CVE-2025-59528

Flowise is a drag & drop user interface to build a customized large language model flow. In version 3.0.5, Flowise is vulnerable to remote code execution. The CustomMCP node allows users to input configuration settings for connecting to an external MCP 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 child_process and fs. This issue has been patched in version 3.0.6.

Flowise is a drag & drop user interface to build a customized large language model flow. In version 3.0.5, Flowise is vulnerable to remote code execution. The CustomMCP node allows users to input configuration settings for connecting to an external MCP 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 child_process and fs. This issue has been patched in version 3.0.6.

## 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 `child_process` 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)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/server/src/routes/node-load-methods/index.ts#L5) - **Taint 02: Controller** [`index.ts` (Line 57–78)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/server/src/controllers/nodes/index.ts#L57-L78) - **Taint 03: Service** [`index.ts` (Line 91–94)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/server/src/services/nodes/index.ts#L91-L94) - **Taint 04: CustomMCP Node Entry Point** [`CustomMCP.ts` (Line 132)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/components/nodes/tools/MCP/CustomMCP/CustomMCP.ts#L132) - **Taint 05: Variable Substitution** [`CustomMCP.ts` (Line 220)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/components/nodes/tools/MCP/CustomMCP/CustomMCP.ts#L220) - **Taint 06: Dangerous Constructor Execution** [`CustomMCP.ts` (Line 262–270)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/components/nodes/tools/MCP/CustomMCP/CustomMCP.ts#L262-L270) ## 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(\"child_process\");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.

## 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 `child_process` 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)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/server/src/routes/node-load-methods/index.ts#L5) - **Taint 02: Controller** [`index.ts` (Line 57–78)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/server/src/controllers/nodes/index.ts#L57-L78) - **Taint 03: Service** [`index.ts` (Line 91–94)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/server/src/services/nodes/index.ts#L91-L94) - **Taint 04: CustomMCP Node Entry Point** [`CustomMCP.ts` (Line 132)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/components/nodes/tools/MCP/CustomMCP/CustomMCP.ts#L132) - **Taint 05: Variable Substitution** [`CustomMCP.ts` (Line 220)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/components/nodes/tools/MCP/CustomMCP/CustomMCP.ts#L220) - **Taint 06: Dangerous Constructor Execution** [`CustomMCP.ts` (Line 262–270)](https://github.com/FlowiseAI/Flowise/blob/5930f1119c655bcf8d2200ae827a1f5b9fec81d0/packages/components/nodes/tools/MCP/CustomMCP/CustomMCP.ts#L262-L270) ## 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(\"child_process\");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.