Mattermost doesn't validate user channel membership when attaching Mattermost posts as comments to Jira issuesGHSA-fmqf-pmcm-8cx9
MEDIUMFix: mattermost/mattermost#34551GHSA-fmqf-pmcm-8cx9 is a medium-severity (CVSS 4.3) CWE-863 vulnerability in github.com/mattermost/mattermost/server/v8. A fix is available for github.com/mattermost/mattermost/server/v8 — see the affected versions and patch details below.
Exploitation Status
No confirmed exploitation observed yet
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-fmqf-pmcm-8cx9.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-serverReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Mattermost versions 11.1.x <= 11.1.0, 11.0.x <= 11.0.5, 10.12.x <= 10.12.3, 10.11.x <= 10.11.7 fails to validate user channel membership when attaching Mattermost posts as comments to Jira issues, which allows an authenticated attacker with access to the Jira plugin to read post content and attachments from channels they do not have access to.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/mattermost/mattermost/server/v8 | all versions | 8.0.0-20251121122154-b57c297c6d7go get github.com/mattermost/mattermost/server/v8@v8.0.0-20251121122154-b57c297c6d7 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 10.11.0&&< 10.11.8 | 10.11.8go get github.com/mattermost/mattermost-server@v10.11.8 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 10.12.0&&< 10.12.4 | 10.12.4go get github.com/mattermost/mattermost-server@v10.12.4 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.0.0&&< 11.0.6 | 11.0.6go get github.com/mattermost/mattermost-server@v11.0.6 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.1.0&&< 11.1.1 | 11.1.1go get github.com/mattermost/mattermost-server@v11.1.1 |
Affected Products
mattermost servermattermostDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/mattermost/mattermost/server/v8, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/mattermost/mattermost/server/v8 to 8.0.0-20251121122154-b57c297c6d7 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fmqf-pmcm-8cx9 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.
Frequently Asked Questions
Is GHSA-fmqf-pmcm-8cx9 in your dependencies?
Find it across Go, including transitive dependencies.