CVE-2025-59528

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)

• 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)

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.

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.

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

CVE-2025-59528 has a CVSS score of 10.0 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.0.6); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2025-59528? CVE-2025-59528 is a critical-severity code injection vulnerability in flowise (npm), affecting versions = 3.0.5. It is fixed in 3.0.6. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2025-59528? CVE-2025-59528 has a CVSS score of 10.0 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of flowise are affected by CVE-2025-59528? flowise (npm) versions = 3.0.5 is affected.
4. Is there a fix for CVE-2025-59528? Yes. CVE-2025-59528 is fixed in 3.0.6. Upgrade to this version or later.
5. Is CVE-2025-59528 exploitable, and should I be worried? Whether CVE-2025-59528 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2025-59528 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2025-59528? Upgrade flowise to 3.0.6 or later.