GHSA-5rq4-664w-9x2c

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`

```typescript 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:

```typescript import { basename } from 'path'

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