What is Cross-Site Scripting

Cross-Site Scripting, also called XXS, is a malicious attack that deliberately injects malicious scripts into a user’s web browser. The attacker executes malicious code in a web application or web page, and as soon as the user visits the web application or web page; the malicious scriptis automatically transmitted to the users’ browser.

Cross Site Scripting

In this way, the attacker takes control of the user’s browser or his account on a specific website. It means that this attack doesn’t harm the web application itself, but affects the users of that application.

Moreover, the malicious scripts usually transmit as JavaScript code. However, the malicious codes can also exploit users’ browsers in other languages including, HTML, Ajax, Flash, and Java.

Summary of the Explanation

Here’s the brief summary of Cross-Site Scripting Explanation:

  • XXS is a web-based attack on vulnerable web pages or web applications
  • It harms users of the web application, not the application itself
  • XXS transmits malicious programs to users via JavaScript.

How Does Cross-Site Scripting (XXS) Attack Work?

Cross-site scripting attacks take place in a way that it first manipulates a vulnerable web application with malicious script, and then the web application delivers that malicious JavaScript to user’s browser.

As soon as the malicious script enters the user browser, the attacker can easily take control of the user’s browser.

Purpose of Cross-Site Scripting

An attacker performs XXS attacks because of the following reasons:

  • To hack an account
  • To transmit malicious software and viruses on systems using the internet
  • To access a user’s clipboard content and browser history
  • To remotely run user’s browser
  • To exploit and access intranet applications

Examples of Cross-Site Scripting

Some of the most vulnerable sources for injecting XXS are self-hosted bulletin-board forums and websites which allow user posting.

Now, we are sharing a simple example of Cross-Site Script below:

It’s a JSP code, in which you can see that an HTTP request is made, and the code reads an employee ID, eid, displaying it to the user.

The code in this example will work fine only if the code (eid) includes standard alphanumeric text.

But, if the same code (eid) uses any Metacharacters or source code value, it means that code will be forced by the web browser to be displayed to the user as HTTP response.

Initially, it doesn’t appear to be a major vulnerability because no one would ever enter a malicious URL. However, the disruption begins, when an attacker creates a malicious link, and trick users to visit the link that’s hidden in the URL.

Usually, the attacker tricks users through social engineering and emails and attracts users to visit a malicious link.

As soon as the user clicks the malicious link, he unintentionally sends back the web application’s malicious content to his own system.

The process of reflecting back the malicious content is called reflected XXS. The XXS attacks cause serious disruptions that often led to tampering and severe data theft.

Types of Cross-Site Scripting

  1. Stored/Persistent XSS

Stored/Persistent XSS attack is the most disrupting form of an XSS attack in which an attacker transmits a permanent script on the web application. Users fall victim to malicious scripts when any request being made on the server.

  1. Reflected XXS

This type of Cross-Site Scripting doesn’t involve attacking the server directly. It uses emails to trick users to execute malicious scripts in the browser. The browser believes that it’s a trusted script and hence all the malicious content reflects back in the user’s browser.

  1. DOM Based Attacks

DOM-based attacks are less common and are different in a way that they never disrupt the server-side code, it only relies on the client-side scripts.

DOM refers to the document object model that is an application programming interface (API) for HTML and XML docs. DOM-based attacks take place only when a web application displays user data into a document object model.

The web application reads the user’s data and transmits it into the browser. If user data is not secure, then an attacker can easily store malicious scripts in the DOM.

How to Determine Your Website’s Vulnerability

You can easily keep a check on your website vulnerability through web vulnerability scanners like Nessus, Nikto, Vega, Grab, WebScarab and much more available.

It is important to carefully conduct a security review of the code and find out all the possible security holes that could allow the input from an HTTP request to make its access into the HTML output.

Bear in mind that a variety of HTML tags can be used to execute a malicious JavaScript. Therefore, it’s significant to scan a website via web security scanners.

In case, if any part of the website is vulnerable, then there are chances that the entire website may fall victim to damage.

How to Prevent Cross-Site Scripting attacks

  1. Escaping User Input

Escaping user input is a method to prevent XXS attacks. In this method, you have to make sure that the data that your web application is about to send back to the users’ web browser is safe.

WordPress and PHP comprise functions that automatically sanitize the data you’re outputting.

  1. Input Validation

Input validation is the process in which any data supplied by a web application is thoroughly checked and verified before sending back to users’ browser.

Web applications must check and validate data before entering into other systems. It helps to detect any malicious link or program that is meant to attack users’ systems.

Conclusion

XXS attacks are common and can disrupt the users’ privacy, however, it is easy to test and prevent your web applications from malicious scripts.

Web applications must constantly sanitize their input before sending it directly to the users’ browser. Also, regular web scans will help web applications to find if there are any vulnerability exists.

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