GHSA-34x7-hfp2-rc4v

Summary

node-tar contains a vulnerability where the security check for hardlink entries uses different path resolution semantics than the actual hardlink creation logic. This mismatch allows an attacker to craft a malicious TAR archive that bypasses path traversal protections and creates hardlinks to arbitrary files outside the extraction directory.

Details

The vulnerability exists in lib/unpack.js. When extracting a hardlink, two functions handle the linkpath differently:

Security check in [STRIPABSOLUTEPATH]:

const entryDir = path.posix.dirname(entry.path);
const resolved = path.posix.normalize(path.posix.join(entryDir, linkpath));
if (resolved.startsWith('../')) { /* block */ }

Hardlink creation in [HARDLINK]:

const linkpath = path.resolve(this.cwd, entry.linkpath);
fs.linkSync(linkpath, dest);

Example: An application extracts a TAR using tar.extract({ cwd: '/var/app/uploads/' }). The TAR contains entry a/b/c/d/x as a hardlink to ../../../../etc/passwd.

• Security check resolves the linkpath relative to the entry's parent directory: a/b/c/d/ + ../../../../etc/passwd = etc/passwd. No ../ prefix, so it passes.

• Hardlink creation resolves the linkpath relative to the extraction directory (this.cwd): /var/app/uploads/ + ../../../../etc/passwd = /etc/passwd. This escapes to the system's /etc/passwd.

The security check and hardlink creation use different starting points (entry directory a/b/c/d/ vs extraction directory /var/app/uploads/), so the same linkpath can pass validation but still escape. The deeper the entry path, the more levels an attacker can escape.

PoC

Setup

Create a new directory with these files:

poc/
├── package.json
├── secret.txt          ← sensitive file (target)
├── server.js           ← vulnerable server
├── create-malicious-tar.js
├── verify.js
└── uploads/            ← created automatically by server.js
    └──...