CVE-2026-40933: Flowiseai Flowise RCE Vulnerability

CVE-2026-40933 Overview

CVE-2026-40933 is a critical command injection vulnerability in Flowise, a drag & drop user interface for building customized large language model (LLM) flows. The vulnerability exists in the Model Context Protocol (MCP) adapter's unsafe serialization of stdio commands, allowing authenticated attackers to execute arbitrary commands on the underlying operating system.

Critical Impact

An authenticated attacker can achieve full remote code execution on the Flowise server by exploiting insufficient input sanitization in the Custom MCP configuration, potentially compromising the entire AI/ML infrastructure and any connected systems.

Affected Products

• Flowise versions prior to 3.1.0
• FlowiseAI Flowise (all configurations using MCP stdio servers)
• Self-hosted Flowise instances with Custom MCP functionality enabled

Discovery Timeline

• 2026-04-21 - CVE CVE-2026-40933 published to NVD
• 2026-04-23 - Last updated in NVD database

Technical Details for CVE-2026-40933

Vulnerability Analysis

This vulnerability represents a command injection flaw (CWE-78) in Flowise's handling of MCP stdio server configurations. When users add a new MCP server through the Custom MCP configuration interface at http://localhost:3000/canvas, the application fails to properly sanitize command inputs despite having several security controls in place.

The root issue lies in the insufficient validation logic when processing stdio commands. While Flowise implements input sanitization functions including validateCommandInjection and validateArgsForLocalFileAccess, along with a predefined allowlist of safe commands, these protections can be bypassed. Attackers can leverage legitimate commands from the allowlist (such as npx) combined with code execution arguments to achieve command injection.

The vulnerability allows authenticated users to chain allowed commands with malicious arguments that execute arbitrary code. This represents a significant security risk as it bridges the gap between authenticated access and full system compromise.

Root Cause

The vulnerability stems from incomplete input validation in the MCP stdio command serialization process. The security controls focus on blocking specific dangerous commands but fail to account for how safe commands can be combined with malicious arguments. The allowlisted commands like npx can accept parameters that enable direct code execution, such as -c followed by arbitrary shell commands.

This is a classic example of a blocklist-based security approach failing to anticipate all attack vectors, where the sanitization checks do not consider the full context of how commands and their arguments interact.

Attack Vector

The attack requires authenticated access to the Flowise application and proceeds through the following mechanism:

1. An authenticated attacker navigates to the Custom MCP configuration interface
2. The attacker adds a new MCP server using the stdio transport type
3. Instead of a legitimate command, the attacker provides an allowlisted command (e.g., npx) combined with code execution arguments
4. The insufficient validation allows arguments like -c touch /tmp/pwn to pass through
5. When the MCP server is initialized, the malicious command executes on the underlying operating system

The attack leverages network accessibility and requires only low-privilege authenticated access. The scope is changed, meaning a successful exploit can impact resources beyond the vulnerable component, affecting the confidentiality, integrity, and availability of the entire host system.

For detailed technical analysis, refer to the OX Security Blog Analysis and the OX Security Blog Advisory.

Detection Methods for CVE-2026-40933

Indicators of Compromise

• Unexpected MCP stdio server configurations added to Flowise instances
• Unusual process spawning from the Flowise application process
• Shell commands executed with suspicious arguments containing -c or similar execution flags
• Unexplained file creation or modification in system directories like /tmp/
• Network connections or outbound traffic originating from the Flowise server to unknown destinations

Detection Strategies

• Monitor Flowise configuration changes, particularly new MCP server additions through audit logs
• Implement process monitoring to detect child processes spawned by the Flowise application with suspicious command-line arguments
• Deploy file integrity monitoring on critical system directories to detect unauthorized file creation or modification
• Review authentication logs for unusual access patterns to the Custom MCP configuration endpoint

Monitoring Recommendations

• Enable detailed logging for all MCP server configuration changes in Flowise
• Configure SIEM rules to alert on command execution patterns involving allowlisted commands with code execution arguments
• Implement behavioral analysis to detect anomalous process chains originating from the Flowise application
• Monitor for reconnaissance activities that may precede exploitation attempts

How to Mitigate CVE-2026-40933

Immediate Actions Required

• Upgrade Flowise to version 3.1.0 or later immediately
• Audit all existing MCP server configurations for suspicious or unauthorized entries
• Review access logs to identify potential exploitation attempts
• Restrict access to the Custom MCP configuration interface to trusted administrators only
• Consider temporarily disabling the Custom MCP functionality until the upgrade is complete

Patch Information

The vulnerability is fixed in Flowise version 3.1.0. Organizations should upgrade to this version or later to remediate the vulnerability. The fix addresses the input sanitization bypass by implementing more comprehensive validation of command arguments in the MCP stdio adapter.

For official patch details and security guidance, refer to the GitHub Security Advisory.

Workarounds

• Implement network segmentation to isolate Flowise instances from critical infrastructure
• Restrict user registration and authentication to only trusted personnel
• Deploy a Web Application Firewall (WAF) with rules to detect and block command injection patterns
• Remove or disable the Custom MCP functionality if not required for operations
• Implement additional access controls requiring multi-factor authentication for administrative functions

# Example: Restrict access to Flowise canvas endpoint via nginx
location /canvas {
    # Restrict to internal network only
    allow 10.0.0.0/8;
    allow 192.168.0.0/16;
    deny all;
    
    # Require additional authentication
    auth_basic "Restricted Access";
    auth_basic_user_file /etc/nginx/.htpasswd;
    
    proxy_pass http://localhost:3000;
}