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)
└─> "../../../etc/passwd" !== "." && !== ".." ✅ PASSES
4. SINK: Client.ts:707 uses filename directly
└─> const localPath = join(localDirPath, file.name)
└─> join("/safe/download", "../../../etc/passwd")
└─> Result: "/safe/download/../../../etc/passwd" → resolves to "/etc/passwd"
5. FILE WRITE: Client.ts:512 opens file
└─> fsOpen(localPath, "w") → writes to /etc/passwd (outside intended directory)
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)
}
}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)References
thecasual Reporter
The
basic-ftplibrary contains a path traversal vulnerability in thedownloadToDir()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
Vulnerable Code
File:
src/Client.ts:707Root Cause:
- Parser validation (
parseListUnix.ts:101) only filters exact.or..entries- No sanitization of
../sequences in filenamespath.join()doesn't prevent traversal,fs.open()resolves pathsImpact
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
Mitigation
Workaround: Do not use
downloadToDir()with untrusted FTP servers.Fix: Sanitize filenames before use:
References