CVE-2026-6957 Overview
CVE-2026-6957 is a path traversal vulnerability in Mattermost Plugins versions <=1.1.5. The flaw stems from inadequate filename sanitization on attachments received from federated peers through the shared-channel attachment sync protocol. An administrator of a remote federated Mattermost server can deliver a malicious filename that, when used to construct export destination paths, writes files to arbitrary locations within the target server's filestore. Mattermost tracks the issue as advisory MMSA-2026-00659 and categorizes the weakness under CWE-22.
Critical Impact
A remote federated administrator can write arbitrary files into the target Mattermost filestore, enabling tampering with server resources and potential follow-on code execution paths.
Affected Products
- Mattermost Plugins versions <=1.1.5
- Mattermost servers configured with shared channels and federated peer connections
- Deployments using the shared-channel attachment sync protocol
Discovery Timeline
- 2026-05-27 - CVE-2026-6957 published to NVD
- 2026-05-27 - Last updated in NVD database
Technical Details for CVE-2026-6957
Vulnerability Analysis
The vulnerability resides in the file-handling logic that processes attachments synchronized from federated Mattermost peers. When a remote server transmits an attachment through the shared-channel sync protocol, the receiving server uses the peer-supplied filename to build the export destination path. The receiving plugin code does not normalize or sanitize traversal sequences such as ../ before joining the filename to the filestore base directory. This allows an attacker-controlled filename to escape the intended attachment directory and resolve to arbitrary paths writable by the Mattermost service account.
Because federated trust relationships authenticate peer servers but do not validate the contents they send, the integrity boundary collapses at the file path layer. The result is a classic relative path traversal flaw described by CWE-22.
Root Cause
The root cause is missing input validation on filenames received over the federation channel. The plugin trusts peer-supplied metadata when constructing local paths and does not enforce that the resolved path remains beneath the intended attachment directory. Standard mitigations such as canonicalizing the path and rejecting paths that fall outside the allowed base directory are absent in versions <=1.1.5.
Attack Vector
Exploitation requires the attacker to control a remote Mattermost server that is federated with the target through shared channels, and to hold administrator privileges on that remote server. The attacker crafts an attachment with a malicious filename containing traversal sequences and delivers it through the shared-channel attachment sync protocol. The target server writes the attachment to an arbitrary location within its filestore, where the Mattermost process has write permissions. Refer to Mattermost Security Updates for vendor-published technical details.
Detection Methods for CVE-2026-6957
Indicators of Compromise
- Files appearing in the Mattermost filestore at paths outside the expected attachment directory structure.
- Attachment sync events from federated peers carrying filenames containing ../, ..\, or absolute path prefixes.
- Unexpected modifications to plugin directories, configuration files, or static assets owned by the Mattermost service account.
Detection Strategies
- Inspect Mattermost server logs for shared-channel sync operations and correlate filenames against a strict allow-list pattern.
- Audit filestore directory contents for files placed outside the configured attachment root.
- Review federation peer trust lists and flag any newly added or low-reputation peers preceding suspicious filestore changes.
Monitoring Recommendations
- Enable file integrity monitoring on the Mattermost filestore base directory and adjacent plugin and configuration paths.
- Alert on writes by the Mattermost service account to paths outside the attachment storage hierarchy.
- Log all inbound shared-channel attachment sync requests with peer identity, original filename, and resolved destination path.
How to Mitigate CVE-2026-6957
Immediate Actions Required
- Upgrade Mattermost Plugins to a version above 1.1.5 as published in the vendor advisory MMSA-2026-00659.
- Review the list of federated peers and remove any that are not strictly necessary for business operations.
- Audit the Mattermost filestore for files placed outside the expected attachment directory before applying the patch.
Patch Information
Mattermost has published a fixed release referenced under advisory MMSA-2026-00659. Administrators should consult Mattermost Security Updates for the corrected plugin version and upgrade instructions.
Workarounds
- Disable shared channels or federation features until the patched plugin version is deployed.
- Restrict federation to a minimal set of fully trusted peer servers operated by your own organization.
- Apply filesystem-level access controls so the Mattermost service account cannot write outside the intended filestore directory.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
