This security release fixes third-party dependencies included in or required by Drupal core. As described in TYPO3-PSA-2019-007: By-passing protection of Phar Stream Wrapper Interceptor:

In order to intercept file invocations like file_exists or stat on compromised Phar archives the base name has to be determined and checked before allowing to be handled by PHP Phar stream handling. [...]

The current implementation is vulnerable to path traversal leading to scenarios where the Phar archive to be assessed is not the actual (compromised) file.

The known vulnerability in Drupal core requires the "administer themes" permission. However, additional vulnerabilities may exist in contributed or custom modules, so site should still update even if they do not grant this permission.