### Impact

When extracting an archive to a directory, a crafted archive can read or write files outside that directory. The flaw is in the code that writes the parsed entries, so it affects every format decompress handles: tar, tar.gz, tar.bz2, and zip by default, plus any others added through the plugins option.

A link (hardlink) or symlink entry is created without checking where its target points. A hardlink can be aimed at any file the running process can read; that file then appears inside the output directory and its contents are exposed. A symlink can point outside the output directory and redirect a later write.

The path containment check used a string prefix comparison (`realPath.indexOf(outputPath) !== 0`). Output `/srv/out` does not contain `/srv/out-old`, but the prefix comparison treats it as inside, so an entry can escape into a sibling directory whose name starts with the output directory name.

File modes were applied as `mode & ~umask`, which does not remove the setuid, setgid, or sticky bits. A crafted entry can create a setuid or setgid file. This matters when extraction runs as root, for example in CI, containers, or install scripts.

Any code that extracts archives from an untrusted or attacker-influenced source is affected. Archives are commonly downloaded before extraction, so this is reachable over the network in many setups.

### Patches

Fixed in `@xhmikosr/decompress` 10.2.1 and 11.1.3. Link targets are now resolved and checked against the output directory, containment uses `path.relative`, and setuid, setgid, and sticky bits are removed.

The upstream `decompress` package is unmaintained, and all versions through its last release (4.2.1) have the same flaws. There is no upstream fix. Migrate to `@xhmikosr/decompress` 11.1.3 or later.

### Workarounds

Extract only archives you trust. Run extraction as a non-root user so the mode issue cannot create a privileged file. After extracting, reject any symlink or hardlink that points outside the target and any file with unexpected mode bits.

### Resources

* Related prior issue in the upstream project this package forks: CVE-2020-12265 / GHSA-qgfr-5hqp-vrw9
* Fix commits and releases:
* https://github.com/XhmikosR/decompress/releases/tag/v10.2.1
* https://github.com/XhmikosR/decompress/releases/tag/v11.1.3
* https://github.com/XhmikosR/decompress/commit/aca5aac
* https://github.com/XhmikosR/decompress/commit/281cefa
* https://github.com/XhmikosR/decompress/commit/60b5299