Affected: npm: tar

### Summary `tar.extract()` in Node `tar` allows an attacker-controlled archive to create a hardlink inside the extraction directory that points to a file outside the extraction root, using default options.

This enables **arbitrary file read and write** as the extracting user (no root, no chmod, no `preservePaths`).

Severity is high because the primitive bypasses path protections and turns archive extraction into a direct filesystem access primitive.

### Details The bypass chain uses two symlinks plus one hardlink:

1. `a/b/c/up -> ../..` 2. `a/b/escape -> c/up/../..` 3. `exfil` (hardlink) -> `a/b/escape/<target-relative-to-parent-of-extract>`

Why this works:

- Linkpath checks are string-based and do not resolve symlinks on disk for hardlink target safety. - See `STRIPABSOLUTEPATH` logic in: - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:255` - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:268` - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:281`

- Hardlink extraction resolves target as `path.resolve(cwd, entry.linkpath)` and then calls `fs.link(target, destination)`. - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:566` - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:567` - `../tar-audit-setuid - CVE/node_modules/tar/dist/commonjs/unpack.js:703`

- Parent directory safety checks (`mkdir` + symlink detection) are applied to the destination p…