Input validation vulnerability in EG-Series 2.1.1

The EG-Series plugin for WordPress has a security vulnerability that allows attackers to inject harmful code through the [series] shortcode. This can happen on any version of the plugin up to 2.1.1. The issue is caused by not properly checking and protecting user input in the shortcode_title function. As a result, attackers who have contributor-level access or higher and have the Classic Editor plugin enabled can add malicious JavaScript code to the titletag attribute, which will run whenever someone views the affected page.