What it is
PHPUnit allows remote attackers to execute arbitrary PHP code via HTTP POST data beginning with a "<?php " substring, as demonstrated by an attack on a site with an exposed /vendor folder, i.e., external access to the /vendor/phpunit/phpunit/src/Util/PHP/eval-stdin.php URI.
What to do
Apply updates per vendor instructions.
Vendor notices and references
Description and required action as published by CISA. Full record at NVD.