Cross-site Scripting (XSS) is a kind of code injection attack that occurs on the client side. The attacker intends to run harmful scripts in the victim's web browser by embedding malicious code in a genuine web page or online application. The primary attack takes place when the victim views the malicious web page or web application. The malicious script is sent to the browser of the user through a web page or web application. Forums, message boards, and online sites that enable comments are vulnerable vehicles that are often utilized for cross-site scripting assaults.
How to Find XSS Vulnerabilities?
The content security policy (CSP) is a browser feature designed to reduce the effect of cross-site scripting and other risks. If a CSP-enabled application has XSS-like behavior, the CSP can hamper or prevent exploitation of the risk. Often, the CSP can be bypassed in order to attack the underlying vulnerability.
Dangling Markup Injection
Dangling markup injection is a method for capturing data across domains when a complete cross-site scripting vulnerability is not available owing to input filters or other protections. It is often used to record sensitive data that is accessible to other users, such as CSRF tokens, which can be used to undertake illegal practices on the user's behalf.
How to Prevent XSS Attacks?
It is difficult to prevent cross-site scripting (XSS). Specific preventive measures vary depending on the subtype of XSS vulnerability, the context of user input, and the programming framework. Yet, there are certain broad strategic concepts you should adhere to in order to make your online application secure.
1. Train and Maintain Awareness
To ensure the security of your online application, everyone engaged in its development must be aware of the dangers associated with XSS vulnerabilities. All of your developers, QA employees, DevOps, and SysAdmins should get appropriate security training. You might begin by directing them to this website.
2. Don’t Trust Any User Input
Consider all user input to be untrustworthy. Any user input utilized as a part of HTML output increases the danger of XSS. Treat input from authorized and/or internal users similarly to public input.
3. Use Escaping/Encoding
4. Sanitize HTML
You can't escape/encode input from the user if it contains HTML since it will break valid tags. Utilize a reliable and validated library to parse and clean HTML in such situations. Select a library based on your development languages, such as HtmlSanitizer for.NET or SanitizeHelper for Ruby on Rails.
5. Set the HTTPOnly Flag
6. Use a Content Security Policy
Implement a Content Security Policy to help limit the effects of a potential XSS issue (CSP). CSP is an HTTP response header that allows you to define the dynamic resources that can be loaded based on the request source.
7. Scan Regularly
XSS vulnerabilities can be created by your developers or by third-party libraries, modules, or software. You should check your online apps on a regular basis with the help of a web vulnerability scanner such as Acunetix.
These preventive solutions cover the majority of XSS attack vectors, but not all. Deploy static and dynamic application scanning technologies in your workflow to discover and address security vulnerabilities.