In GnuPG before 2.5.5, if a user chooses to import a certificate with certain crafted subkey data that lacks a valid backsig or that has incorrect usage flags, the user loses the ability to verify signatures made from certain other signing keys, aka a "verification DoS."
https://lists.gnupg.org/pipermail/gnupg-announce/2025q1/000491.html
https://dev.gnupg.org/rG48978ccb4e20866472ef18436a32744350a65158