XSS – Cross-Site Scripting

Cross-site scripting (XSS) is a type of vulnerability in web applications where attackers inject malicious code, usually JavaScript, into websites that are accessed by other users. XSS attacks aim to steal data, deceive users or even take control of a user’s session. The vulnerability usually arises because a web application does not properly validate or sanitize user input before it is sent to the browser.

What types of XSS are there?

There are three main types of XSS attacks:

• Reflected XSS: The malicious code is passed as part of a URL and sent directly from the website back to the victim’s browser. The attack is only carried out when a targeted visit is made to the prepared URL.
• Persistent (stored) XSS: The malicious code is permanently stored on the server, for example in a database, and delivered to all users every time the affected page is accessed. This is particularly dangerous as every user who visits the page can be affected.
• DOM-based XSS: Here, the malicious code is executed within the browser DOM without the server being directly involved. The attack takes place entirely on the client side by manipulating content in the browser.

How does an XSS attack work?

In an XSS attack, the attacker injects JavaScript code or other executable scripts into a website. These scripts can then be executed in the browsers of unsuspecting users. The attacker usually exploits vulnerabilities in the input fields or in the way websites process user input. A typical sequence of a reflected XSS attack could look like this:

1. A user clicks on a prepared link that contains malicious code.
2. The web application accepts this link, does not perform sufficient validation and displays the code in a response page.
3. The user’s browser executes the malicious code as if it were a legitimate part of the website.

What effects can an XSS attack have?

The effects of XSS attacks can be serious and depend on the capabilities of the attacker. Typical consequences are

• Data theft: Attackers can access cookies and use them to steal session tokens, for example, in order to impersonate the affected user (session hijacking).
• Phishing: The malicious website can manipulate content to simulate legitimate forms and input fields in order to collect confidential data such as passwords.
• Defacement: Attackers can manipulate the content of the website and display false messages or content, for example.
• Malware distribution: Attackers can use XSS to insert code that redirects users to malicious websites or downloads malware.

How do you recognize an XSS vulnerability?

XSS vulnerabilities can be detected using various methods:

• Manual testing: Security researchers try to inject user input directly into various fields (e.g. search fields, contact forms) of the web application to see if they are sent back to the browser unfiltered.
• Automated scans: Tools such as OWASP ZAP or Burp Suite perform automated tests to identify potentially vulnerable input fields.
• Code reviews: When reviewing the source code, vulnerabilities can become visible, especially in places where user input is output directly in HTML or JavaScript without prior cleanup.

How can you protect yourself against XSS attacks?

The prevention of XSS attacks requires several measures:

• Validate and escape input: All user input should be validated and escaped on both the client and server side before it is output. Escaping HTML and JavaScript prevents embedded code from being executed.
• Content Security Policy (CSP): A well-configured CSP helps to prevent the execution of untrusted JavaScript by explicitly defining the permitted sources for JavaScript.
• Use secure APIs: APIs such as innerHTML should be avoided if there are alternatives such as textContent or setAttribute that do not allow code execution.
• Output Encoding: Use libraries such as OWASP Java Encoder or ESAPI to ensure that user input is correctly decoded.
• Use of HTTPOnly and Secure Cookies: This protects cookies from JavaScript access and prevents sensitive information from being read by XSS attacks.

How does reflected XSS differ from persistent XSS?

Reflected XSS differs from persistent XSS in the way in which the malicious code is executed:

• Reflected XSS: The malicious code is sent directly to the victim’s browser via a URL. This usually requires user interaction, e.g. clicking on a crafted link.
• Persistent XSS: The malicious code is stored on the server (e.g. in a database) and automatically delivered to all users who visit a particular page. This makes persistent XSS more dangerous as it affects multiple users without their active interaction.

Which tools are used to prevent XSS?

Various tools help to detect and prevent XSS vulnerabilities:

• OWASP ZAP: An open source web application scanner that can identify vulnerabilities, including XSS.
• Burp Suite: A comprehensive toolset for analyzing and vulnerability testing of web applications.
• Content Security Policy (CSP): A server-side tool that can restrict the execution of malicious scripts.
• SAST (Static Application Security Testing) tools: These tools check the source code of an application for security vulnerabilities, including XSS.

How does a user recognize whether a website is vulnerable to XSS?

Users performing security checks can determine whether a website is vulnerable to XSS in several ways:

• Unusual behavior: Unexpected pop-ups or redirects may indicate an XSS attack.
• User input reflected: If user input (e.g. search terms) is returned unfiltered on the website, this is a potential sign.
• Use test scripts: Manually entering typical XSS test scripts such as "<script>alert('XSS')</script>" in forms or URLs can uncover vulnerabilities.

Which programming languages or frameworks are particularly susceptible to XSS?

In principle, any web application can be vulnerable to XSS, regardless of the programming language used. However, there are some languages and frameworks that are more likely to have XSS vulnerabilities built in:

• PHP: Often vulnerable, especially older or poorly maintained applications, as PHP often processes direct user input.
• JavaScript/Node.js: Since JavaScript itself is often the target of XSS, developers must pay particular attention to the correct handling of user input.
• Frameworks such as Angular or React: Although modern frameworks offer standard protection (e.g. through automatic escaping), security gaps can also arise here if these mechanisms are deliberately circumvented.

Are there any known examples of XSS attacks in the past?

Yes, there are many documented cases of XSS attacks. A well-known example is an attack on Twitter in 2010, where a malicious script spread rapidly via tweets by forcing users to automatically propagate the malicious code. Within minutes, thousands of accounts were affected. Another example is the Samy worm of 2005, which affected MySpace and spread virally in a similar way.

What is a Content Security Policy (CSP) and how does it help with XSS prevention?

A Content Security Policy (CSP) is a security feature that allows web applications to control from which sources scripts, stylesheets and other resources may be loaded. By defining a CSP, developers can specify which scripts are allowed to be executed, which helps to minimize the impact of XSS attacks. Even if an attacker manages to inject malicious code into a website, a correctly configured CSP will prevent this code from being executed.