Description

Cause of the Vulnerability

The CustomMCP node allows users to input configuration settings for connecting to an external MCP (Model Context Protocol) server. This node parses the user-provided mcpServerConfig string to build the MCP server configuration. However, during this process, it executes JavaScript code without any security validation.

Specifically, inside the convertToValidJSONString function, user input is directly passed to the Function() constructor, which evaluates and executes the input as JavaScript code. Since this runs with full Node.js runtime privileges, it can access dangerous modules such as childprocess and fs.

Vulnerability Flow

1. User Input Received: Input is provided via the API endpoint /api/v1/node-load-method/customMCP through the mcpServerConfig parameter. 2. Variable Substitution: The substituteVariablesInString function replaces template variables like $vars.xxx, but no security filtering is applied during this step. 3. Dangerous Code Execution: The convertToValidJSONString function executes the input using Function('return ' + inputString)(). If the inputString contains malicious code, it gets executed in the global Node.js context, allowing actions such as command execution and file system access.

Taint Flow

- Taint 01: Route Registration index.ts (Line 5)

- Taint 02: Controller index.ts (Line 57–78)

- Taint 03: Service index.ts (Line 91–94)

- Taint 04: CustomMCP Node Entry Point CustomMCP.ts (Line 132)

- Taint 05: Variable Substitution CustomMCP.ts (Line 220)

- Taint 06: Dangerous Constructor Execution CustomMCP.ts (Line 262–270)

Proof of Concept (PoC)

bash curl -X POST http://localhost:3000/api/v1/node-load-method/customMCP \ -H "Content-Type: application/json" \ -H "Authorization: Bearer tmY1fIjgqZ6-nWUuZ9G7VzDtlsOiSZlDZjFSxZrDd0Q" \ -d '{ "loadMethod": "listActions", "inputs": { "mcpServerConfig": "({x:(function(){const cp = process.mainModule.require(\"childprocess\");cp.execSync(\"echo !!RCE-OK!! >/tmp/RCE.txt\");return 1;})()})" } }' <img width="1907" height="958" alt="image" src="https://github.com/user-attachments/assets/78b50eb1-67af-4c8b-97ea-7e2c05426962" />

When executed, this creates a file /tmp/RCE.txt on the server, confirming command execution.

Impact

Complete System Takeover and Infrastructure Threat

This vulnerability allows attackers to execute arbitrary JavaScript code on the Flowise server, leading to:

- Full system compromise - File system access - Command execution - Sensitive data exfiltration

As only an API token is required, this poses an extreme security risk to business continuity and customer data.