Description

Basic FTP has Path Traversal Vulnerability in its downloadToDir() method The basic-ftp library contains a path traversal vulnerability in the downloadToDir() method. A malicious FTP server can send directory listings with filenames containing path traversal sequences (../) that cause files to be written outside the intended download directory.

Source-to-Sink Flow

1. SOURCE: FTP server sends LIST response
└─> "-rw-r--r-- 1 user group 1024 Jan 20 12:00 ../../../etc/passwd"

2. PARSER: parseListUnix.ts:100 extracts filename
└─> file.name = "../../../etc/passwd"

3. VALIDATION: parseListUnix.ts:101 checks
└─> if (name === "." || name === "..") ❌ (only filters exact matches)...
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Vulnerable Code

File: src/Client.ts:707

protected async _downloadFromWorkingDir(localDirPath: string): Promise<void> {
await ensureLocalDirectory(localDirPath)
for (const file of await this.list()) {
const localPath = join(localDirPath, file.name) // ⚠️ VULNERABLE
// file.name comes from untrusted FTP server, no sanitization
await this.downloadTo(localPath, file.name)
}
}...
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Root Cause: - Parser validation (parseListUnix.ts:101) only filters exact . or .. entries - No sanitization of ../ sequences in filenames

path.join() doesn't prevent traversal, fs.open() resolves paths

Impact

A malicious FTP server can: - Write files to arbitrary locations on the client filesystem - Overwrite critical system files (if user has write access) - Potentially achieve remote code execution

Affected Versions

Tested: v5.1.0

Likely: All versions (code pattern exists since initial implementation)

Mitigation

Workaround: Do not use downloadToDir() with untrusted FTP servers.

Fix: Sanitize filenames before use:

import { basename } from 'path'

// In _downloadFromWorkingDir:
const sanitizedName = basename(file.name) // Strip path components
const localPath = join(localDirPath, sanitizedName)

Aliases

References