20 Cross-Site Scripting Interview Questions and Answers

Cross-Site Scripting (XSS) is a type of security vulnerability that allows attackers to inject malicious code into webpages. This malicious code can then be executed by unsuspecting users who visit the infected site. As a web developer, it is important to be familiar with XSS and how to prevent it. During a job interview, you may be asked questions about XSS and how to mitigate it. In this article, we review some of the most common XSS interview questions and provide guidance on how to answer them.

Cross-Site Scripting Interview Questions and Answers

Here are 20 commonly asked Cross-Site Scripting interview questions and answers to prepare you for your interview:

1. What is XSS?

Cross-site scripting (XSS) is a type of computer security vulnerability typically found in web applications. XSS enables attackers to inject malicious code into webpages viewed by other users. When a user views the page, the malicious code is executed by the web browser, resulting in the execution of the attacker’s code.

2. What are the types of XSS attacks?

There are three types of XSS attacks:

1. Reflected XSS attacks occur when an attacker injects malicious code into a web page, which is then reflected back to the user who visits the page.

2. Stored XSS attacks occur when an attacker injects malicious code into a web page, which is then stored by the web server. When another user visits the page, the malicious code is executed.

3. DOM-based XSS attacks occur when an attacker manipulates the Document Object Model (DOM) of a web page to inject malicious code. The code is then executed by the browser.

3. How do you find out if an application is susceptible to XSS?

In order to find out if an application is susceptible to XSS, you need to look for any places where user input is being accepted without proper sanitation or encoding. This can happen in places like search fields, comments sections, or contact forms. If user input is being accepted and displayed on the page without being properly sanitized, then it is possible for an attacker to inject malicious code that will be executed by the browser.

4. Can you explain what a reflected XSS attack is?

A reflected XSS attack is one where an attacker injects malicious code into a web page, which is then reflected back to the user who visits the page. This can happen if the web page takes user input and displays it back on the page without first sanitizing it. The code is then executed by the user’s browser, and can be used to steal information or perform other malicious actions.

5. Can you explain what a stored XSS attack is?

A stored XSS attack is when an attacker is able to inject malicious code into a website that is then stored by the website. This means that when other users visit the website, they will also be exposed to the malicious code. This can be done by storing the code in a database that the website uses, or by uploading a malicious file to the website.

6. Can you explain what a DOM based XSS attack is?

A DOM based XSS attack is one where the attacker is able to exploit a vulnerability in the way that the browser processes JavaScript. This type of attack can be used to inject malicious code into a page that will then be executed by the browser. The code can be used to steal information from the user or to redirect them to a malicious site.

7. What’s the difference between persistent and non-persistent XSS attacks?

The main difference between persistent and non-persistent XSS attacks is that persistent XSS attacks are saved on the server, while non-persistent XSS attacks are not. This means that persistent XSS attacks can affect multiple users, while non-persistent XSS attacks can only affect the user who is currently viewing the page.

8. Are all web applications vulnerable to XSS attacks? Why or why not?

No, not all web applications are vulnerable to XSS attacks. However, many are, because XSS attacks exploit a common vulnerability in web applications that allow untrusted input to be executed as code. This can happen when a web application does not properly validate or escape user input before displaying it. When this happens, an attacker can inject malicious code into the web page that will be executed by unsuspecting users who visit the page.

9. Is it possible to prevent XSS in most cases without any user interaction?

Yes, it is possible to prevent XSS in most cases without any user interaction. The best way to do this is to use a web application firewall (WAF) that can detect and block malicious requests. Additionally, you can also sanitize all user input to strip out any potentially dangerous code before it is processed by your application.

10. What measures can be taken to protect against XSS attacks?

There are a few different measures that can be taken in order to protect against XSS attacks. One is to make sure that all input is validated before it is processed by the server. This means that any potentially malicious code should be removed or escaped before it is stored or displayed. Another measure is to use a content security policy that can help to prevent malicious code from being executed in the first place.

11. If there are no sanitization functions available, how would you ensure that your code isn’t vulnerable to XSS attacks?

One way to protect against XSS attacks when there are no sanitization functions available is to use a whitelist of approved characters. This means that only certain characters are allowed in input fields, and any input that contains characters outside of the whitelist is automatically rejected. This can help to prevent malicious code from being injected into your website.

12. What does a black box test for XSS involve?

A black box test for XSS involves trying to inject malicious code into a web page in order to see if the code is executed. This can be done by trying to input code into a form field on the page or by modifying the URL parameters. If the code is executed, then the page is considered vulnerable to XSS attacks.

13. What does a white box test for XSS involve?

A white box test for XSS involves testing all of the input fields in an application to see if they are vulnerable to cross-site scripting attacks. This includes testing both GET and POST parameters, as well as cookies and hidden form fields.

14. What makes XSS so powerful compared to other forms of cyberattacks?

XSS is powerful because it can be used to inject malicious code into a website, which can then be executed by unsuspecting users who visit the site. This can be used to steal sensitive information, redirect users to malicious websites, or even take control of their computers.

15. What steps do you take when trying to fix XSS vulnerabilities?

The first step is to identify where the vulnerability is located. Once you have found the vulnerable code, you need to determine how the attacker is injecting malicious code. Once you know how the attacker is injecting code, you can then start to work on fixing the vulnerability. This usually involves either sanitizing user input or encoding output.

16. Do you think XSS issues can be fixed by just changing client side validation?

No, I do not think that XSS issues can be fixed by just changing client side validation. Client side validation is important, but it is not the only defense against XSS attacks. Server side validation, input filtering, and output encoding should also be used in order to prevent XSS attacks.

17. In what ways has the OWASP Top 10 changed since 2013?

The OWASP Top 10 is a list of the most common web security risks, and it is updated every few years to reflect the changing landscape of web threats. The 2013 version of the OWASP Top 10 included items such as Injection, Cross-Site Scripting (XSS), and Broken Authentication and Session Management. The 2017 version, which is the most recent at the time of writing, added items such as Insufficient Logging and Monitoring, and Broken Access Control.

18. Which organizations are at risk from XSS attacks?

Any organization that relies on a website or web application is at risk from XSS attacks. This includes e-commerce businesses, banks, and even government agencies. If an attacker can find a way to inject malicious code into a website, they can potentially gain access to sensitive information or even take control of the entire website.

19. What’s the relationship between XSS and CSRF attacks? How can one be prevented while the other can’t?

Both XSS and CSRF attacks exploit vulnerabilities in web applications to inject malicious code, but they differ in how the code is injected. In an XSS attack, the attacker injects malicious code into a web page that is then executed by the browser when the page is loaded. In a CSRF attack, the attacker tricks the victim’s browser into making a request to a web application on the attacker’s behalf, without the victim’s knowledge or consent.

The best way to prevent XSS attacks is to sanitize all user input to strip out any potentially malicious code. This can be done using a whitelist of allowed characters, or by encoding the input so that it can’t be executed by the browser. CSRF attacks can be prevented by using a security token that is required for all sensitive requests. The token is generated by the server and is unique to the user’s session. The token is then included in all sensitive requests, and the server verifies that the token is valid before processing the request.

20. What was the first recorded instance of an XSS attack?

The first recorded instance of an XSS attack was in 1998, when a hacker used a vulnerability in Microsoft Internet Explorer to redirect users to a malicious website. This website then used an ActiveX control to install a Trojan horse on the user’s computer.