Mattermost allows attackers to manipulate administrator terminals via crafted messages containing ANSI and OSC escape sequencesGHSA-3439-vqgj-2gcf
HIGHGHSA-3439-vqgj-2gcf is a high-severity (CVSS 8) CWE-150 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
- 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 GHSA-3439-vqgj-2gcf.
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/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 11.2.x <= 11.2.2, 10.11.x <= 10.11.10, 11.4.x <= 11.4.0, 11.3.x <= 11.3.1 fail to sanitize user-controlled post content in the mmctl commands terminal output which allows attackers to manipulate administrator terminals via crafted messages containing ANSI and OSC escape sequences that enable screen manipulation, fake prompts, and clipboard hijacking. Mattermost Advisory ID: MMSA-2026-00599.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 11.4.0-rc1&&< 11.4.1 | 11.4.1go get github.com/mattermost/mattermost/server/v8@v11.4.1 |
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 11.3.0-rc1&&< 11.3.2 | 11.3.2go get github.com/mattermost/mattermost/server/v8@v11.3.2 |
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 11.2.0-rc1&&< 11.2.3 | 11.2.3go get github.com/mattermost/mattermost/server/v8@v11.2.3 |
| 🐹Go | github.com/mattermost/mattermost/server/v8 | ≥ 10.11.0-rc1&&< 10.11.11 | 10.11.11go get github.com/mattermost/mattermost/server/v8@v10.11.11 |
| 🐹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/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 11.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-3439-vqgj-2gcf 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-3439-vqgj-2gcf in your dependencies?
Find it across Go, including transitive dependencies.