Input validation vulnerability in GTM Server Side 1.1.1

The GTM Server Side plugin for WordPress is vulnerable to a type of cyber attack called Stored Cross-Site Scripting. Attackers who have access to the administrative level of a WordPress website can inject malicious code, such as web scripts, into pages of the website. When a user visits these pages, the injected code will execute. This vulnerability only affects multi-site installations and installations where additional security measures have been disabled. The most recent version of the GTM Server Side plugin (1.1.1) is vulnerable to this type of attack due to problems with input sanitization and output escaping.