This security advisory fixes multiple vulnerabilities in both Drupal 7 and Drupal 8. See below for a list.
The PHP functions which Drupal provides for HTML escaping are not affected.
Private file access bypass - Moderately Critical - Drupal 7
When using Drupal's private file system, Drupal will check to make sure a user has access to a file before allowing the user to view or download it. This check fails under certain conditions in which one module is trying to grant access to the file and another is trying to deny it, leading to an access bypass vulnerability.
jQuery vulnerability with untrusted domains - Moderately Critical - Drupal 7
A jQuery cross site scripting vulnerability is present when making Ajax requests to untrusted domains. This vulnerability is mitigated by the fact that it requires contributed or custom modules in order to exploit.
For Drupal 8, this vulnerability was already fixed in Drupal 8.4.0 as a side effect of upgrading Drupal core to use a newer version of jQuery. For Drupal 7, it is fixed in the current release (Drupal 7.57) for jQuery 1.4.4 (the version that ships with Drupal 7 core) as well as for other newer versions of jQuery that might be used on the site, for example using the jQuery Update module.
Language fallback can be incorrect on multilingual sites with node access restrictions - Moderately Critical - Drupal 8
When using node access controls with a multilingual site, Drupal marks the untranslated version of a node as the default fallback for access queries. This fallback is used for languages that do not yet have a translated version of the created node. This can result in an access bypass vulnerability.
This issue is mitigated by the fact that it only applies to sites that a) use the Content Translation module; and b) use a node access module such as Domain Access which implement hook_node_access_records().
Note that the update will mark the node access tables as needing a rebuild, which will take a long time on sites with a large number of nodes.
External link injection on 404 pages when linking to the current page - Less Critical - Drupal 7
Drupal core has an external link injection vulnerability when the language switcher block is used. A similar vulnerability exists in various custom and contributed modules. This vulnerability could allow an attacker to trick users into unwillingly navigating to an external site.