Auth bypass in Mattermost
CVE-2026-16049
Mattermost Plugins versions <=11.8 10.20.11 11.5.7.0 _The Mattermost GitLab plugin fails to verify channel permissions when processing API requests with a caller-supplied_ {{post_id}}_, and fails to validate the_ {{web_url}} _parameter against the configured GitLab instance, which allows an authenticated attacker to inject bot-authored messages containing arbitrary URLs into channels they do not have access to via the_ {{createIssue}} _and_ {{attachCommentToIssue}} _API endpoints._ Mattermost Advisory ID: MMSA-2026-00673
Vulnerability class: Broken Access Control
CVSS v3 metric
CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.
Affected products
- Mattermost — versions 0, 11.9.0, 10.11.21
Weakness classification (CWE)
References
- responsibledisclosure@mattermost.com (vendor-advisory)
Frequently asked questions
- What is CVE-2026-16049?
- CVE-2026-16049 is a medium-severity vulnerability in Mattermost, classified under Missing Authorization. CVSS score: 4.3/10. Published 2026-08-17.
- How severe is CVE-2026-16049?
- Medium severity. CVSS v3 base score is 4.3 out of 10.