Incorrect authorization in Mattermost
Priority LOW 19CWE-863Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- Low
- User interaction
- None
- Attack complexity
- Low
- Impact type
- Security bypass
- Confidentiality · integrity · availability
- None · Low · None
- Fix status
- Fixed
Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub. Impact type from CWE-863.
Summary
Mattermost versions 11.5.x <= 11.5.1, 10.11.x <= 10.11.13, 11.4.x <= 11.4.3 fail to validate that a remote cluster has access to a channel before processing membership removal requests during shared channel membership sync, which allows a malicious remote cluster to remove any user from any channel, including private channels, via crafted membership sync messages targeting channels the remote cluster is not authorized to access. Mattermost Advisory ID: MMSA-2026-00576
Why it matters now
Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00152 4th percentileScored 2026-11-07
Fix Fixed Mattermost (Mattermost): 11.6.0, 11.5.2, 10.11.14 +1github.com/mattermost/mattermost/server/v8 (Go): 11.5.2, 10.11.14, 11.4.4 +1+1 more product
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- No
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| Mattermost | Mattermost | 11.5.0 – 11.5.1; 10.11.0 – 10.11.13; 11.4.0 – 11.4.3 | 11.6.0, 11.5.2, 10.11.14, 11.4.4 | Patch available | CNA |
| — | github.com/mattermost/mattermost/server/v8 (Go) | 11.5.0 to before 11.5.2 | 11.5.2 | Patch available | GHSA-8h9w-w78c-vvr3 |
| — | github.com/mattermost/mattermost/server/v8 (Go) | 10.11.0 to before 10.11.14 | 10.11.14 | Patch available | GHSA-8h9w-w78c-vvr3 |
| — | github.com/mattermost/mattermost/server/v8 (Go) | 11.4.0 to before 11.4.4 | 11.4.4 | Patch available | GHSA-8h9w-w78c-vvr3 |
| — | github.com/mattermost/mattermost/server/v8 (Go) | before 8.0.0-20260216150504-8738f8c4b3d4 | 8.0.0-20260216150504-8738f8c4b3d4 | Patch available | GHSA-8h9w-w78c-vvr3 |
| — | github.com/mattermost/mattermost-server (Go) | before 5.3.2-0.20260216150504-8738f8c4b3d4 | 5.3.2-0.20260216150504-8738f8c4b3d4 | Patch available | GHSA-8h9w-w78c-vvr3 |
- Mattermost VendorMattermost Affected 11.5.0 – 11.5.1; 10.11.0 – 10.11.13; 11.4.0 – 11.4.3 Fixed in 11.6.0, 11.5.2, 10.11.14, 11.4.4 Patch available CNA
- github.com/mattermost/mattermost/server/v8 (Go) Vendor— Affected 11.5.0 to before 11.5.2 Fixed in 11.5.2 Patch available GHSA-8h9w-w78c-vvr3
- github.com/mattermost/mattermost/server/v8 (Go) Vendor— Affected 10.11.0 to before 10.11.14 Fixed in 10.11.14 Patch available GHSA-8h9w-w78c-vvr3
- github.com/mattermost/mattermost/server/v8 (Go) Vendor— Affected 11.4.0 to before 11.4.4 Fixed in 11.4.4 Patch available GHSA-8h9w-w78c-vvr3
- github.com/mattermost/mattermost/server/v8 (Go) Vendor— Affected before 8.0.0-20260216150504-8738f8c4b3d4 Fixed in 8.0.0-20260216150504-8738f8c4b3d4 Patch available GHSA-8h9w-w78c-vvr3
- github.com/mattermost/mattermost-server (Go) Vendor— Affected before 5.3.2-0.20260216150504-8738f8c4b3d4 Fixed in 5.3.2-0.20260216150504-8738f8c4b3d4 Patch available GHSA-8h9w-w78c-vvr3
CPE configurations NVD · 1
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* | 10.11.0 to before 10.11.14; 11.4.0 to before 11.4.4; 11.5.0 to before 11.5.2 | Vulnerable | Mattermost |
- cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* Versions 10.11.0 to before 10.11.14; 11.4.0 to before 11.4.4; 11.5.0 to before 11.5.2 Vulnerable ·Mattermost
Remediation
- Fix · CNA Mattermost (Mattermost) Fixed in 11.6.0, 11.5.2, 10.11.14, 11.4.4
- Fix · GHSA-8h9w-w78c-vvr3 github.com/mattermost/mattermost/server/v8 (Go) Fixed in 11.5.2 Advisory →
- Fix · GHSA-8h9w-w78c-vvr3 github.com/mattermost/mattermost/server/v8 (Go) Fixed in 10.11.14 Advisory →
- Fix · GHSA-8h9w-w78c-vvr3 github.com/mattermost/mattermost/server/v8 (Go) Fixed in 11.4.4 Advisory →
- Fix · GHSA-8h9w-w78c-vvr3 github.com/mattermost/mattermost/server/v8 (Go) Fixed in 8.0.0-20260216150504-8738f8c4b3d4 Advisory →
- Fix · GHSA-8h9w-w78c-vvr3 github.com/mattermost/mattermost-server (Go) Fixed in 5.3.2-0.20260216150504-8738f8c4b3d4 Advisory →
Timeline
- 2026-10-02 Threatist record created from CVE-2026-28759
- 2026-10-03 Alias added: GHSA-8h9w-w78c-vvr3
References
More Mattermost Server vulnerabilities
- TI-2025-18933 Arbitrary file read in Mattermost Boards via import & export board archive CVSS 3.1CRITICAL 9.9
- TI-2025-04465 Path Traversal Leading to RCE by Any Authenticated Mattermost User CVSS 3.1CRITICAL 9.9
- TI-2017-14068 An issue was discovered in Mattermost Server before 3.6.2. CVSS 3.1CRITICAL 9.8
- TI-2017-14060 An issue was discovered in Mattermost Server before 3.8.2, 3.7.5, and 3.6.7. CVSS 3.1CRITICAL 9.8
- TI-2017-14048 An issue was discovered in Mattermost Server before 4.1.0, 4.0.4, and 3.10.3. CVSS 3.1CRITICAL 9.8
- TI-2025-14360 Arbitrary file read via block duplication in Mattermost Boards CVSS 3.1CRITICAL 9.9
Sources
- CVE-2026-28759 · CVE Program
- CVE-2026-28759 · NVD
- FIRST EPSS
- GHSA-8h9w-w78c-vvr3 · GitHub Advisory Database · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences