Private channel enumeration via /mute slash command
Priority LOW 19CWE-203Published · Updated
Key facts
- Attack vector
- Network
- Privileges required
- Low
- User interaction
- None
- Attack complexity
- Low
- Impact type
- Information disclosure
- Confidentiality · integrity · availability
- Low · None · None
- Fix status
- Fixed
Attack profile from CVSS 3.1 · CNA. Fix status from CNA and GitHub. Impact type from CWE-203.
Summary
Mattermost versions 11.3.x <= 11.3.0, 11.2.x <= 11.2.2, 10.11.x <= 10.11.10 fail to use consistent error responses when handling the /mute command which allows an authenticated team member to enumerate private channels they are not authorized to know about via differing error messages for nonexistent versus private channels. Mattermost Advisory ID: MMSA-2026-00588
Why it matters now
Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00191 8th percentileScored 2026-11-07
Fix Fixed Mattermost (Mattermost): 11.4.0, 11.3.1, 11.2.3 +1github.com/mattermost/mattermost/server/v8 (Go): 8.0.0-20260130144323-5bb5261c72fa+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 | 11.3.0 – 11.3.0; 11.2.0 – 11.2.2; 10.11.0 – 10.11.10 | 11.4.0, 11.3.1, 11.2.3, 10.11.11 | Patch available | CNA |
| — | github.com/mattermost/mattermost/server/v8 (Go) | before 8.0.0-20260130144323-5bb5261c72fa | 8.0.0-20260130144323-5bb5261c72fa | Patch available | GHSA-5mr9-crcg-8wh2 |
| — | github.com/mattermost/mattermost-server (Go) | before 5.3.2-0.20260130144323-5bb5261c72fa | 5.3.2-0.20260130144323-5bb5261c72fa | Patch available | GHSA-5mr9-crcg-8wh2 |
| — | github.com/mattermost/mattermost-server (Go) | 10.11.0-rc1 to before 10.11.11 | 10.11.11 | Patch available | GHSA-5mr9-crcg-8wh2 |
| — | github.com/mattermost/mattermost-server (Go) | 11.2.0-rc1 to before 11.2.3 | 11.2.3 | Patch available | GHSA-5mr9-crcg-8wh2 |
| — | github.com/mattermost/mattermost-server (Go) | 11.3.0-rc1 to before 11.3.1 | 11.3.1 | Patch available | GHSA-5mr9-crcg-8wh2 |
- Mattermost VendorMattermost Affected 11.3.0 – 11.3.0; 11.2.0 – 11.2.2; 10.11.0 – 10.11.10 Fixed in 11.4.0, 11.3.1, 11.2.3, 10.11.11 Patch available CNA
- github.com/mattermost/mattermost/server/v8 (Go) Vendor— Affected before 8.0.0-20260130144323-5bb5261c72fa Fixed in 8.0.0-20260130144323-5bb5261c72fa Patch available GHSA-5mr9-crcg-8wh2
- github.com/mattermost/mattermost-server (Go) Vendor— Affected before 5.3.2-0.20260130144323-5bb5261c72fa Fixed in 5.3.2-0.20260130144323-5bb5261c72fa Patch available GHSA-5mr9-crcg-8wh2
- github.com/mattermost/mattermost-server (Go) Vendor— Affected 10.11.0-rc1 to before 10.11.11 Fixed in 10.11.11 Patch available GHSA-5mr9-crcg-8wh2
- github.com/mattermost/mattermost-server (Go) Vendor— Affected 11.2.0-rc1 to before 11.2.3 Fixed in 11.2.3 Patch available GHSA-5mr9-crcg-8wh2
- github.com/mattermost/mattermost-server (Go) Vendor— Affected 11.3.0-rc1 to before 11.3.1 Fixed in 11.3.1 Patch available GHSA-5mr9-crcg-8wh2
CPE configurations NVD · 1
| CPE | Versions | Role | Vendor |
|---|---|---|---|
| cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* | 10.11.0 to before 10.11.11; 11.2.0 to before 11.2.3; 11.3.0 to before 11.3.1 | Vulnerable | Mattermost |
- cpe:2.3:a:mattermost:mattermost_server:*:*:*:*:*:*:*:* Versions 10.11.0 to before 10.11.11; 11.2.0 to before 11.2.3; 11.3.0 to before 11.3.1 Vulnerable ·Mattermost
Remediation
- Fix · CNA Mattermost (Mattermost) Fixed in 11.4.0, 11.3.1, 11.2.3, 10.11.11
- Fix · GHSA-5mr9-crcg-8wh2 github.com/mattermost/mattermost/server/v8 (Go) Fixed in 8.0.0-20260130144323-5bb5261c72fa Advisory →
- Fix · GHSA-5mr9-crcg-8wh2 github.com/mattermost/mattermost-server (Go) Fixed in 5.3.2-0.20260130144323-5bb5261c72fa Advisory →
- Fix · GHSA-5mr9-crcg-8wh2 github.com/mattermost/mattermost-server (Go) Fixed in 10.11.11 Advisory →
- Fix · GHSA-5mr9-crcg-8wh2 github.com/mattermost/mattermost-server (Go) Fixed in 11.2.3 Advisory →
- Fix · GHSA-5mr9-crcg-8wh2 github.com/mattermost/mattermost-server (Go) Fixed in 11.3.1 Advisory →
Timeline
- 2026-10-02 Threatist record created from CVE-2026-21386
- 2026-10-03 Alias added: GHSA-5mr9-crcg-8wh2
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-2026-21386 · CVE Program
- CVE-2026-21386 · NVD
- FIRST EPSS
- GHSA-5mr9-crcg-8wh2 · 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