The Cross-site Scripting (XSS)

Cross-site scripting, also known as XSS, is a web security vulnerability that enables hackers to manipulate user interactions with compromised applications. Through cross-site scripting, the perpetrator can impersonate a user, execute any actions the user is able to, also can access and manipulate their data. If the user has privileges within the application, the perpetrator may gain complete control over all functionalities and data associated with that application.

Briefly, cross-site scripting involves manipulating a website to run malicious scripts. When the code executes within the victim’s browser, the attacker can fully compromise the interaction with the application.

Types of XSS Attacks

There are three main types of XSS attacks.

Reflected XSS occurs when an application receives data in an HTTP request and includes that data within its response in an unsecure way.

Here is a simple example of a reflected XSS vulnerability:

https://website.com/status?message=Everything+is+fine.

<p>Status: Everything is fine.</p>

Instead, a hacker can easily perform an attack like this:

https://website.com/status?message=<script>/*+malicious+code+here…+*/</script>

<p>Status: <script>/*malicious code here*/</script></p>

If the user visits the URL generated by the attacker, then the perpetrator’s script executes in the user’s browser. At that point, the script can carry out any action and retrieve any data to which the user has access.

Stored XSS occurs when a vulnerable application receives data from a malicious source and includes that data within its subsequent HTTP responses. This data might be submitted via HTTP requests. For example, comments on a forum/blog post, usernames in a chat room, etc.

An example of a stored XSS attack is an application for exchanging text messages which allows users to submit any messages which are publicly displayed to other users, such as:

<p>Hello, this is my text.</p>

The application won’t perform any other verification over the submitted message, so the hacker can easily send a malicious message:

<p><script>/*malicious code here*/</script></p>

DOM-based XSS occurs when an application contains some client-side JavaScript that unsafely processes data from an untrusted source.

If the perpetrator controls the value of the input field, they can easily construct a malicious value to eventually execute their own script.

What can XSS be used for?

A hacker who exploits a cross-site scripting vulnerability is able to:

• Impersonate the victim user.
• Inject malware into a website.
• Perform any action that the user can perform.
• Capture the user’s login credentials or other sensitive data.

The impact of an XSS attack will depend on the functionality of the targeted application, the captured data, and the access-level of the compromised user.

So, in the case of a simple application where all users are anonymous and all information is displayed publicly, the consequences will be minimal. On the other hand, an application holding sensitive data, such as banking transactions or healthcare records will be massively impacted.

Also, if the compromised user has elevated privileges within the application, then the impact will be serious, allowing the hacker to take full control of the application getting access to users and data.

Test For Such Vulnerabilities

Testing for reflected and stored XSS involves submitting simple unique input – a short alphanumeric string into every entry point in the application and identifying where the submitted input is returned in HTTP responses. Next, test each location to find out if an input can be used to execute malicious scripts.

Manually testing for DOM-based XSS can be done by placing an unique input in the parameter, then using the browser’s developer tools to search the DOM for this input and testing it to determine whether it is exploitable. Other types of DOM XSS are harder to detect, but achievable with the right support team.

Prevention Methods

Preventing XSS vulnerabilities will involve a combination of the following measures.

In case user-controllable data is output in HTTP responses, encode the output to prevent it from being interpreted as script.

Also, when the user input is received, filter as strictly as possible based on what data you expect to receive.

Make sure to utilize appropriate response headers. To prevent cross-site scripting in HTTP responses that aren’t intended to contain any HTML or JavaScript, you can use the Content-Type and X-Content-Type-Options headers to ensure that browsers interpret the responses the way you intend.

Finally, one of the most effective measures for safeguarding the client side of your business is to implement a content security policy (CSP). This security measure can be easily integrated into any website, providing an additional layer of protection.