CVE-2026-53906: MCO Path Traversal Vulnerability

CVE-2026-53906 Overview

CVE-2026-53906 is a path traversal and path disclosure vulnerability affecting My Compliance Office (MCO). The flaw exists in file handling functionality tied to data export and upload operations. Improper validation of the filename parameter allows authenticated attackers to write files to arbitrary locations on the server. The same weakness leaks absolute server paths through error messages, aiding further reconnaissance. The vulnerability is confirmed in MCO version 25.3.3.1, though other versions may be affected. Vendor contact attempts were unsuccessful, so no coordinated patch is available at publication time. The issue is categorized under CWE-22: Improper Limitation of a Pathname to a Restricted Directory.

Critical Impact
Authenticated attackers can write arbitrary files to the underlying server filesystem and enumerate absolute paths through error responses.

Affected Products

My Compliance Office (MCO) version 25.3.3.1 (confirmed)
Other MCO versions potentially affected (unconfirmed)
Deployments exposing MCO data export or upload endpoints

Discovery Timeline

2026-07-01 - CVE-2026-53906 published to NVD
2026-07-01 - Last updated in NVD database

Technical Details for CVE-2026-53906

Vulnerability Analysis

The vulnerability resides in MCO endpoints that handle file export and upload workflows. These endpoints accept a filename parameter from the client and use it to construct filesystem paths without adequate sanitization. Attackers can supply traversal sequences such as ../ or absolute paths to redirect file write operations outside the intended directory. The same weakness surfaces two distinct impacts: unauthorized file placement and disclosure of internal filesystem structure through verbose error responses. The vulnerability requires high privileges to exploit, which limits the attacker pool to authenticated users with export or upload rights. However, once exploited, an attacker can stage malicious files in web-accessible locations or overwrite configuration files. Path disclosure also assists in chaining this issue with other weaknesses.

Root Cause

The root cause is improper input validation of the filename parameter in file handling routines. The application concatenates user-supplied filenames into filesystem paths without canonicalizing the result or restricting writes to an allowlisted directory. Error handling additionally returns raw filesystem paths in exception messages, exposing internal directory structure to the caller.

Attack Vector

An authenticated attacker submits a crafted request to the export or upload endpoint with a filename value containing directory traversal sequences (for example, ../../../var/www/app/shell.jsp). The server resolves the path relative to the export directory and writes attacker-controlled content to the target location. When path resolution fails, the server returns an error message that reveals the absolute base path, letting the attacker refine subsequent payloads. See the CERT Polska Vulnerability Report for technical details on the disclosed behavior.

Detection Methods for CVE-2026-53906

Indicators of Compromise

HTTP requests to MCO export or upload endpoints containing ../, ..\, URL-encoded traversal sequences (%2e%2e%2f), or absolute path prefixes in the filename parameter.
Unexpected file creations outside the designated MCO export or upload directory, particularly in web-accessible or configuration paths.
Application error responses containing absolute filesystem paths returned to unauthenticated or low-context clients.

Detection Strategies

Inspect web server and application logs for filename values that deviate from expected naming patterns or contain traversal metacharacters.
Alert on file integrity monitoring events in directories adjacent to the MCO installation root when writes originate from the MCO process.
Correlate authenticated MCO sessions with anomalous file write telemetry on the host to identify abuse by privileged users.

Monitoring Recommendations

Enable verbose access logging on MCO endpoints handling data export and upload, capturing full query parameters and request bodies.
Forward host-based file creation events and web server logs to a centralized analytics platform for correlation.
Baseline legitimate MCO export locations and alert on writes to any path outside that baseline.

How to Mitigate CVE-2026-53906

Immediate Actions Required

Restrict access to MCO export and upload functionality to the minimum set of trusted administrative users.
Place MCO behind a web application firewall configured to block directory traversal payloads in the filename parameter.
Audit the MCO installation directory and adjacent paths for unauthorized files written since the deployment of version 25.3.3.1.

Patch Information

No vendor patch is available. According to the CERT Polska Vulnerability Report, vendor contact attempts were unsuccessful. Contact My Compliance Office directly through the official product portal for remediation status and consult vendor communications for updated guidance.

Workarounds

Enforce strict input validation at a reverse proxy, rejecting requests where filename contains path separators or encoded traversal sequences.
Run the MCO application under a least-privileged service account with write access limited to the intended export directory only.
Suppress verbose error messages at the proxy layer to prevent absolute path disclosure in responses.

bash

# Example NGINX rule to block traversal patterns in the filename parameter
location /mco/ {
    if ($arg_filename ~* "(\.\./|\.\.\\|%2e%2e|/etc/|c:\\)") {
        return 403;
    }
    proxy_pass http://mco_backend;
    proxy_intercept_errors on;
    error_page 500 502 503 504 /generic_error.html;
}