The EG-Series plugin for WordPress is vulnerable to Stored Cross-Site Scripting via the plugin's [series] shortcode in all versions up to, and including, 2.1.1 due to insufficient input sanitization and output escaping on user supplied attributes in the shortcode_title function. This makes it possible for authenticated attackers - with contributor-level access and above, on sites with the Classic Editor plugin activated - to inject arbitrary JavaScript code in the titletag attribute that will execute whenever a user access an injected page.
The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special characters such as "<", ">", and "&" that could be interpreted as web-scripting elements when they are sent to a downstream component that processes web pages.