Cross-Site Scripting (XSS)

June 1, 2023

Cross-Site Scripting (XSS) is a common security vulnerability within web applications. It allows attackers to inject malicious scripts into web pages viewed by other users. These attacks can have serious consequences, such as stealing user credentials or spreading malware. This vulnerability often results from a web application failing to properly validate and sanitize user input.

How Does XSS Work?

At its core, XSS attacks involve injecting malicious code into a web page, which the user’s browser then executes. Let’s consider a simple example:

Assume a blog site where users can post comments. If the site is not properly sanitizing user input, an attacker could post a comment that looks something like this:


When other users view this comment, the script executes, sending their cookies to the attacker’s server.

XSS attacks typically fall into two categories:

  1. Reflected XSS: This attack happens when malicious script injected by the attacker is immediately returned by a web page where it’s executed by the victim’s browser. For example, an attacker might send a phishing email with a link containing malicious code. If the victim clicks on the link, the web application reflects the attacker’s script back to the victim’s browser.
  2. Stored XSS (Persistent XSS): These attacks are more dangerous because the injected script is permanently stored on the targeted server (e.g., in a database, comment field, etc.). The malicious script is then served to users within regular site content. The comment example given above is an instance of Stored XSS.

How Can XSS be Prevented?

Preventing XSS revolves around proper input validation, sanitization, and rendering of user input. Here are some strategies to mitigate XSS risks:

  • Input Validation: Use a well-known library like OWASP’s ESAPI or a programming language feature to properly sanitize all user inputs before processing.
  • Input Filtering: Implement a whitelist of allowed input and filter out everything else. Libraries such as OWASP’s Java Encoder for Java applications can help with this.
  • Output Encoding: Encode user input when it’s rendered on the web page to prevent it from being executed as a script. This means replacing characters like < and > with their HTML encoded equivalents (&lt;, &gt;).
  • Content Security Policy (CSP): CSP is an HTTP header that allows site operators to specify trusted sources of content. It can prevent browsers from loading code from untrusted sources.
  • HttpOnly Cookies: Flag cookies as HttpOnly to prevent them from being accessed via JavaScript. This significantly reduces the risk of stolen session cookies.


Cross-Site Scripting (XSS) poses a serious threat to web applications, potentially leading to stolen user data, spread of malware, and more. However, it is possible to mitigate these risks by implementing robust input validation, output encoding, and applying security headers. It’s crucial for web developers and site operators to keep abreast of best practices and emerging threats to maintain the security of their applications.

Remember, a secure application is an ongoing commitment, not a one-time task. Regular updates, security audits, and patching vulnerabilities are essential to maintain the security of a web application.