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Mattermost ServerApplication · Mattermost

CVE-2026-4635

MEDIUM · 5.3 CVSS v3.1 Published 2026-05-22
Fix available
A fix is available. Upgrade to 10.11.15 / 11.4.5 or later.
See remediation →
59/100
Remediation priority · Elevated
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 archive the channel before removing persistent notifications which allows authenticated user to crash the server via timing the creation of persistent notification message between the server deleting existing persistent notifications and archiving the channel.. Mattermost Advisory ID: MMSA-2026-00637

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 archive channels before removing persistent notifications, creating a race condition where an authenticated user can trigger a server crash by creating a persistent notification message between the server deleting existing persistent notifications and archiving the channel. This is a denial-of-service vulnerability exploiting improper synchronization between channel archival and notification cleanup logic.

MitigationUpgrade to Mattermost versions beyond 11.6.0, 11.5.3, 11.4.4, and 10.11.14 once patches are released. Implement proper channel archival before notification removal to eliminate the race condition.

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, < 11.6.1

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
High
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS:3.1/AV:N/AC:H/PR:L/UI:N/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 installed Mattermost Server version
    Run 'mattermost version' from the command line, or log into the System Console and navigate to About > Mattermost to view the exact version number
    Affected if Version is 10.11.0 through 10.11.14, 11.4.0 through 11.4.4, 11.5.0 through 11.5.3, or 11.6.0 (versions prior to the patched releases 10.11.15, 11.4.5, 11.5.4, or 11.6.1)
  2. Confirm persistent notifications are enabled
    In the System Console, go to Notifications > Persistent Notifications and verify whether the feature is turned on, or check the config.json setting for EnablePersistentNotifications
    Affected if Persistent notifications feature is enabled for the server or for specific users/channels

You are affected if your Mattermost Server version falls within any of the vulnerable ranges AND persistent notifications are enabled, since the race condition requires both conditions to be present.

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 to Mattermost versions beyond 11.6.0, 11.5.3, 11.4.4, and 10.11.14 once patches are released. Implement proper channel archival before notification removal to eliminate the race condition.

Recommended fix High confidence

Upgrade to 10.11.15+ (if on 10.11.x), 11.4.5+ (if on 11.4.x), 11.5.4+ (if on 11.5.x), or 11.6.1+ (if on 11.6.x)

  1. 1. Identify the currently installed Mattermost Server version using the system console or CLI
  2. 2. Determine which version branch you are on (10.11.x, 11.4.x, 11.5.x, or 11.6.x)
  3. 3. If on 10.11.x branch, upgrade to version 10.11.15 or later
  4. 4. If on 11.4.x branch, upgrade to version 11.4.5 or later
  5. 5. If on 11.5.x branch, upgrade to version 11.5.4 or later
  6. 6. If on 11.6.x branch, upgrade to version 11.6.1 or later
  7. 7. Follow standard Mattermost upgrade procedures including backing up the database and configuration files
  8. 8. Test the upgrade in a staging environment before deploying to production
Caveat Review Mattermost release notes for any breaking changes between your current version and the target fixed version

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

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation3.0 h
  • Implementation12.0 h
  • Testing6.0 h
  • Review / QA3.0 h
24.0 hours of engineering $4,200
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $6,720.

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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-2026-4635 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