This security release fixes third-party dependencies included in or required by Drupal core.
Validation messages were not escaped when using the form theme of the PHP templating engine which, when validation messages may contain user input, could result in an XSS.
Service IDs derived from unfiltered user input could result in the execution of any arbitrary code, resulting in possible remote code execution.
This fixes situations where part of an expiry time in a cookie could be considered part of the username, or part of the username could be considered part of the expiry time. An attacker could modify the remember me cookie and authenticate as a different user. This attack is only possible if remember me functionality is enabled and the two users share a password hash or the password hashes (e.g. UserInterface::getPassword()) are null for all users (which is valid if passwords are checked by an external system, e.g. an SSO).