dbcve.org
Mattermost ServerApplication · Mattermost

CVE-2026-5755

MEDIUM · 6.5 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 →
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 11.6.x <= 11.6.0, 11.5.x <= 11.5.2, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate the TIFF IFD offset in the image header before allocating memory, which allows authenticated users with file upload or posting permissions to cause a denial of service (server OOM) via uploading a crafted TIFF file or posting a URL that serves one.. Mattermost Advisory ID: MMSA-2026-00648

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 · moderate confidence

Mattermost fails to validate the TIFF Image File Directory (IFD) offset in the image header before allocating memory, allowing authenticated users with file upload or posting permissions to trigger excessive memory allocation by uploading a crafted TIFF file or posting a URL serving one, resulting in server out-of-memory (OOM) condition and denial of service.

MitigationRestrict or disable TIFF file uploads until a patch is applied. Implement file type restrictions and consider memory/CPU limits on file processing operations.

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, < 11.5.3>= 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
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. Check Mattermost Server version
    Run 'grep -i version /opt/mattermost/config/config.json' or check the system package manager (e.g., 'mattermost --version' or 'rpm -q mattermost-server')
    Affected if The installed version falls within any of these ranges: >=10.11.0 and <10.11.15, >=11.4.0 and <11.4.5, >=11.5.0 and <11.5.3, or >=11.6.0 and <11.6.1
  2. Verify file upload capability is enabled
    In Mattermost System Console, go to Posts > File Attachments, or inspect the config file at /opt/mattermost/config/config.json for 'EnableFileAttachments' set to true
    Affected if File attachments are enabled for users who can post or upload files
  3. Check for users with file upload permissions
    In Mattermost System Console, go to User Management > Users and review roles. Users with 'Team User', 'Channel User', or custom roles allowing file uploads can exploit this. Alternatively, query the database: SELECT username, roles FROM Users WHERE deleteat=0;
    Affected if Any authenticated user (excluding deactivated accounts) has permissions to upload or post files containing attachments
  4. Identify uploaded TIFF files on the server
    Search the data directory (typically /opt/mattermost/data/) for .tiff or .tif files using 'find /opt/mattermost/data -type f \( -name "*.tiff" -o -name "*.tif" \)'
    Affected if TIFF files exist in the system, indicating potential exploitation vectors (though the vulnerability is triggered at upload time, not by existing files)

A user is affected if their Mattermost Server version is within the affected ranges AND file upload/posting permissions are enabled for any authenticated user.

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 / 11.5.3 or later
Fixed in 10.11.1511.4.511.5.3
Interim mitigation

Restrict or disable TIFF file uploads until a patch is applied. Implement file type restrictions and consider memory/CPU limits on file processing operations.

Recommended fix High confidence

10.11.15, 11.4.5, 11.5.3, or 11.6.1 depending on your current major version branch

  1. 1. Identify the currently installed Mattermost Server version using the system console or CLI
  2. 2. Based on your current version, plan upgrade to the corresponding fixed release: 10.11.15, 11.4.5, 11.5.3, or 11.6.1
  3. 3. Back up your Mattermost database and configuration files before upgrading
  4. 4. Download the appropriate Mattermost Server fixed release from the official mattermost.com downloads page
  5. 5. Follow the standard Mattermost upgrade procedure for your deployment method (docker, omnibus, or manual)
  6. 6. After upgrade, verify the server starts successfully and test file upload functionality
Caveat Standard Mattermost upgrade considerations apply - review release notes for your version branch 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
  • Consultation2.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
11.0 hours of engineering $1,930
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2026-5755 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.

Get it on GitHub Scan in the browser

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-2026-5755 in production — separate from our analysis above.

No notes yet

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

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