This is a security release.

• SECURITY Fixes CVE-2021-34551, a complex RCE affecting Windows hosts. See SECURITY.md for details.
• The fix for this issue changes the way that language files are loaded. While they remain in the same PHP-like format, they are processed as plain text, and any code in them will not be run, including operations such as concatenation using the . operator.
• Deprecation The current translation file format using PHP arrays is now deprecated; the next major version will introduce a new format.
• SECURITY Fixes CVE-2021-3603 that may permit untrusted code to be run from an address validator. See SECURITY.md for details.
• The fix for this issue includes a minor BC break: callables injected into validateAddress, or indirectly through the $validator class property, may no longer be simple strings. If you want to inject your own validator, provide a closure instead of a function name.
• Haraka message ID strings are now recognised

Thanks to Vikrant Singh Chauhan, listensec.com, and the WordPress security team for reporting and assistance with this release.