Cross-Site Scripting (XSS)

Cross-Site Scripting (XSS) is a common and dangerous type of security vulnerability typically found in web applications. XSS attacks occur when an attacker injects malicious scripts into trusted websites, which then execute within the browsers of users who visit the site. This can lead to unauthorized access, data theft, session hijacking, and more.

XSS vulnerabilities arise when user-supplied input is not properly validated or escaped before being included in web pages. These attacks exploit the trust users have in a legitimate website, making them particularly harmful and hard to detect.

Variants

There are three main types of XSS attacks:

• Stored XSS: Also known as persistent XSS, this variant occurs when malicious input is permanently stored on the target server, such as in a database or message forum, and later served to users.
• Reflected XSS: In this case, the malicious script is not stored but is immediately reflected off a web server, typically via a URL parameter, and executed in the user's browser.
• DOM-Based XSS: This type occurs when the vulnerability exists in client-side scripts. The attack is executed as the Document Object Model (DOM) is modified in the victim's browser.

Impact

The consequences of XSS attacks can be severe. Attackers can steal sensitive information such as cookies, session tokens, or other credentials. They can also perform actions on behalf of users, redirect users to malicious websites, or deface content. In some cases, XSS can lead to complete account takeovers or significant reputational and financial damage for organizations.

Discoverability

Discovering XSS vulnerabilities can be challenging. While automated security scanners can catch many instances, sophisticated or DOM-based XSS often requires manual testing and code review to identify. Well-crafted attacks can appear as normal user behavior, making detection even harder without proper monitoring and sanitization of input and output data.

Types

Stored XSS

• Mechanism: Malicious payload is stored in the server (e.g., in a database) and served to users when they access the affected content.
• Example: An attacker submits a blog comment containing a malicious script, which is later displayed to readers.
• Tools: Burp Suite, OWASP ZAP

Reflected XSS

• Mechanism: Malicious payload is part of the request (like URL parameters) and is immediately reflected in the HTTP response.
• Example: A crafted URL sent via email that, when clicked, executes a script in the user's browser.
• Tools: XSSer, XSStrike

DOM-Based XSS

• Mechanism: Vulnerability lies in the client-side JavaScript code that dynamically updates the webpage based on user input.
• Example: A web page that reads URL parameters and writes them into the page without proper sanitization.
• Tools: DOM Invader (Burp Suite extension)