Cross-site scripting (XSS) is a type of injection attack where malicious scripts are injected into otherwise benign and trusted websites. These scripts are then executed by other users' browsers when they visit the website, allowing the attacker to access any cookies, session tokens, or other sensitive information retained by the browser for that site.
XSS attacks work by exploiting a lack of input validation on the website. Untrusted user input containing script code is echoed back to users without being properly encoded. This causes the users' browsers to execute the malicious script when they encounter it.
There are three main types of XSS attacks:
Reflected XSS
In reflected XSS, the malicious script comes from the current HTTP request. The attacker crafts a URL containing the script and tricks victims into visiting that URL. When the website reflects the URL back in its response, the script executes in the victims' browsers.
Stored XSS
In stored XSS, the malicious script comes from the website's database. The attacker injects the script into some input field, which is then stored in the database. When other users view that stored input, the script executes in their browsers.
DOM-based XSS
DOM-based XSS occurs when client-side JavaScript processes untrusted input in an unsafe way. The script reads input from the URL or document.cookie and writes it to the DOM without proper sanitization, causing the input to be executed as a script.
Impact of XSS
The impact of an XSS vulnerability depends on the privileges of the compromised users. At a minimum, the attacker can impersonate users and read their data. If an administrator is compromised, the attacker may gain complete control of the website.
The consequence of an XSS attack is that the attacker can impersonate the victim user, perform any actions the victim can perform, and access any data the victim can access. If the victim has administrative privileges, the attacker can completely compromise the website.
The primary defenses against XSS are:
Filtering input to allow only "safe" characters
Encoding output contexts to render scripts harmless
Using Content Security Policy headers as an additional layer of defense
Preventing XSS Attacks
To prevent XSS, websites must sanitize untrusted input by escaping HTML, JavaScript, and CSS. They can also use Content Security Policy (CSP) headers as an additional defense. Input validation and output encoding are essential to securely handling data and preventing XSS vulnerabilities.
Finding and preventing XSS vulnerabilities requires a combination of input validation, output encoding, and security headers. Automated scanners and code reviews can help identify vulnerabilities, but manual testing is often required to find them all.