Cross-site Scripting (also known as XSS) is a web security vulnerability that allows an attacker to compromise the interactions that users have with a vulnerable application. XSS is a client-side code injection attack. The attacker aims to execute malicious scripts in a web browser of the victim by including malicious code in a legitimate web page or web application.
Cross Site Scripting vulnerabilities are the most common vulnerability found in WordPress plugins by a significant margin.
Preventing cross-site scripting is trivial in some cases but can be much harder depending on the complexity of the application and the ways it handles user-controllable data.
The following suggestions can help protect your users against XSS attacks:
Sanitize user input:
Validate to catch potentially malicious user-provided input.
Encode output to prevent potentially malicious user-provided data from triggering automatic load-and-execute behavior by a browser.
Limit use of user-provided data:
Only use where it’s necessary.
Utilize the Content Security Policy:
Provides additional levels of protection and mitigation against XSS attempts.
Regularly use a web vulnerability scanning tool to identify XSS vulnerabilities in your software.
While thease type of attacks continue to grow and have success, especialy on WordPress web sites, a bit od testing and usage of methods such as the above listed, is a great way to protect users of your application from the majority of XSS attacks.