CVE-2026-6961 Overview
CVE-2026-6961 is a path traversal vulnerability [CWE-22] affecting Mattermost Server. The flaw exists in the shared channel file synchronization feature used between federated Mattermost peers. The server fails to sanitize the FileInfo.Name value received from federated peers during file sync operations. An attacker who controls a federated server can supply path traversal sequences in the filename field. This allows writing files to arbitrary locations within the target server's filestore.
Affected releases include Mattermost 11.6.x <= 11.6.1, 11.5.x <= 11.5.4, 10.11.x <= 10.11.15, and 10.11.x <= 10.11.16. The issue is tracked under Mattermost Advisory ID MMSA-2026-00661.
Critical Impact
A malicious federated peer can write arbitrary files into the victim server's filestore, enabling integrity compromise and potential follow-on code execution paths.
Affected Products
- Mattermost Server 11.6.x up to and including 11.6.1
- Mattermost Server 11.5.x up to and including 11.5.4
- Mattermost Server 10.11.x up to and including 10.11.15 and 10.11.16
Discovery Timeline
- 2026-06-12 - CVE-2026-6961 published to NVD
- 2026-06-18 - Last updated in NVD database
Technical Details for CVE-2026-6961
Vulnerability Analysis
Mattermost supports shared channels, a federation feature that allows users on different Mattermost servers to participate in the same channel. As part of this feature, file attachments are synchronized between peer servers. Each synced file carries a FileInfo structure containing metadata such as the filename. The receiving server uses this filename to determine where the file is stored on disk.
The vulnerability stems from missing sanitization of the FileInfo.Name field. A federated peer under attacker control can populate this field with directory traversal sequences such as ../. The receiving server resolves these sequences relative to its filestore root and writes the file outside the intended directory.
Exploitation requires the attacker to operate a federated Mattermost server that is connected to the target via a shared channel. The attack does not require user interaction on the target side. The scope changes because the vulnerable component writes outside its trust boundary, impacting other components on the host filesystem.
Root Cause
The root cause is improper limitation of a pathname to a restricted directory [CWE-22]. The shared channel sync handler trusts the filename supplied by the remote peer and concatenates it with the local filestore path without normalizing or rejecting traversal sequences.
Attack Vector
The attacker must control a federated server with an established shared channel relationship to the victim. During a file sync operation, the attacker submits a FileInfo payload where the Name field contains traversal sequences and an attacker-chosen suffix path. When the victim server processes the sync, it writes the file contents to the resolved location, which can be any path the Mattermost process has write access to within its filestore mount or beyond if the configuration permits.
See the Mattermost Security Updates advisory for additional technical details.
Detection Methods for CVE-2026-6961
Indicators of Compromise
- Files appearing in the Mattermost filestore outside expected per-channel or per-team directory structures.
- Filenames in shared channel sync logs containing ../, ..\, URL-encoded %2e%2e, or absolute path prefixes.
- Unexpected modifications to configuration files, plugin directories, or static asset paths under the Mattermost data directory.
Detection Strategies
- Inspect Mattermost application logs for shared channel file sync events with anomalous FileInfo.Name values.
- Monitor filesystem write events on the Mattermost host targeting paths outside the standard data/ filestore hierarchy.
- Audit established remote cluster connections and shared channel federations for unexpected or untrusted peers.
Monitoring Recommendations
- Enable filesystem integrity monitoring on the Mattermost data directory and adjacent application paths.
- Forward Mattermost audit and application logs to a centralized analytics platform for traversal pattern matching.
- Alert on creation of executable file types or plugin manifests within filestore subdirectories.
How to Mitigate CVE-2026-6961
Immediate Actions Required
- Upgrade Mattermost Server to a fixed release as published in the Mattermost security bulletin.
- Review the list of federated peers and disconnect any untrusted or unnecessary shared channel relationships.
- Audit the filestore for files written outside expected directories since the federation was established.
Patch Information
Mattermost has published fixed versions addressing the unsanitized filename handling in shared channel file sync. Administrators should consult the Mattermost Security Updates page for the specific patched versions corresponding to the 11.6.x, 11.5.x, and 10.11.x branches and apply the upgrade promptly.
Workarounds
- Disable shared channels and remote cluster federation until the server is upgraded.
- Restrict outbound and inbound federation traffic to known, trusted Mattermost peers using network controls.
- Run the Mattermost process under a least-privilege account with write access limited strictly to the intended filestore path.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
