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Mattermost CVE-2026-16047

| EUVDEUVD-2026-60208 MEDIUM
Missing Authorization (CWE-862)
2026-08-17 responsibledisclosure@mattermost.com GHSA-7xj3-8pxr-p8pc
Mattermost Authentication Bypass Mattermost Server
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

PR:L reflects mandatory team-member authentication; C:L captures membership-only disclosure with no message content or integrity/availability impact.

3.1 AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
4.0 AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/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 - 15:35 vuln.today

DescriptionCVE.org

Mattermost versions 11.7.x <= 11.7.6, 10.11.x <= 10.11.21, 11.8.x <= 11.8.3 fail to validate that users have read access to a channel before linking a board to it, which allows an authenticated attacker to discover the membership of private channels on the same team via creating, patching, importing, or bulk-creating boards with an arbitrary channelId. Mattermost Advisory ID: MMSA-2026-00674

AnalysisAI

Private channel membership disclosure in Mattermost's Boards feature allows any authenticated team member to enumerate who belongs to private channels they are not authorized to view. Affected across three concurrently maintained release trains - 11.7.x through 11.7.6, 10.11.x through 10.11.21, and 11.8.x through 11.8.3 - the flaw is exploitable by any valid team account via standard board management API calls. …

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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata

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Vulnerability AssessmentAI

Exploitation Exploitation requires an authenticated Mattermost account with at minimum standard team membership, as confirmed by the CVSS PR:L metric. … Additional conditions and limiting factors are described in the full assessment.
Risk Assessment The CVSS 4.3 vector (AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N) is internally consistent with the vulnerability's behavior: network exploitable with no complexity, requiring only a low-privilege authenticated account, with impact limited to low confidentiality. … 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 Mattermost release per the vendor's security advisory at https://mattermost.com/security-updates; the available data specifies affected ranges (11.7.x ≤ 11.7.6, 10.11.x ≤ 10.11.21, 11.8.x ≤ 11.8.3) but does not name explicit fixed versions - confirm the precise target version directly from the advisory before deploying. … Detailed patch versions, workarounds, and compensating controls in full report.
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CVE-2026-16047 vulnerability details – vuln.today
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