CVE-2026-9157: Gmission Web Fax RCE Vulnerability

CVE-2026-9157 Overview

CVE-2026-9157 is a high-severity vulnerability affecting Gmission Web Fax versions 3.0 up to (but not including) 3.1. The flaw combines improper input validation [CWE-20] with unrestricted upload of files with dangerous types, enabling remote code inclusion. Attackers with local access can upload malicious files that the application processes as executable code. The vulnerability carries a CVSS 4.0 score of 8.6, reflecting high impact on confidentiality, integrity, and availability.

Critical Impact

Successful exploitation allows attackers to execute arbitrary code within the Web Fax application context, leading to full compromise of the affected system.

Affected Products

• Gmission Web Fax 3.0
• Gmission Web Fax versions after 3.0 and before 3.1
• Systems running vulnerable Web Fax deployments referenced in the Gmission Fax Service Overview

Discovery Timeline

• 2026-05-21 - CVE-2026-9157 published to NVD
• 2026-05-21 - Last updated in NVD database

Technical Details for CVE-2026-9157

Vulnerability Analysis

The vulnerability stems from two related weaknesses in Gmission Web Fax. The application fails to validate user-supplied input properly and accepts uploads of files with dangerous types without enforcing restrictions on extension, MIME type, or content. An attacker who can interact with the upload functionality can supply a file containing executable server-side code. When the application later includes or processes the uploaded file, the embedded code executes within the Web Fax process context.

Remote code inclusion attacks of this class typically target server-side scripting engines used to render fax management interfaces. The combination of unrestricted upload and improper input validation removes the safeguards that would normally prevent malicious files from being treated as trusted application components.

Root Cause

The root cause is the absence of allow-list validation on uploaded file types in Web Fax 3.0. The application does not constrain file extensions, verify content signatures, or store uploaded files outside the web-accessible execution path. Input parameters that reference uploaded files are not sanitized before inclusion, allowing attacker-controlled paths or filenames to reach file inclusion logic.

Attack Vector

The attack vector is local, requiring no privileges and no user interaction according to the CVSS 4.0 metrics. An attacker with local access to the Web Fax interface uploads a crafted file containing executable code. The attacker then triggers inclusion of that file by invoking the relevant application function or URL. The code runs with the privileges of the Web Fax service, leading to compromise of confidentiality, integrity, and availability on the host.

No verified public exploit code is currently available. Refer to the Gmission Fax Service Overview for product context.

Detection Methods for CVE-2026-9157

Indicators of Compromise

• Unexpected files with server-executable extensions (such as .php, .jsp, .asp, .aspx) inside Web Fax upload directories
• Web server process spawning child processes such as shells, cmd.exe, or powershell.exe from the Web Fax application path
• Outbound network connections originating from the Web Fax service to unfamiliar external hosts
• New or modified files in fax storage directories with timestamps that do not align with normal fax activity

Detection Strategies

• Inspect Web Fax upload directories for files whose MIME type or content does not match a fax document
• Monitor process lineage where the Web Fax service is the parent process of interpreters or system utilities
• Review HTTP access logs for POST requests to upload endpoints followed by GET requests to attacker-supplied filenames
• Apply file integrity monitoring to Web Fax installation directories to detect unauthorized code deployment

Monitoring Recommendations

• Enable verbose logging on the Web Fax application and forward logs to a centralized analytics platform for correlation
• Alert on creation of executable file types within any directory writable by the Web Fax service account
• Track authentication events and upload activity on Web Fax to baseline normal usage and detect deviations

How to Mitigate CVE-2026-9157

Immediate Actions Required

• Upgrade Gmission Web Fax to version 3.1 or later, which addresses the unrestricted upload and input validation flaws
• Restrict network and local access to the Web Fax interface to trusted administrative users only
• Audit existing upload directories for unauthorized files and remove any artifacts left by exploitation attempts
• Rotate credentials and service account secrets that may have been exposed if exploitation is suspected

Patch Information

Gmission addresses CVE-2026-9157 in Web Fax version 3.1. Organizations running Web Fax 3.0 should plan an upgrade to version 3.1 or higher. Consult the Gmission Fax Service Overview for vendor contact and update channels.

Workarounds

• Block write access to Web Fax upload directories from the web server execution context where feasible
• Place the Web Fax service behind a reverse proxy that enforces file type and size restrictions on uploaded content
• Disable script execution within fax storage directories through web server configuration until the patch is applied

# Example: deny script execution inside the Web Fax uploads directory (Apache)
<Directory "/var/www/webfax/uploads">
    Options -ExecCGI
    RemoveHandler .php .phtml .php5 .jsp .asp .aspx
    AddType text/plain .php .phtml .php5 .jsp .asp .aspx
    php_admin_flag engine off
</Directory>