The Custom CSS, JS & PHP plugin for WordPress is vulnerable to Reflected Cross-Site Scripting due to the use of add_query_arg & remove_query_arg without appropriate escaping on the URL in all versions up to, and including, 2.3.0. This makes it possible for unauthenticated attackers to inject arbitrary web scripts in pages that execute if they can successfully trick a user into performing an action such as clicking on a link.

Improper Neutralization of Input During Web Page Generation (XSS or 'Cross-site Scripting') vulnerability in Harpreet Singh Ajax Custom CSS/JS allows Reflected XSS.This issue affects Ajax Custom CSS/JS: from n/a through 2.0.4.

The Add Custom CSS and JS WordPress plugin through 1.20 does not have CSRF check in some places, and is missing sanitisation as well as escaping, which could allow attackers to make logged in as author and above add Stored XSS payloads via a CSRF attack

The Custom CSS, JS & PHP plugin for WordPress is vulnerable to Cross-Site Request Forgery in versions up to, and including, 2.0.7. This is due to missing or incorrect nonce validation on the save() function. This makes it possible for unauthenticated attackers to save code snippets via a forged request granted they can trick a site administrator into performing an action such as clicking on a link.

Cross-site scripting vulnerability in Simple Custom CSS and JS prior to version 3.4 allows remote attackers to inject arbitrary web script or HTML via unspecified vectors.