FlowiseAI/Flowise · error · Error

Environment variable '${key}' contains null byte

Error message

Environment variable '${key}' contains null byte

What it means

Thrown by validateEnvironmentVariables after a key passes the allow-list but its string value contains a null byte ('\0'). Null bytes can truncate or manipulate strings in C-based process environments and are a classic poisoning vector, so the validator rejects them outright.

Source

Thrown at packages/components/nodes/tools/MCP/core.ts:305

/**
 * Validates user-supplied env vars against the operator-controlled allow-list in
 * `CUSTOM_MCP_ALLOWED_ENV_VARS` (comma-separated names). Empty = none allowed.
 */
export const validateEnvironmentVariables = (env: Record<string, any>): void => {
    const allowedEnvVars = new Set(
        (process.env.CUSTOM_MCP_ALLOWED_ENV_VARS ?? '')
            .split(',')
            .map((s) => s.trim())
            .filter(Boolean)
    )

    for (const [key, value] of Object.entries(env)) {
        if (!allowedEnvVars.has(key)) {
            throw new Error(`Environment variable '${key}' is not allowed. Permitted: ${[...allowedEnvVars].join(', ') || '(none)'}`)
        }

        if (typeof value === 'string' && value.includes('\0')) {
            throw new Error(`Environment variable '${key}' contains null byte`)
        }
    }
}

/**
 * Validates that command arguments don't contain flags that enable arbitrary code execution
 * This prevents attacks where whitelisted commands are used with dangerous flags
 * (e.g., "npx -c malicious-command" or "python -c malicious-code")
 * @param command The command to validate
 * @param args The arguments to validate
 */
export const validateCommandFlags = (command: string, args: string[]): void => {
    // Define dangerous flags for each command that enable code execution
    const dangerousFlagsByCommand: Record<string, string[]> = {
        npx: [
            '-c', // Execute shell commands
            '--call', // Execute shell commands
            '--shell-auto-fallback', // Shell execution fallback

View on GitHub (pinned to abe4a8601a)

Solutions

1. Sanitize env values by stripping null bytes before assigning: value.replace(/\0/g, '').
2. Validate input encoding at the source (reject non-printable characters in secret fields).
3. Re-issue the token/secret if it legitimately cannot be represented without null bytes.

Example fix

// before
env['API_KEY'] = rawToken // contains '\0'

// after
env['API_KEY'] = String(rawToken).replace(/\0/g, '')

for (const [k, v] of Object.entries(env)) {
  if (typeof v === 'string' && v.includes('\0')) {
    throw new Error(`Refusing to forward env var '${k}': contains null byte`)
  }
}

const envValuesHaveNoNullBytes = (env: Record<string, any>): boolean =>
  Object.values(env).every(v => typeof v !== 'string' || !v.includes('\0'))

try {
  validateEnvironmentVariables(env)
} catch (e) {
  if (e.message.includes('contains null byte')) {
    // strip null bytes from the named value before retrying
  }
  throw e
}

When it happens

Trigger: A string value in serverParams.env includes the literal '\0' character. Could come from binary/encoded user input, malformed base64 decoding, or a malicious payload designed to terminate a string early.

Common situations: User pastes a value with embedded control characters; a token or secret was decoded from base64/hex with trailing null bytes; test fixtures containing raw bytes.

Related errors

• Argument contains potentially dangerous characters: "${arg}"
• Environment variable '${key}' is not allowed. Permitted: ${[
• Argument '${arg}' is not allowed for command '${command}'.
• Argument '${arg}' contains flag '${flag}' that is not allowe
• Argument '${arg}' contains dangerous flag '-${ch}' for comma

AI-assisted analysis of FlowiseAI/Flowise@abe4a8601a (2026-08-12). Data as JSON: /api/errors/55511c3e0d6cb20f. Report an issue: GitHub.