Mattermost fails to use consistent error responses when handling the /mute commandGHSA-5mr9-crcg-8wh2
MEDIUMFix: mattermost/mattermost@5bb5261GHSA-5mr9-crcg-8wh2 is a medium-severity (CVSS 4.3) CWE-203 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-5mr9-crcg-8wh2.
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 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexistent versus private channels. Mattermost Advisory ID: MMSA-2026-00588
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/mattermost/mattermost/server/v8 | all versions | 8.0.0-20260130144323-5bb5261c72fago get github.com/mattermost/mattermost/server/v8@v8.0.0-20260130144323-5bb5261c72fa |
| 🐹Go | github.com/mattermost/mattermost-server | all versions | 5.3.2-0.20260130144323-5bb5261c72fago get github.com/mattermost/mattermost-server@v5.3.2-0.20260130144323-5bb5261c72fa |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 10.11.0-rc1&&< 10.11.11 | 10.11.11go get github.com/mattermost/mattermost-server@v10.11.11 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.2.0-rc1&&< 11.2.3 | 11.2.3go get github.com/mattermost/mattermost-server@v11.2.3 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.3.0-rc1&&< 11.3.1 | 11.3.1go get github.com/mattermost/mattermost-server@v11.3.1 |
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 8.0.0-20260130144323-5bb5261c72fa or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-5mr9-crcg-8wh2 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
Frequently Asked Questions
Is GHSA-5mr9-crcg-8wh2 in your dependencies?
Find it across Go, including transitive dependencies.