Affected: npm: tar

### Summary

`tar` (node-tar) applies a PAX extended header's `size=` record (and other PAX overrides) to the **next header entry of any type**, including intermediary metadata headers such as a GNU long-name (`L`) or long-link (`K`) entry. Per POSIX pax, a PAX extended header (`x`) describes the *next file entry*, not the intermediary extension headers that may sit between the `x` header and the file it annotates. Because node-tar lets the PAX `size` override the byte length of an intervening `L`/`K`/`x` header, an attacker can desynchronize node-tar's stream cursor relative to every other mainstream tar implementation (GNU tar, libarchive/bsdtar, Python `tarfile`, and the now-fixed `tar-rs` / `astral-tokio-tar`).

The result is a tar parser **interpretation differential** (CWE-436): a single crafted archive yields a different set of members under node-tar than under the reference tar tools. An attacker can use this to hide a member from one parser while it is visible to another, which defeats security tooling whose scanner and extractor disagree on archive contents (e.g. a malware/secret scanner that lists entries with one library while a downstream step extracts with another). node-tar is one of the most widely deployed JavaScript tar libraries (it backs `npm`'s own package-tarball handling and is a transitive dependency of a very large fraction of the npm ecosystem), so the blast radius for "files that extract differently depending on the tool" is broad.

This is the sam…