CVE-2026-4646 is a medium-severity (CVSS 4.3) CWE-1287 vulnerability in github.com/mattermost/mattermost-server. A fix is available for github.com/mattermost/mattermost-server — see the affected versions and patch details below.
Insufficient input validation in GitHub plugin API causes denial of service
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 CVE-2026-4646.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2026-4646 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 384,534 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-plugin-githubReal-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 11.6.x <= 11.6.0, 11.5.x <= 11.5.3, 11.4.x <= 11.4.4, 10.11.x <= 10.11.14 fail to validate user-supplied input in API request handlers which allows an authenticated attacker to crash the plugin process via a crafted HTTP request to the PR details endpoint.. Mattermost Advisory ID: MMSA-2026-00638
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.6.0&&< 11.6.1 | 11.6.1go get github.com/mattermost/mattermost-server@v11.6.1 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.5.0&&< 11.5.4 | 11.5.4go get github.com/mattermost/mattermost-server@v11.5.4 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.4.0&&< 11.4.5 | 11.4.5go get github.com/mattermost/mattermost-server@v11.4.5 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 10.11.0&&< 10.11.15 | 10.11.15go get github.com/mattermost/mattermost-server@v10.11.15 |
| 🐹Go | github.com/mattermost/mattermost-plugin-github | all versions | 1.0.1-0.20260330164815-c2840e980b3cgo get github.com/mattermost/mattermost-plugin-github@v1.0.1-0.20260330164815-c2840e980b3c |
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, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/mattermost/mattermost-server to 11.6.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-4646 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 CVE-2026-4646 in your dependencies?
Find it across Go, including transitive dependencies.