XSS Vulnerability Prevention

Cross-Site Scripting (XSS) vulnerabilities are among the most common vulnerabilities found in web applications. However, they are also among the most preventable ones. This section will discuss a few measures that can be implemented to protect a web application from XSS vulnerabilities.

Output Encoding

Output encoding is the primary defensive measure against XSS vulnerabilities. By encoding all user-supplied input that is dynamically rendered by the web application in its web pages, we can prevent JavaScript code from being executed. However, different contexts in web applications require different encoding schemes. The OWASP Cheat Sheet Series provides an excellent overview of the different contexts and their corresponding encoding schemes. For instance, HTML entities should be encoded in HTML contexts, JavaScript escape encoding should be used in JavaScript contexts, and URL encoding in URL contexts.

Input Sanitization

Another defensive measure is input sanitization. By removing or transforming dangerous user input, we can prevent XSS vulnerabilities from occurring. For instance, in a web application that implements a rich text editor, users may be allowed to provide HTML tags in their input. Input sanitization can be used to remove JavaScript event handlers such as onload from img tags. Alternatively, instead of writing a custom input sanitization function, a well-tested library should be used. However, input sanitization should never be the only defense against XSS vulnerabilities, as it is often possible to bypass it.

Content Security Policy

Content Security Policy (CSP) is another defense-in-depth security measure against XSS vulnerabilities. As discussed in previous sections, a strict CSP can significantly limit the impact of an XSS vulnerability, as it restricts the resources a web application can load. However, a CSP is not a primary defensive measure but should only be implemented as a second layer of defense. Since it does not prevent the XSS payload from being injected into the web page, the CSP is ineffective if it contains any misconfigurations.

HttpOnly Cookie Flag

As discussed, stealing victims' session cookies is the most widely exploited technique that threat actors carry out using XSS vulnerabilities. The HttpOnly attribute on the session cookie prevents access to the cookie from JavaScript code, thus effectively preventing the exfiltration of the victim's session cookie. However, it does not lessen the severity of XSS vulnerabilities, as an attacker can still execute arbitrary JavaScript code in the victim's browser within the vulnerable web application and in the context of the victim. This enables the attacker to perform the same actions as if they knew the session cookie.