpected API calls made via integration actions.

You are affected if your Mattermost Server version is within the vulnerable ranges AND integrations/webhooks are enabled and configured.

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 10.11.15 / 11.4.5 / 11.5.4 or later
Fixed in 10.11.1511.4.511.5.4
Interim mitigation

Upgrade Mattermost to versions beyond 11.6.0, 11.5.3, 11.4.4, and 10.11.14 to obtain the patched URL validation logic that prevents path traversal in integration URLs.

Recommended fix High confidence

10.11.15 (for 10.11.x); 11.4.5 (for 11.4.x); 11.5.4 (for 11.5.x); 11.6.1 or latest 11.x (for 11.6.0)

  1. 1. Identify the current Mattermost Server version by checking the system console or running 'mattermost version'
  2. 2. For Mattermost 10.11.x (versions 10.11.0 through 10.11.14): Upgrade to version 10.11.15 or later
  3. 3. For Mattermost 11.4.x (versions 11.4.0 through 11.4.4): Upgrade to version 11.4.5 or later
  4. 4. For Mattermost 11.5.x (versions 11.5.0 through 11.5.3): Upgrade to version 11.5.4 or later
  5. 5. For Mattermost 11.6.0: Upgrade to version 11.6.1 or later if available, or upgrade to the latest stable 11.x release
  6. 6. After upgrade, verify the integration settings in the system console to ensure no unauthorized integrations were added
  7. 7. Consider reviewing audit logs for any suspicious integration activity
Caveat Standard Mattermost minor/patch upgrade precautions apply - review release notes for any configuration or feature changes

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

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA4.0 h
22.0 hours of engineering $3,860
Get the upgrade done

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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-2026-4858 in production — separate from our analysis above.

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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