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.
A web page or web app is susceptible to XSS if the output it creates contains unsanitized user input. The victim's browser must then parse this user input. In VBScript, ActiveX, Flash, and even CSS, XSS attacks are conceivable. But they are most common in JavaScript because JavaScript is an important part of most browser experiences.
How to Find XSS Vulnerabilities?
XSS vulnerabilities in web applications can be difficult to detect and prevent. The easiest method to detect problems is to undertake a security assessment of the code and look for any places where HTTP request input could potentially find its way into the HTML output. It should be noted that numerous HTML elements can be used to send malicious JavaScript. Although Nessus, Nikto, and other such tools can assist in scanning a website for these issues, they only scratch the surface. If one component of a website is susceptible, there is a good chance that other parts are as well.
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.
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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
Based on where user input is to be utilized, employ the proper escaping/encoding technique: HTML escape, JavaScript escape, CSS escape, URL escape, and so on. For escape, use existing libraries rather than write your own unless strictly necessary.
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
Enable the HttpOnly flag for cookies to prevent the effects of a potential XSS vulnerability. Once you do, client-side JavaScript will not be able to access such cookies.
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.
Closing Lines
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.