Full Report
Mattermost security advisory (AV26-828)
Analysis Summary
# Vulnerability: Mattermost Improper Access Control in API
## CVE Details
- **CVE ID:** CVE-2026-9816
- **CVSS Score:** 8.1 (High)
- **CWE:** CWE-284 (Improper Access Control)
## Affected Systems
- **Products:** Mattermost Self-Hosted Server
- **Versions:**
- Versions ≤ 10.11.21
- Versions ≤ 11.7.6
- Versions ≤ 11.8.3
- **Configurations:** Systems utilizing default API permissions for authenticated users.
## Vulnerability Description
Mattermost suffers from an improper access control vulnerability within specific API endpoints. The flaw allows an authenticated user to perform unauthorized actions or access sensitive information that should be restricted to administrative roles. The vulnerability stems from insufficient validation of user permissions when processing requests to the server's backend logic.
## Exploitation
- **Status:** Not currently reported as exploited in the wild; PoC status is restricted.
- **Complexity:** Low
- **Attack Vector:** Network (Remote)
## Impact
- **Confidentiality:** High (Potential unauthorized access to private channels or user data)
- **Integrity:** High (Potential unauthorized modification of system settings or data)
- **Availability:** Low (Minimal direct impact on service uptime)
## Remediation
### Patches
Mattermost has released the following security updates to address this vulnerability. It is recommended to upgrade to the latest version within your respective release branch:
- **Upgrade to version 10.11.22** or higher
- **Upgrade to version 11.7.7** or higher
- **Upgrade to version 11.8.4** or higher
### Workarounds
There are no official functional workarounds that maintain full feature parity. Access should be restricted to trusted users only until patches can be applied.
## Detection
- **Indicators of Compromise:** Unusual API request patterns in server logs, specifically successful `POST` or `PUT` requests to administrative endpoints originating from non-admin accounts.
- **Detection methods:** Audit Mattermost server logs for `status=200` responses on protected API routes where the `user_id` does not correspond to a System Administrator role.
## References
- **Vendor Advisory:** hxxps[://]mattermost[.]com/security-updates/
- **NVD Detail:** hxxps[://]nvd[.]nist[.]gov/vuln/detail/CVE-2026-9816
- **Canadian Centre for Cyber Security:** hxxps[://]www[.]cyber[.]gc[.]ca/en/alerts-advisories/mattermost-security-advisory-av26-828