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Mattermost GitLab Plugin CVE-2026-16049

| EUVDEUVD-2026-60209 MEDIUM
Missing Authorization (CWE-862)
2026-08-17 responsibledisclosure@mattermost.com GHSA-vr3j-2q9w-h26r
Mattermost Gitlab Authentication Bypass
4.3
CVSS 3.1 · Vendor: mattermost
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Severity by source

Vendor (mattermost) PRIMARY
4.3 MEDIUM
AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
vuln.today AI
4.3 MEDIUM

Primary impact is unauthorized message injection (integrity, I:L); no confidentiality loss occurs, distinguishing this from the official C:L/I:N assignment.

3.1 AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
4.0 AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

Primary rating from Vendor (mattermost).

CVSS VectorVendor: mattermost

Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

Lifecycle Timeline

1
Analysis Generated
Aug 17, 2026 - 15:36 vuln.today

DescriptionCVE.org

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

AnalysisAI

Unauthorized message injection in the Mattermost GitLab Plugin (<=11.8) allows authenticated users to post bot-authored messages containing arbitrary URLs into restricted channels by exploiting two missing authorization checks: channel permission verification on the caller-supplied post_id parameter, and GitLab instance validation of the web_url parameter. Exploitation targets the createIssue and attachCommentToIssue API endpoints without requiring elevated privileges. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

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Vulnerability AssessmentAI

Exploitation The Mattermost GitLab Plugin must be installed and active on the Mattermost instance. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment With a CVSS base score of 4.3 (Medium) and vector AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N, this vulnerability sits at the lower end of the risk spectrum in absolute terms. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in.
Exploit Scenario Full exploit scenario with step-by-step reproduction available after sign-in.
Remediation Upgrade to a patched version of the Mattermost GitLab Plugin and corresponding Mattermost platform release as detailed in the vendor security advisory at https://mattermost.com/security-updates (MMSA-2026-00673). … Detailed patch versions, workarounds, and compensating controls in full report.
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CVE-2026-16049 vulnerability details – vuln.today
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