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Mattermost ServerApplication · Mattermost

CVE-2026-6961

HIGH · 7.6 CVSS v3.1 Published 2026-06-12
Fix available
A fix is available. Upgrade to 10.11.17 / 11.5.5 or later.
See remediation →
82/100
Remediation priority · High
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 <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, 10.11.x <= 10.11.16 Mattermost fails to sanitize FileInfo.Name received from federated peers during shared channel file sync, which allows an attacker who controls a federated server to write files to arbitrary locations within the target server's filestore via path traversal sequences in the filename field.. Mattermost Advisory ID: MMSA-2026-00661

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

A path traversal vulnerability in Mattermost's federated shared channel file sync allows an attacker controlling a federated server to inject path traversal sequences (e.g., ../../../) into the FileInfo.Name field, enabling arbitrary file writes to locations within the target server's filestore.

MitigationUpgrade Mattermost to versions newer than 11.6.1, 11.5.4, 10.11.15, and 10.11.16 as specified in MMSA-2026-00661. Until patched, consider disabling federated shared channels as a temporary workaround.

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, < 10.11.17>= 11.5.0, < 11.5.5>= 11.6.0, < 11.6.2

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

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:N/I:H/A:L

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
    Retrieve the installed Mattermost Server version from the system (typically via 'mattermost version' command or the system admin console). Compare it to the affected ranges: 10.11.0 through 10.11.16, 11.5.0 through 11.5.4, and 11.6.0 through 11.6.1.
    Affected if The installed version falls within any of the following ranges: >=10.11.0 and <10.11.17, >=11.5.0 and <11.5.5, or >=11.6.0 and <11.6.2.
  2. Verify federated shared channels status
    In the Mattermost admin console, navigate to the shared channels or federation settings panel and confirm whether the federated shared channels feature is enabled for any team or channel.
    Affected if Federated shared channels are currently enabled and the server is connected to one or more federated instances.
  3. Inspect recent file writes in filestore
    Examine the Mattermost filestore (local disk or object storage bucket) for any files written outside the expected /mattermost/ directory structure, particularly files with path traversal patterns (containing ../ sequences) in their paths.
    Affected if Unexpected files exist outside the standard filestore directory hierarchy or files contain traversal sequences in their names.
  4. Review federation-related audit logs
    Search the Mattermost audit logs for entries related to file transfers from federated sources, looking for anomalies such as unusual file paths or unexpected file creation events.
    Affected if Log entries show file creation requests from federated sources with atypical file path values.

A user is affected if their Mattermost Server version is one of the affected versions listed AND federated shared channels are enabled and connected to federated servers.

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 10.11.17 / 11.5.5 / 11.6.2 or later
Fixed in 10.11.1711.5.511.6.2
Interim mitigation

Upgrade Mattermost to versions newer than 11.6.1, 11.5.4, 10.11.15, and 10.11.16 as specified in MMSA-2026-00661. Until patched, consider disabling federated shared channels as a temporary workaround.

Recommended fix High confidence

10.11.17 (for 10.11.x branch), 11.5.5 (for 11.5.x branch), or 11.6.2 (for 11.6.x branch)

  1. Identify your current Mattermost Server version by checking the system console or running 'mattermost version'
  2. Determine which version branch you are on (10.11.x, 11.5.x, or 11.6.x)
  3. Create a complete backup of your Mattermost database and filestore before upgrading
  4. Stop the Mattermost Server service
  5. Download the appropriate fixed version for your branch: 10.11.17, 11.5.5, or 11.6.2
  6. Install the new Mattermost Server version using your existing deployment method (e.g., Docker, mattermost binary, package manager)
  7. Start the Mattermost Server service
  8. Verify the upgrade was successful by checking the server version in the system console

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
  • Testing4.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,080
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,328.

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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-2026-6961 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