CVE-2026-1126: lwj flow Unrestricted Upload RCE Vulnerability

CVE-2026-1126 Overview

CVE-2026-1126 is an unrestricted file upload vulnerability affecting lwj flow, an open-source workflow management application. The vulnerability exists in the uploadFile function within the FormResource.java file, specifically in the SVG File Handler component. An attacker can exploit this flaw to upload arbitrary files without proper validation, potentially leading to remote code execution or other malicious outcomes.

Critical Impact

Remote attackers with low privileges can upload malicious files through the SVG File Handler, potentially compromising the application server and gaining unauthorized access to the system.

Affected Products

• lwj flow (up to commit a3d2fe8133db9d3b50fda4f66f68634640344641)
• SVG File Handler component in flow-front-rest module
• FormResource.java file upload functionality

Discovery Timeline

• January 18, 2026 - CVE-2026-1126 published to NVD
• January 18, 2026 - Last updated in NVD database

Technical Details for CVE-2026-1126

Vulnerability Analysis

This vulnerability stems from improper access control (CWE-284) in the file upload functionality of lwj flow. The uploadFile function in FormResource.java fails to properly validate and restrict the types of files that can be uploaded through the SVG File Handler. This allows attackers to bypass intended security restrictions and upload files that should otherwise be blocked.

The vulnerability is exploitable remotely over the network and requires only low-level privileges to execute. While the attack complexity is low and no user interaction is required, the impact on confidentiality, integrity, and availability is limited in scope.

The exploit for this vulnerability has been publicly disclosed through a Gitee Issue report, increasing the risk of exploitation in the wild. The project maintainers were notified of the issue but have not yet responded.

Root Cause

The root cause of CVE-2026-1126 is improper access control in the file upload mechanism. The uploadFile function in \flow-master\flow-front-rest\src\main\java\com\dragon\flow\web\resource\flow\FormResource.java does not implement adequate file type validation or restrictions. When processing the File argument in the SVG File Handler, the application fails to:

1. Validate the file extension against an allowlist of permitted types
2. Verify the file content matches the declared file type (MIME type validation)
3. Enforce proper access controls on who can upload files and what types are permitted

Attack Vector

The attack can be carried out remotely over the network by authenticated users with low privileges. The attacker leverages the SVG File Handler endpoint to submit malicious files disguised as legitimate SVG content or simply bypasses any client-side validation to upload arbitrary file types.

The exploitation flow typically involves:

1. Authenticating to the lwj flow application with minimal privileges
2. Crafting a malicious file payload (e.g., web shell, executable, or malicious script)
3. Submitting the file through the SVG File Handler upload endpoint
4. Accessing the uploaded file to trigger execution or further exploitation

Due to the nature of unrestricted file uploads, attackers may upload executable code that can be triggered server-side, leading to remote code execution, data exfiltration, or complete system compromise.

Detection Methods for CVE-2026-1126

Indicators of Compromise

• Unusual file uploads with unexpected extensions (e.g., .jsp, .php, .exe) through the SVG File Handler endpoint
• Web server logs showing POST requests to the FormResource upload endpoint with suspicious file names
• Presence of unexpected files in the upload directory that do not match legitimate SVG content
• Failed authentication attempts followed by successful file uploads from the same source

Detection Strategies

• Monitor HTTP traffic for file upload requests to /flow/resource/form/uploadFile or similar endpoints with non-SVG file types
• Implement file integrity monitoring on upload directories to detect unauthorized or suspicious file additions
• Deploy web application firewall (WAF) rules to inspect file upload requests and block potentially malicious content
• Review application logs for anomalous upload activity patterns from low-privilege accounts

Monitoring Recommendations

• Enable verbose logging for the FormResource.java component to capture detailed file upload activity
• Configure alerts for file uploads containing executable code signatures or script content
• Monitor server resource utilization for signs of malicious code execution following file uploads
• Implement network traffic analysis to detect data exfiltration attempts following successful exploitation

How to Mitigate CVE-2026-1126

Immediate Actions Required

• Restrict access to the file upload functionality to only trusted, high-privilege users until a patch is available
• Implement server-side file type validation using allowlists that only permit legitimate SVG files
• Deploy web application firewall rules to block suspicious file uploads at the network perimeter
• Monitor the Gitee Issue tracker for updates from the project maintainers

Patch Information

As of the last update, no official patch has been released for CVE-2026-1126. The lwj flow project uses a rolling release strategy, which means version-specific patch information is not available. The project was notified of the vulnerability through an issue report but has not yet responded.

Organizations should monitor the VulDB entry #341718 and the official lwj flow repository for patch announcements. Consider upgrading to the latest commit once a fix is merged, or apply the recommended workarounds until an official remediation is available.

Workarounds

• Implement a reverse proxy or WAF rule that validates file uploads and blocks non-SVG content types before reaching the application
• Modify server configuration to disable execution of uploaded files in the upload directory
• Restrict network access to the file upload endpoint using IP allowlisting or VPN requirements
• Consider temporarily disabling the SVG File Handler functionality if it is not critical to business operations

# Example: Disable execution in upload directory (Apache)
<Directory "/path/to/upload/directory">
    Options -ExecCGI
    php_flag engine off
    <FilesMatch "\.(php|jsp|asp|aspx|exe|sh|bat)$">
        Require all denied
    </FilesMatch>
</Directory>