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How CVE-2026-6957 Works
CVE-2026-6957 is a path traversal vulnerability affecting Mattermost Plugins, specifically the Legal Hold plugin, versions 1.1.5 and earlier. The flaw resides in the way the plugin handles filenames received from federated Mattermost peers during the shared‑channel attachment synchronization protocol.
When a remote federated Mattermost server shares a channel with a target server, the attachment sync process transfers filenames as part of the protocol. The affected plugin versions fail to sanitize or validate these filenames before using them to construct file export destination paths.
An attacker who is an administrator of a remote federated Mattermost server can craft a malicious filename containing directory traversal sequences (e.g., ../, ..\\) or absolute path elements. When this filename is delivered through the sync protocol, the plugin concatenates it with the intended export directory without any restrictions. As a result, the plugin writes files to arbitrary locations within the target server’s filestore.
Because the plugin operates with the privileges of the Mattermost server process, a successful exploit can overwrite configuration files, plant malicious content, or alter system data. The attack does not require authentication on the target server—it leverages the trust relationship established during federation. The root cause is the absence of input sanitization combined with the use of unsanitized user input in filesystem operations, as described by CWE‑22 (Improper Limitation of a Pathname to a Restricted Directory).
DailyCVE Form
Platform: Mattermost Plugins
Version: <=1.1.5
Vulnerability: Path Traversal
Severity: High
Date: 2026-06-02
Prediction: Patch by 2026-06-27
What Undercode Say
Check plugin version:
curl -s http://target-mattermost:8065/api/v4/plugins | jq '.[] | select(.id=="com.mattermost.plugin-legal-hold") | .version'
Simulate malicious filename:
Create a filename with path traversal echo "malicious_content" > "$(echo -e '../../../config/config.json')"
Send via shared‑channel sync (conceptual):
import requests
malicious_payload = "../../../config/config.json"
requests.post("https://target-mattermost/api/v4/shared-channels/sync",
json={"filename": malicious_payload, "content": "Hacked!"})
Exploit
- Establish a federated channel between an attacker‑controlled Mattermost instance and the target server.
- As the remote administrator, upload an attachment with a filename containing path traversal sequences (e.g.,
../../../../config/config.json). - Trigger the shared‑channel attachment sync protocol (e.g., by sharing a message with the attachment).
- The target plugin uses the unsanitized filename to construct the export path, writing the attachment content outside the intended directory—overwriting or creating arbitrary files in the target’s filestore.
Protection
- Upgrade: Update the Legal Hold plugin to version 1.1.6 or later once released. In the absence of a fixed version, disable the Legal Hold plugin until a patch is available.
- Input validation: Implement strict filename sanitization that rejects any path containing
..,/,\, or absolute path prefixes. Use a whitelist of allowed characters (e.g., alphanumerics, dots, hyphens). - Filesystem confinement: Ensure the Mattermost server runs with the minimum required filesystem permissions. Use containerization or a restricted service account to limit the impact of arbitrary file writes.
- Network controls: Restrict shared‑channel federation to trusted peers only, and monitor shared‑channel traffic for anomalous path patterns.
Impact
A successful exploit allows an attacker to:
- Overwrite critical configuration files (e.g.,
config.json), leading to server compromise or denial of service. - Inject malicious scripts or binaries into the target server’s filesystem, potentially gaining remote code execution.
- Exfiltrate sensitive data by writing files to world‑readable locations.
- Disrupt normal operation by filling the filesystem with garbage data or deleting essential files.
The vulnerability is particularly dangerous because it crosses trust boundaries—an administrator on one federated server can compromise another server without any credentials on the target.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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