Contao is an Open Source CMS. In versions starting from 5.0.0 and prior to 5.3.38 and 5.6.1, if a news feed contains protected news archives, their news items are not filtered and become publicly available in the RSS feed. This issue has been patched in versions 5.3.38 and 5.6.1. A workaround involves not adding protected news archives to the news feed page.
https://github.com/contao/contao/security/advisories/GHSA-w53m-gxvg-vx7p
https://github.com/contao/contao/commit/e75f46b11974fbf7a4652e65c19ad6ca84c59271
https://contao.org/en/security-advisories/information-disclosure-in-the-news-module