It was discovered that php-phpseclib, a pure-PHP implementation of various cryptographic and arithmetic algorithms (v2), mishandles RSA PKCS#1 v1.5 signature verification. An attacker may get invalid signatures accepted, bypassing authorization control in specific situations.
For Debian 10 buster, this problem has been fixed in version 2.0.30-2~deb10u1.
We recommend that you upgrade your php-phpseclib packages.
For the detailed security status of php-phpseclib please refer to its security tracker page at: https://security-tracker.debian.org/tracker/php-phpseclib
Further information about Debian LTS security advisories, how to apply these updates to your system and frequently asked questions can be found at: https://wiki.debian.org/LTS