Cross-site scripting (XSS)

Definition

Cross-site scripting (XSS) is a type of security vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. This vulnerability can lead to unauthorized actions on behalf of users, session hijacking, and exposure of sensitive information. Understanding XSS is crucial for identifying common application vulnerabilities and implementing effective mitigation strategies, ensuring robust web application security.

5 Must Know Facts For Your Next Test

1. XSS attacks can be classified into three main types: stored XSS, reflected XSS, and DOM-based XSS, each exploiting different vectors to inject scripts.
2. Stored XSS involves injecting malicious scripts that are permanently stored on the target server, affecting every user that accesses the infected page.
3. Reflected XSS occurs when an injected script is reflected off a web server, typically through a URL or form input, and executed immediately in the user's browser.
4. To mitigate XSS vulnerabilities, developers can use input validation, output encoding, and implement CSP to control the sources of executable scripts.
5. XSS can lead to serious consequences, including the theft of cookies or session tokens, redirecting users to phishing sites, and spreading malware.

Review Questions

• What are the different types of cross-site scripting attacks and how do they differ from each other?
  • Cross-site scripting attacks can be categorized into three types: stored XSS, reflected XSS, and DOM-based XSS. Stored XSS involves embedding malicious scripts on a server so that they execute when any user accesses the page. Reflected XSS happens when the attack is reflected off a web server and executed immediately based on user input. DOM-based XSS occurs when the vulnerability is present in the client-side script rather than server-side code, manipulating the Document Object Model (DOM) directly.
• Discuss the role of Content Security Policy (CSP) in protecting against cross-site scripting attacks.
  • Content Security Policy (CSP) is a powerful tool used to prevent cross-site scripting attacks by allowing developers to define which resources are permitted to load and execute on their web applications. By specifying trusted sources for scripts and blocking all inline scripts or untrusted external sources, CSP significantly reduces the risk of successful XSS attacks. It acts as a security layer that helps to mitigate vulnerabilities by limiting the ability of attackers to run their injected scripts.
• Evaluate the effectiveness of input validation and output encoding as strategies to mitigate cross-site scripting vulnerabilities.
  • Input validation and output encoding are essential strategies for mitigating cross-site scripting vulnerabilities. Input validation ensures that only properly formatted data is accepted by an application, rejecting potentially harmful inputs before they can cause harm. Output encoding transforms potentially dangerous characters into a safe format for display in a browser, preventing them from being executed as code. Together, these techniques create a robust defense against XSS by ensuring that user input is handled securely at both entry and exit points within an application.

Related terms

Injection Attack: A broad category of attacks where an attacker sends untrusted data to a web application, causing it to execute unintended commands.

Content Security Policy (CSP): A security feature that helps prevent XSS attacks by allowing web developers to specify which content sources are trusted.

Same-Origin Policy: A security measure that restricts how documents or scripts loaded from one origin can interact with resources from another origin.