CVE-2026-27656 — mattermost-server
MEDIUMCVE-2026-27656 is a medium-severity (CVSS 5.7) CWE-303 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.
Account Takeover via Substring Matching in OpenID Connect Authentication
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- 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-2026-27656.
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🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost-server🐹github.com/mattermost/mattermost/server/v8Real-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 11.4.x <= 11.4.0, 11.3.x <= 11.3.1, 11.2.x <= 11.2.3, 10.11.x <= 10.11.11 fail to properly validate user identity in the OpenID {{IsSameUser()}} comparison logic, which allows an attacker to take over arbitrary user accounts via an overly permissive substring matching flaw in the user discovery flow.. Mattermost Advisory ID: MMSA-2026-00590
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.4.0-rc1&&< 11.4.1 | 11.4.1go get github.com/mattermost/mattermost-server@v11.4.1 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.3.0-rc1&&< 11.3.2 | 11.3.2go get github.com/mattermost/mattermost-server@v11.3.2 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 11.2.0-rc1&&< 11.2.4 | 11.2.4go get github.com/mattermost/mattermost-server@v11.2.4 |
| 🐹Go | github.com/mattermost/mattermost-server | ≥ 10.11.0-rc1&&< 10.11.12 | 10.11.12go get github.com/mattermost/mattermost-server@v10.11.12 |
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 8.0.0-20260105080200-d27a2195068d&&< 8.0.0-20260217110922-b7d4a1f1f59b | 8.0.0-20260217110922-b7d4a1f1f59bgo get github.com/mattermost/mattermost/server/v8@v8.0.0-20260217110922-b7d4a1f1f59b |
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 11.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27656 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-2026-27656 in your dependencies?
Find it across Go, including transitive dependencies.