CVE-2026-41269: Flowise File Upload RCE Vulnerability

CVE-2026-41269 Overview

CVE-2026-41269 is an unrestricted file upload vulnerability (CWE-434) in Flowise, a drag & drop user interface for building customized large language model flows. Prior to version 3.1.0, the Chatflow configuration file upload settings can be modified to allow the application/javascript MIME type, enabling attackers to upload malicious .js files despite frontend restrictions. This vulnerability allows persistent storage of Node.js web shells on the server, potentially leading to Remote Code Execution (RCE).

Critical Impact

Attackers can bypass file upload restrictions to deploy persistent Node.js web shells, enabling Remote Code Execution on Flowise servers hosting LLM workflows.

Affected Products

• Flowise versions prior to 3.1.0
• Flowise Chatflow configuration file upload component
• Self-hosted Flowise LLM workflow deployments

Discovery Timeline

• April 23, 2026 - CVE-2026-41269 published to NVD
• April 23, 2026 - Last updated in NVD database

Technical Details for CVE-2026-41269

Vulnerability Analysis

This vulnerability stems from insufficient validation of MIME types in the Chatflow configuration file upload functionality. The application implements frontend restrictions that prevent JavaScript file uploads through the user interface. However, these restrictions are client-side only and can be bypassed by directly manipulating the upload configuration or intercepting and modifying HTTP requests.

When an attacker successfully modifies the allowed MIME types to include application/javascript, they can upload arbitrary .js files to the server. Because Flowise operates on Node.js, these uploaded JavaScript files can contain fully functional web shells that execute server-side code when accessed.

The persistence aspect of this vulnerability is particularly concerning. Unlike memory-based attacks, uploaded web shells remain on the server filesystem, providing attackers with continued access even after the initial exploitation vector is patched.

Root Cause

The root cause is improper server-side validation of file upload configurations. The application relies on client-side MIME type restrictions without implementing corresponding server-side enforcement. This allows authenticated users to modify the acceptable file types through configuration manipulation, bypassing intended security controls.

Additionally, the application fails to validate or sanitize uploaded file contents against potentially dangerous patterns, such as executable JavaScript code containing shell commands or system calls.

Attack Vector

The attack vector is network-based and requires low-privilege authentication. An attacker with valid credentials can exploit this vulnerability through the following sequence:

1. Authenticate to the Flowise application
2. Access the Chatflow configuration settings
3. Modify the allowed MIME types to include application/javascript
4. Upload a malicious Node.js web shell disguised as a legitimate JavaScript file
5. Access the uploaded file to execute arbitrary commands on the server

The uploaded web shell can leverage Node.js built-in modules such as child_process to execute system commands, fs to read/write files, or net to establish reverse shell connections. This provides full control over the underlying server where Flowise is deployed.

Detection Methods for CVE-2026-41269

Indicators of Compromise

• Unexpected .js files appearing in Flowise upload directories
• Configuration changes to MIME type allowlists adding application/javascript
• HTTP requests to uploaded JavaScript files with unusual query parameters
• Child processes spawned by the Node.js Flowise process executing shell commands
• Network connections originating from the Flowise server to unknown external hosts

Detection Strategies

• Monitor Flowise configuration files for unauthorized modifications to upload settings
• Implement file integrity monitoring on upload directories to detect new JavaScript files
• Review web server access logs for requests to uploaded files with suspicious patterns
• Deploy network monitoring to identify anomalous outbound connections from Flowise servers
• Analyze process trees to detect shell commands spawned by the Node.js runtime

Monitoring Recommendations

• Enable verbose logging for file upload operations in Flowise
• Configure alerts for configuration changes to file upload MIME type settings
• Implement real-time file system monitoring on directories where uploads are stored
• Monitor Node.js process behavior for unexpected child process execution

How to Mitigate CVE-2026-41269

Immediate Actions Required

• Upgrade Flowise to version 3.1.0 or later immediately
• Audit existing upload directories for any suspicious JavaScript files
• Review Chatflow configurations to ensure MIME type restrictions are properly set
• Restrict network egress from Flowise servers to prevent reverse shell connections
• Implement web application firewall rules to block JavaScript file uploads

Patch Information

This vulnerability is fixed in Flowise version 3.1.0. Organizations should upgrade to this version or later to remediate the vulnerability. The patch implements proper server-side validation of file upload MIME types, preventing the bypass of frontend restrictions.

For detailed information about the security fix, refer to the GitHub Security Advisory.

Workarounds

• Deploy a reverse proxy or WAF to enforce strict MIME type validation before requests reach Flowise
• Remove or restrict access to the file upload functionality until patching is complete
• Implement network segmentation to isolate Flowise servers from sensitive internal resources
• Configure read-only permissions on upload directories where possible to prevent file writes

# Configuration example: Restrict file permissions on upload directory
chmod 755 /path/to/flowise/uploads
chown -R flowise:flowise /path/to/flowise/uploads

# Example: Block JavaScript MIME type at nginx reverse proxy level
location /api/v1/upload {
    if ($content_type ~* "application/javascript") {
        return 403;
    }
    proxy_pass http://flowise_backend;
}