Three smaller security issues are solved by this update
- The input filter fpr PHAR stubs was hardened
- XSS-vulberability at the frontend was fixed
- custom fields access rights on disabled fields are more secure
Bugfixes are concerning especially the tag component and the news module showing featured articles.
See detailed information on https://github.com/joomla/joomla-cms/milestone/38?closed=1
The update 3.8.12 was published on August 28, 2018.