dbcve.org
Mattermost ServerApplication · Mattermost

CVE-2026-6673

MEDIUM · 6.4 CVSS v3.1 Published 2026-06-22
Fix available
A fix is available. Upgrade to 10.11.18 / 11.5.6 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable

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 11.7.x <= 11.7.0, 11.6.x <= 11.6.2, 11.5.x <= 11.5.5, 10.11.x <= 10.11.17 fail to authenticate Atlassian Connect installed callbacks, allowing a remote unauthenticated attacker to inject a rogue sharedSecret and disrupt the Jira integration via POST to /ac/installed during the pending-install window.. Mattermost Advisory ID: MMSA-2026-00654

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

This is an authentication bypass vulnerability in Mattermost's Atlassian Connect integration. The /ac/installed callback endpoint fails to properly authenticate requests during the pending-install window, allowing unauthenticated remote attackers to inject a rogue sharedSecret by sending POST requests to /ac/installed, thereby compromising the Jira integration.

MitigationUpdate Mattermost to versions 11.7.1, 11.6.3, 11.5.6, 10.11.18 or later to patch the authentication bypass in the Atlassian Connect integration.

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, < 10.11.18>= 11.5.0, < 11.5.6>= 11.6.0, < 11.6.3>= 11.7.0, < 11.7.1

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
Required
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
High

CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:N/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. Check Mattermost Server version
    Run 'mattermost version' or inspect the server's version information through the system package manager or about page in the admin console
    Affected if The installed version falls within any of these ranges: 10.11.0 through 10.11.17.x, 11.5.0 through 11.5.5.x, 11.6.0 through 11.6.2.x, or 11.7.0 through 11.7.0.x
  2. Verify Atlassian Connect plugin status
    Navigate to the Mattermost plugin management section in the system console or use the CLI tool to list installed plugins, and confirm whether the Atlassian Connect / Jira integration plugin is present and enabled
    Affected if The Atlassian Connect plugin is installed and enabled on an affected Mattermost version
  3. Inspect Atlassian Connect configuration
    Access the plugin configuration settings for Atlassian Connect in the Mattermost system console or inspect the plugin's configuration file in the plugins directory
    Affected if Configuration for the /ac/installed callback endpoint exists and the plugin has been in pending-install state at any point
  4. Review sharedSecret values
    Examine the Atlassian Connect plugin configuration for any sharedSecret entries, comparing them against expected legitimate values documented in your Atlassian Cloud instance
    Affected if A sharedSecret value is present that does not match the legitimate secret generated by your Atlassian Cloud organization

You are affected if your Mattermost Server version is within the affected ranges AND the Atlassian Connect / Jira integration plugin is or was ever enabled.

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 10.11.18 / 11.5.6 / 11.6.3 or later
Fixed in 10.11.1811.5.611.6.3
Interim mitigation

Update Mattermost to versions 11.7.1, 11.6.3, 11.5.6, 10.11.18 or later to patch the authentication bypass in the Atlassian Connect integration.

Recommended fix High confidence

Upgrade to the latest patch version in your current release branch: 10.11.18+, 11.5.6+, 11.6.3+, or 11.7.1+

  1. 1. Identify the currently running Mattermost Server version by checking the system console or running `mattermost version`
  2. 2. Determine which version branch your current installation falls into (10.11.x, 11.5.x, 11.6.x, or 11.7.x)
  3. 3. For 10.11.x branches: upgrade to version 10.11.18 or later
  4. 4. For 11.5.x branches: upgrade to version 11.5.6 or later
  5. 5. For 11.6.x branches: upgrade to version 11.6.3 or later
  6. 6. For 11.7.x branches: upgrade to version 11.7.1 or later
  7. 7. Follow Mattermost standard upgrade procedures: back up the database and config files, stop the server, install the new version, restart the server
  8. 8. After upgrade, verify the Atlassian Connect integration is functioning correctly and monitor for any unauthorized changes to the sharedSecret
Caveat Standard Mattermost minor/patch upgrades typically do not introduce breaking changes; however, review release notes before upgrading

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
  • Implementation3.0 h
  • Testing2.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,420
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,272.

Scan for this in your stack

Free · runs locally
dbcve dependency scanner

Check whether your project pulls in CVE-2026-6673 — 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-2026-6673 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