In PHP versions 7.3.x below 7.3.16 and 7.4.x below 7.4.4, while using mb_strtolower() function with UTF-32LE encoding, certain invalid strings could cause PHP to overwrite stack-allocated buffer. This could lead to memory corruption, crashes and potentially code execution.
https://www.tenable.com/security/tns-2021-14
https://www.php.net/ChangeLog-7.php#7.4.4
https://www.oracle.com/security-alerts/cpuoct2021.html
https://www.debian.org/security/2020/dsa-4719
https://usn.ubuntu.com/4330-2/
https://usn.ubuntu.com/4330-1/