CVE-2026-3113: 
vulnerability analysis and mitigation

CVE-2026-3113 is an information disclosure vulnerability in Mattermost Server caused by improper file permissions on bulk export files (Mattermost Advisory ID: MMSA-2026-00593). Affected versions include 11.4.x ≤ 11.4.0, 11.3.x ≤ 11.3.1, 11.2.x ≤ 11.2.3, and 10.11.x ≤ 10.11.11. The vulnerability was disclosed on March 26, 2026, with patches made available shortly after. It carries a CVSS v3.1 base score of 5.5 (Medium) (Red Hat Advisory, Red Hat Bugzilla).

The root cause is classified under CWE-732 (Incorrect Permission Assignment for Critical Resource) and CWE-279 (Incorrect Execution-Assigned Permissions). When Mattermost generates and downloads bulk export files, it fails to apply restrictive file system permissions, leaving the exported files readable by other local users on the same server. Exploitation requires only low-privileged local access and no user interaction, making it straightforward for any authenticated OS-level user on the host to access the export files directly from the file system (Red Hat Advisory, Red Hat Bugzilla).

Successful exploitation allows low-privileged local users on the Mattermost server to read the full contents of bulk export files without authorization. These exports may contain sensitive user communications, channel messages, and other confidential organizational data. The impact is limited to confidentiality (no integrity or availability impact), and exploitation is constrained to users with local OS access to the server, reducing but not eliminating risk in multi-tenant or shared hosting environments (Red Hat Advisory).

1. Gain local access: Obtain a low-privileged local user account on the operating system hosting the Mattermost server (e.g., via SSH access, a shared hosting environment, or another vulnerability).
2. Locate bulk export files: Navigate to the Mattermost data directory (typically configured in config.json under FileSettings.Directory) and search for bulk export files, commonly stored in a subdirectory such as export/.
3. Read export contents: Because the files lack restrictive permissions, use standard OS commands (e.g., cat, less, or cp) to read or copy the bulk export files without requiring elevated privileges.
4. Extract sensitive data: Parse the exported files (typically in JSON or ZIP format) to access user messages, channel contents, and other organizational data that was intended to be restricted (Red Hat Bugzilla, Red Hat Advisory).

• File System: Unexpected access timestamps on bulk export files in the Mattermost data/export directory by OS users other than the Mattermost service account; bulk export files with world-readable permissions (e.g., -rw-r--r-- or -rw-rw-r--).
• Logs: OS-level audit logs (e.g., Linux auditd) showing read access to Mattermost export files by non-Mattermost service accounts; inotifywait or similar file monitoring alerts on the export directory.
• Process: Shell processes (e.g., bash, cat, cp) initiated by local users accessing the Mattermost data directory outside of normal administrative activity.

Mattermost has released patched versions: 11.4.1, 11.3.2, 11.2.4, and 10.11.12. Administrators should upgrade to one of these versions immediately. As a temporary workaround prior to patching, restrict local server access to authorized users only and manually apply restrictive file system permissions (e.g., chmod 600 or chmod 640 with appropriate group ownership) to existing bulk export files in the Mattermost data directory (Mattermost Security Updates, Red Hat Advisory).