CVE-2025-4981: Mattermost Server Path Traversal Flaw

CVE-2025-4981 Overview

CVE-2025-4981 is a critical path traversal vulnerability in Mattermost Server that allows authenticated users to write files to arbitrary locations on the filesystem. The vulnerability exists in the archive extractor component, which fails to properly sanitize filenames when processing uploaded archive files. By crafting malicious archives containing path traversal sequences (e.g., ../) in filenames, attackers can escape the intended upload directory and write files anywhere on the filesystem, potentially leading to remote code execution.

This vulnerability is particularly concerning because it affects default Mattermost configurations where both FileSettings.EnableFileAttachments and FileSettings.ExtractContent are set to true, meaning most Mattermost deployments are vulnerable out of the box.

Critical Impact

Authenticated attackers can achieve remote code execution by writing malicious files to arbitrary filesystem locations, potentially compromising the entire Mattermost server and any connected systems.

Affected Products

• Mattermost Server versions 10.5.x through 10.5.5
• Mattermost Server versions 9.11.x through 9.11.15
• Mattermost Server versions 10.8.x through 10.8.0 (including RC1, RC2, RC3)
• Mattermost Server versions 10.7.x through 10.7.2
• Mattermost Server versions 10.6.x through 10.6.5

Discovery Timeline

• 2025-06-20 - CVE-2025-4981 published to NVD
• 2025-07-08 - Last updated in NVD database

Technical Details for CVE-2025-4981

Vulnerability Analysis

The vulnerability resides in Mattermost's archive extraction functionality, specifically in how the server handles document search indexing. When users upload archive files (such as ZIP files), Mattermost extracts the contents to enable content-based document searching. The extraction routine fails to validate and sanitize filenames within the archive before writing them to disk.

This allows an attacker to craft a malicious archive where filenames contain directory traversal sequences like ../ or absolute paths. When the archive is processed, the extraction routine blindly follows these path sequences, writing files outside the intended extraction directory. This can result in overwriting critical system files, planting web shells, or modifying application configurations.

The vulnerability is classified under CWE-427 (Uncontrolled Search Path Element), though the core issue is improper input validation of archive filenames leading to path traversal.

Root Cause

The root cause of CVE-2025-4981 is insufficient input validation in the archive extraction component. The filename sanitization logic does not properly handle or reject path traversal sequences embedded within archive entry names. When processing uploaded archives for content indexing, the server extracts files using the provided filenames without normalizing paths or validating that the resulting file path remains within the designated extraction directory.

This is a classic Zip Slip vulnerability pattern where archive formats allow embedded path information that, if not properly validated, can be exploited to write files outside the intended directory structure.

Attack Vector

The attack vector is network-based and requires low-privileged authentication. An attacker needs only a valid Mattermost account to exploit this vulnerability. The attack flow involves:

1. Creating a malicious archive file containing entries with path traversal sequences in their filenames
2. Uploading the crafted archive to Mattermost through normal file attachment functionality
3. The server's content extraction process automatically triggers, parsing the archive
4. Files are written to attacker-controlled locations on the filesystem

The exploitation path from arbitrary file write to remote code execution depends on the server environment but commonly involves writing web shells to accessible directories, overwriting cron jobs or scheduled tasks, modifying application configuration files, or planting SSH authorized keys.

Since no user interaction is required beyond the initial upload, and the extraction happens automatically when document search is enabled, this vulnerability is highly exploitable in default configurations.

Detection Methods for CVE-2025-4981

Indicators of Compromise

• Unexpected files appearing in system directories such as /tmp, /var, web roots, or cron directories
• Archive uploads containing suspicious path patterns like ../, ..\\, or absolute paths in server logs
• File creation events in directories outside the Mattermost data directory during archive processing
• Web shells or unauthorized executable files appearing on the server
• Unusual process execution originating from the Mattermost server process

Detection Strategies

• Monitor file system activity for write operations outside expected Mattermost directories, particularly during file upload events
• Implement file integrity monitoring (FIM) on critical system directories to detect unauthorized modifications
• Review Mattermost server logs for archive upload activities and correlate with file system events
• Deploy endpoint detection and response (EDR) solutions to identify suspicious file creation patterns

Monitoring Recommendations

• Enable detailed logging for file upload and extraction operations in Mattermost
• Configure alerts for file writes containing path traversal sequences in filenames
• Monitor for new files created in web-accessible directories or system configuration locations
• Implement network-level monitoring for unusual outbound connections from the Mattermost server that could indicate post-exploitation activity

How to Mitigate CVE-2025-4981

Immediate Actions Required

• Upgrade Mattermost Server to the latest patched version immediately
• If patching is not immediately possible, disable content extraction by setting FileSettings.ExtractContent to false
• Review server filesystem for any unauthorized files that may have been planted via exploitation
• Audit user upload activity for suspicious archive files
• Consider temporarily restricting file upload permissions to trusted users until patching is complete

Patch Information

Mattermost has released security updates addressing this vulnerability. Organizations should upgrade to patched versions as documented in the Mattermost Security Updates page. Ensure you are running a version newer than:

• 10.5.5 for the 10.5.x branch
• 9.11.15 for the 9.11.x branch
• 10.8.0 for the 10.8.x branch
• 10.7.2 for the 10.7.x branch
• 10.6.5 for the 10.6.x branch

Workarounds

• Disable document content extraction by setting FileSettings.ExtractContent = false in the Mattermost configuration
• Restrict file upload permissions to reduce the attack surface until patching is completed
• Implement network segmentation to limit potential lateral movement if the server is compromised
• Deploy web application firewall (WAF) rules to inspect and block archive uploads containing path traversal patterns

# Mattermost configuration workaround
# Edit config.json or set via environment variable

# Option 1: Modify config.json
# Set ExtractContent to false under FileSettings:
# "FileSettings": {
#     "ExtractContent": false
# }

# Option 2: Environment variable
export MM_FILESETTINGS_EXTRACTCONTENT=false

# Restart Mattermost service after configuration change
sudo systemctl restart mattermost