A php7.4 security update has been released for Debian GNU/Linux 11 to address an out-of-bounds read and write flaw was discovered in the PHP-FPM code.

DSA 4992-1: php7.4 security update

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Debian Security Advisory DSA-4992-1 security@debian.org
  https://www.debian.org/security/ Salvatore Bonaccorso
October 25, 2021   https://www.debian.org/security/faq
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Package : php7.4
CVE ID : CVE-2021-21703
Debian Bug : 997003

An out-of-bounds read and write flaw was discovered in the PHP-FPM code,
which could result in escalation of privileges from local unprivileged
user to the root user.

For the stable distribution (bullseye), this problem has been fixed in
version 7.4.25-1+deb11u1.

We recommend that you upgrade your php7.4 packages.

For the detailed security status of php7.4 please refer to its security
tracker page at:
  https://security-tracker.debian.org/tracker/php7.4

Further information about Debian Security Advisories, how to apply
these updates to your system and frequently asked questions can be
found at:   https://www.debian.org/security/