The release notes state: “Props to Alex Concha of the WordPress Security Team for their work in hardening deserialization requests.”
blog.wpscan.com/2020/10/30/wordpress-5.5.2-security-release.html
github.com/WordPress/wordpress-develop/commit/add6bedf3a53b647d0ebda2970057912d3cd79d3
wordpress.org/news/2020/10/wordpress-5-5-2-security-and-maintenance-release/