CVE-2026-59948: Composer: Arbitrary file write outside vendor via malicious transitive package name

A maliciously crafted package, published on an untrusted third party repository other than Packagist.org or Private Packagist, can cause Composer to write files outside the vendor/ directory and outside your project, with attacker-controlled content, during a normal install or update through using an invalid package name, which was not correctly validated by Composer.

This is an arbitrary file write that can be used to execute code outside the Composer project’s context in which you expected the package code to execute (for example by writing shell startup files, SSH authorized_keys, or a cron entry). It is a supply-chain issue: it requires a malicious or compromised package to be present in the dependency graph, it is not otherwise remotely exploitable against a machine.

The fix makes Composer validate every package produced by dependency resolution before anything is written to composer.lock or installed, and abort with a security error if a package name is not a valid vendor/package name.