CVE-2026-13346: pip would incorrectly handle doubly-encoded package URLs from indexes

pip would incorrectly handle doubly-encoded package URLs from indexes allowing for files to be installed to arbitrary locations on disk even when installing wheels.

This vulnerability requires downloading or installing a package from a malicious package index to succeed, malicious packages alone are not able to exploit this vulnerability. Note that this vulnerability only materially impacts users running pip download with the --only-binary option as installing source distributions from an untrusted index is already an unsafe operation that executes code during install time.