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Mattermost ServerApplication · Mattermost

CVE-2026-26233

MEDIUM · 6.5 CVSS v3.1 Published 2026-03-25
Fix available
A fix is available. Upgrade to 10.11.12 / 11.2.4 or later.
See remediation →
71/100
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 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to rate limit login requests which allows unauthenticated remote attackers to cause denial of service (server crash and restart) via HTTP/2 single packet attack with 100+ parallel login requests.. Mattermost Advisory ID: MMSA-2025-00566

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 implement rate limiting on login endpoints, allowing unauthenticated remote attackers to send 100+ parallel login requests via HTTP/2 single packet attack. This causes the server to crash and restart, resulting in denial of service.

MitigationImplement rate limiting on authentication endpoints (e.g., fail2ban, web application firewall, or Mattermost configuration) to restrict login attempts per IP, or upgrade to patched versions when available.

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, < 10.11.12>= 11.2.0, < 11.2.4>= 11.3.0, < 11.3.2>= 11.4.0, < 11.4.1

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

CVSS:3.1/AV:N/AC:L/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. Determine installed Mattermost Server version
    Run 'mattermost version' or check the system package manager for the installed Mattermost version, or look in the Mattermost configuration file for the version field
    Affected if The installed version falls within any of these ranges: 10.11.0 to 10.11.11, 11.2.0 to 11.2.3, 11.3.0 to 11.3.1, or 11.4.0 to 11.4.0 (meaning version is >=10.11.0 and <10.11.12, or >=11.2.0 and <11.2.4, or >=11.3.0 and <11.3.2, or >=11.4.0 and <11.4.1)
  2. Locate Mattermost configuration file
    Find the config.json or config.d directory in the Mattermost installation directory, typically under /opt/mattermost/config/ or /etc/mattermost/
    Affected if The configuration file exists and can be inspected for rate limiting settings
  3. Check rate limiting configuration for authentication endpoints
    Examine the Mattermost configuration file for rate limit settings, specifically look for 'RateLimitSettings' or similar sections that control login request rate limiting
    Affected if Rate limiting is disabled, set to false, set to 0, or not configured at all for login/authentication endpoints (the 'Enable' or 'PerIP' settings are missing or set to allow unlimited requests)
  4. Verify rate limiting is not externally implemented
    Check if fail2ban, a web application firewall, or other external rate limiting is configured specifically for Mattermost login endpoints
    Affected if No external rate limiting mechanism (fail2ban, WAF, iptables rules) is configured to limit login attempts to the Mattermost server

You are affected if your Mattermost Server version is within the affected ranges AND rate limiting is not enabled on login endpoints, either through Mattermost configuration or an external mechanism.

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 10.11.12 / 11.2.4 / 11.3.2 or later
Fixed in 10.11.1211.2.411.3.2
Interim mitigation

Implement rate limiting on authentication endpoints (e.g., fail2ban, web application firewall, or Mattermost configuration) to restrict login attempts per IP, or upgrade to patched versions when available.

Recommended fix High confidence

Mattermost Server 10.11.12, 11.2.4, 11.3.2, or 11.4.1 (choose the version appropriate for your release branch)

  1. Identify your current Mattermost Server version by checking the system console or running 'mattermost version'
  2. Backup your Mattermost database and configuration files before upgrading
  3. Upgrade to Mattermost Server version 10.11.12 or later if running 10.11.x
  4. Upgrade to version 11.2.4 or later if running 11.2.x
  5. Upgrade to version 11.3.2 or later if running 11.3.x
  6. Upgrade to version 11.4.1 or later if running 11.4.x
  7. After upgrade, verify the server is running and accessible
  8. Confirm rate limiting is functioning by testing login endpoint throttling
Caveat Standard upgrade considerations apply - review release notes for database migrations and any deprecated features

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
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
15.0 hours of engineering $2,640
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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-2026-26233 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