FlowiseAI/Flowise · error · Error

Custom MCP script execution disabled. Configure CUSTOM_MCP_A

Error message

Custom MCP script execution disabled. Configure CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS environment variable.

What it means

Thrown by validateArgsForLocalFileAccess when CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS is unset or empty. The function splits that env var on commas to build allowedScriptPaths; an empty list means script-style local MCP execution is fully disabled by default, so any call to a script-path-based MCP server is refused.

Source

Thrown at packages/components/nodes/tools/MCP/core.ts:257

        },
        {
            name: name,
            description: description,
            schema: argsSchema
        }
    )
}

export const validateArgsForLocalFileAccess = (args: string[]): void => {
    const allowedScriptPaths = (process.env.CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS ?? '')
        .split(',')
        .map((s) => s.trim())
        .filter(Boolean)

    const scriptArg = args[0]

    if (allowedScriptPaths.length === 0)
        throw new Error('Custom MCP script execution disabled. Configure CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS environment variable.')

    if (!allowedScriptPaths.includes(scriptArg)) throw new Error('Custom MCP script path not in allowed list.')
}

export const validateCommandInjection = (args: string[]): void => {
    const dangerousPatterns = [
        // Shell metacharacters
        /[;&|`$(){}[\]<>]/,
        // Command chaining
        /&&|\|\||;;/,
        // Redirections
        />>|<<|>/,
        // Backticks and command substitution
        /`|\$\(/,
        // Process substitution
        /<\(|>\(/
    ]

View on GitHub (pinned to abe4a8601a)

Solutions

1. Set CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS to a comma-separated list of absolute script paths you trust (e.g. /opt/mcp/server.js).
2. Confirm the script path passed as args[0] exactly matches an entry in that list (no trailing slash, absolute).
3. If script execution is not required, route the MCP server through an allowed command instead and leave the list empty.

Example fix

# before
# CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS=

# after
CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS=/opt/mcp/server.js,/opt/mcp/other.js

const allowed = (process.env.CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS ?? '').split(',').map(s => s.trim()).filter(Boolean)
if (allowed.length === 0) {
  throw new Error('Configure CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS before enabling script-based MCP servers')
}

const scriptExecutionEnabled = (): boolean => (process.env.CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS ?? '').split(',').map(s => s.trim()).filter(Boolean).length > 0

try {
  validateArgsForLocalFileAccess(args)
} catch (e) {
  if (e.message.includes('Custom MCP script execution disabled')) {
    // operator must populate CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS
  }
  throw e
}

When it happens

Trigger: validateArgsForLocalFileAccess is invoked with any args array while CUSTOM_MCP_ALLOWED_ABSOLUTE_SCRIPT_PATHS is unset or empty. Reached via validateMCPServerConfig when serverParams.args is a non-empty array.

Common situations: Default deployment where the operator has not opted into local script MCP servers; env var misspelled; a custom MCP node pointing at a local .js/.py script before the allow-list is configured.

Related errors

• Environment variable '${key}' is not allowed. Permitted: ${[
• Command '${serverParams.command}' is not allowed. Permitted:
• Security validation failed: ${error.message}
• Custom MCP script path not in allowed list.
• Invalid server configuration

AI-assisted analysis of FlowiseAI/Flowise@abe4a8601a (2026-08-12). Data as JSON: /api/errors/67c946a381ce8627. Report an issue: GitHub.