CVE-2026-3112: Mattermost Path Traversal Vulnerability

CVE-2026-3112 Overview

CVE-2026-3112 is a path traversal vulnerability in Mattermost that allows system administrators to read arbitrary host files through malicious Advanced Logging configuration. The vulnerability stems from insufficient validation of file target paths in the AdvancedLoggingJSON configuration during support packet generation.

This vulnerability is classified as CWE-22 (Improper Limitation of a Pathname to a Restricted Directory), commonly known as Path Traversal or Directory Traversal. An authenticated attacker with system administrator privileges can exploit this flaw to access sensitive files on the host system that should be restricted.

Critical Impact

System administrators can read arbitrary files from the host system by manipulating the Advanced Logging file target paths, potentially exposing sensitive configuration files, credentials, and system data.

Affected Products

• Mattermost 11.4.x versions <= 11.4.0
• Mattermost 11.3.x versions <= 11.3.1
• Mattermost 11.2.x versions <= 11.2.3
• Mattermost 10.11.x versions <= 10.11.11

Discovery Timeline

• 2026-03-26 - CVE-2026-3112 published to NVD
• 2026-03-26 - Last updated in NVD database

Technical Details for CVE-2026-3112

Vulnerability Analysis

This vulnerability exists in Mattermost's Advanced Logging functionality, specifically in how file target paths are processed during support packet generation. The application fails to properly validate or sanitize the AdvancedLoggingJSON configuration parameter before using it to determine file paths for logging operations.

When generating support packets, the system accepts user-controlled logging configuration that specifies file targets. Without proper path validation, an attacker can inject path traversal sequences (such as ../) or absolute paths to escape the intended logging directory and read arbitrary files from the host filesystem.

The vulnerability requires administrator-level privileges to exploit, which limits the attack surface but remains a significant concern in multi-tenant or shared administrative environments. Malicious insiders or compromised administrator accounts could leverage this flaw to access sensitive system files including /etc/passwd, application configuration files containing database credentials, or private keys.

Root Cause

The root cause is improper input validation in the file path handling logic for Advanced Logging configuration. The application does not adequately:

1. Sanitize user-provided file paths to remove path traversal sequences
2. Validate that specified paths remain within the allowed directory boundaries
3. Implement proper canonicalization of paths before file access operations

The AdvancedLoggingJSON configuration accepts file target paths without verifying they remain within expected boundaries, allowing directory traversal attacks.

Attack Vector

The attack is conducted over the network and requires high privileges (system administrator access). The exploitation flow involves:

1. An authenticated system administrator accesses the Mattermost configuration interface
2. The attacker modifies the AdvancedLoggingJSON configuration to include a malicious file target path with traversal sequences (e.g., ../../../etc/passwd)
3. The attacker triggers support packet generation
4. The vulnerable code processes the malicious path without proper validation
5. The contents of the targeted file are included in the generated support packet or exposed through logging output

The vulnerability mechanism relies on manipulating the logging configuration to point to sensitive files outside the intended logging directory. When support packet generation occurs, the application follows the configured paths without proper boundary checking, effectively enabling arbitrary file read capabilities.

For detailed technical information, see the Mattermost Security Updates advisory (MMSA-2025-00562).

Detection Methods for CVE-2026-3112

Indicators of Compromise

• Unusual modifications to AdvancedLoggingJSON configuration containing path traversal sequences (../)
• Support packet generation requests from unexpected administrator accounts
• Log entries showing file access attempts outside the standard Mattermost logging directories
• Configuration changes that reference system files like /etc/passwd, /etc/shadow, or application credential files

Detection Strategies

• Monitor Mattermost configuration change events for modifications to Advanced Logging settings
• Implement file integrity monitoring on sensitive system files to detect unauthorized access
• Review audit logs for support packet generation activities, particularly from new or unusual administrator accounts
• Deploy endpoint detection rules to identify path traversal patterns in configuration parameters

Monitoring Recommendations

• Enable verbose logging for configuration changes in Mattermost administrative interfaces
• Configure SIEM alerts for configuration modifications containing path traversal indicators
• Monitor for unusual file read operations by the Mattermost process outside its standard directories
• Track support packet generation frequency and correlate with expected administrative activities

How to Mitigate CVE-2026-3112

Immediate Actions Required

• Upgrade Mattermost to a patched version that addresses this vulnerability
• Review recent AdvancedLoggingJSON configuration changes for signs of exploitation
• Audit administrator account access and remove unnecessary administrative privileges
• Implement additional access controls for support packet generation functionality

Patch Information

Mattermost has released security updates to address this vulnerability. Organizations should upgrade to the following minimum versions:

• Mattermost 11.4.x: Upgrade beyond 11.4.0
• Mattermost 11.3.x: Upgrade beyond 11.3.1
• Mattermost 11.2.x: Upgrade beyond 11.2.3
• Mattermost 10.11.x: Upgrade beyond 10.11.11

Consult the Mattermost Security Updates page for the latest patched versions and detailed upgrade instructions. The advisory ID for this vulnerability is MMSA-2025-00562.

Workarounds

• Restrict system administrator access to trusted personnel only until patches can be applied
• Disable Advanced Logging functionality if not required for operations
• Implement network segmentation to limit administrator access to Mattermost configuration interfaces
• Monitor and alert on any support packet generation requests as a compensating control

# Review current Advanced Logging configuration for suspicious paths
grep -r "AdvancedLoggingJSON" /opt/mattermost/config/
# Check for path traversal patterns in configuration
grep -E "\.\./|/etc/|/var/" /opt/mattermost/config/config.json