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Mattermost ServerApplication · Mattermost

CVE-2026-3108

HIGH · 8.8 CVSS v3.1 Published 2026-03-26
Fix available
A fix is available. Upgrade to 10.11.11 / 11.2.3 or later.
See remediation →
94/100
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 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to sanitize user-controlled post content in the mmctl commands terminal output which allows attackers to manipulate administrator terminals via crafted messages containing ANSI and OSC escape sequences that enable screen manipulation, fake prompts, and clipboard hijacking.. Mattermost Advisory ID: MMSA-2026-00599

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 sanitize ANSI and OSC escape sequences from user-controlled post content when displayed through the mmctl command-line interface. Attackers who can post messages can inject terminal escape codes to manipulate screen output, create fake prompts, and hijack clipboard operations.

MitigationUpdate Mattermost to a patched version beyond the affected releases. Until patched, avoid using mmctl in shared terminal sessions or consider piping output through a sanitization filter.

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, < 10.11.11>= 11.2.0, < 11.2.3>= 11.3.0, < 11.3.2= 11.4.0

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
Unchanged
Confidentiality
High
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/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. Identify Mattermost Server installation and version
    Run 'mattermost version' or check the Mattermost installation directory for version information. On Linux, this may be in /opt/mattermost or via a package manager query like 'dpkg -l mattermost-server'.
    Affected if The installed version falls within >= 10.11.0, < 10.11.11; >= 11.2.0, < 11.2.3; >= 11.3.0, < 11.3.2; or = 11.4.0
  2. Locate mmctl command-line tool
    Check if mmctl is installed on the system. Common locations include /opt/mattermost/bin/mmctl, or run 'which mmctl' to find it in PATH. This tool is bundled with Mattermost Server installations.
    Affected if mmctl is present on the system and accessible to administrators who manage the Mattermost instance
  3. Determine if mmctl is used to view post content
    Review administrator workflows and logs to see if mmctl is used to retrieve, list, or display channel messages or posts. Check commands such as 'mmctl post list', 'mmctl channel messages', or similar post retrieval operations.
    Affected if Administrators use mmctl to view user-generated post content from channels
  4. Check for terminal output rendering of post content
    Observe whether mmctl output containing post messages is rendered in a terminal emulator. The vulnerability exploits terminal rendering of ANSI/OSC escape sequences.
    Affected if mmctl is used to display post content in a terminal window or console

You are affected if your Mattermost Server version is in the vulnerable ranges AND administrators use mmctl to view post content in a terminal, allowing malicious escape sequences from user messages to manipulate screen output or hijack clipboard contents.

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 10.11.11 / 11.2.3 / 11.3.2 or later
Fixed in 10.11.1111.2.311.3.2
Interim mitigation

Update Mattermost to a patched version beyond the affected releases. Until patched, avoid using mmctl in shared terminal sessions or consider piping output through a sanitization filter.

Recommended fix High confidence

Mattermost Server 10.11.11 / 11.2.3 / 11.3.2 / 11.4.1 (depending on your current branch)

  1. Identify the currently running Mattermost Server version using system administration tools or the Mattermost system console
  2. Determine which version branch your current installation belongs to (10.11.x, 11.2.x, 11.3.x, or 11.4.x)
  3. For 10.11.x branches: upgrade to Mattermost Server version 10.11.11 or later
  4. For 11.2.x branches: upgrade to Mattermost Server version 11.2.3 or later
  5. For 11.3.x branches: upgrade to Mattermost Server version 11.3.2 or later
  6. For 11.4.0: upgrade to Mattermost Server version 11.4.1 or later
  7. After upgrading, verify the mmctl terminal output properly sanitizes user-controlled post content by testing with messages containing ANSI escape sequences
Caveat Review Mattermost release notes for any breaking changes between your current version and the target fixed version before upgrading

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

Fix this in Mattermost Server Scoped from the published advisory
  • Consultation1.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
6.0 hours of engineering $1,040
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,664.

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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-2026-3108 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