The Media Library module has a security vulnerability whereby it doesn't sufficiently restrict access to media items in certain configurations.

Solution:

If you are using Drupal 8.7.x, you should upgrade to Drupal 8.7.11.
If you are using Drupal 8.8.x, you should upgrade to Drupal 8.8.1.
Versions of Drupal 8 prior to 8.7.x are end-of-life and do not receive security coverage.

Alternatively, you may mitigate this vulnerability by unchecking the "Enable advanced UI" checkbox on /admin/config/media/media-library. (This mitigation is not available in 8.7.x.)

References

• https://github.com/FriendsOfPHP/security-advisories/blob/master/drupal/drupal/2019-12-18-3.yaml
• https://www.drupal.org/sa-core-2019-011