CVE-2026-28759 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.
Insufficient authorization in shared channel membership sync allows remote cluster to remove users from arbitrary channels
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-2026-28759.
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/v8🐹github.com/mattermost/mattermost/server/v8🐹github.com/mattermost/mattermost/server/v8🐹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 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 11.5.0&&< 11.5.2 | 11.5.2go get github.com/mattermost/mattermost/server/v8@v11.5.2 |
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 10.11.0&&< 10.11.14 | 10.11.14go get github.com/mattermost/mattermost/server/v8@v10.11.14 |
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 11.4.0&&< 11.4.4 | 11.4.4go get github.com/mattermost/mattermost/server/v8@v11.4.4 |
| 🐹Go | github.com/mattermost/mattermost/server/v8 | all versions | 8.0.0-20260216150504-8738f8c4b3d4go get github.com/mattermost/mattermost/server/v8@v8.0.0-20260216150504-8738f8c4b3d4 |
| 🐹Go | github.com/mattermost/mattermost-server | all versions | 5.3.2-0.20260216150504-8738f8c4b3d4go get github.com/mattermost/mattermost-server@v5.3.2-0.20260216150504-8738f8c4b3d4 |
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 11.5.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-28759 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 CVE-2026-28759 in your dependencies?
Find it across Go, including transitive dependencies.