What is cross-site scripting?

Cross-site scripting (XSS) refers to a type of attack wherein an attacker injects code into a legitimate website, which then executes when the website is loaded by the victim. The insertion of this malicious code can occur through various means, with the most common methods being appending it to the end of a URL or directly posting it on a page that showcases user-generated content. From a technical perspective, cross-site scripting can be categorized as a client-side code injection attack.

What is client-side code?

Client-side code refers to JavaScript code that is executed on a user’s device. In the context of websites, this code runs within the web browser after the browser loads a web page. It operates differently from server-side code, which is executed on the web server hosting the site. Client-side code plays a crucial role in creating interactive web pages, enabling faster and more reliable execution of interactive content. Unlike server-side code, it eliminates the need for constant communication with the web server during user interactions. This efficiency makes it particularly suitable for browser-based games, ensuring smooth gameplay even in the presence of connectivity issues.

The use of client-side code is prevalent in modern web development and is widely employed on most contemporary websites. However, with the increasing reliance on cross-site code, a vulnerability known as cross-site scripting has become a commonly reported cyber-security concern. Major websites like YouTube, Facebook, and Twitter have experienced cross-site scripting attacks, emphasizing the importance of safeguarding against this type of vulnerability.

What is an example of cross-site scripting?

An illustrative instance of cross-site scripting attacks can often be observed in websites hosting unvalidated comment forums. In such scenarios, attackers exploit the vulnerability by submitting comments that contain executable code enclosed within ‘’ tags. By utilizing these tags, the attacker instructs the web browser to interpret the content between them as JavaScript code. Subsequently, when any other user visits the website and loads the page, their web browser executes the malicious code encapsulated within the script tags, thereby making them a target of the attack.

How can an attacker use cross-site scripting to cause harm?

Cross-site scripting (XSS) attacks in JavaScript are widely employed due to the language’s capability to access sensitive data, which can be exploited for malicious purposes such as identity theft. A primary concern is JavaScript’s access to cookies*, as an attacker can employ an XSS attack to pilfer a user’s cookies and assume their online identity. Another alarming capability of JavaScript is its ability to generate HTTP requests, facilitating the transmission of data, including stolen cookies, to the attacker. Moreover, by leveraging client-side JavaScript, an attacker can exploit APIs containing sensitive information like geolocation coordinates and webcam data.

A typical flow of a cross-site scripting attack unfolds as follows:

1. The victim visits a webpage, and the malicious code copies the user’s cookies.

2. Subsequently, the code dispatches an HTTP request to the attacker’s webserver, embedding the pilfered cookies in the request’s body.

3. Armed with these cookies, the attacker can masquerade as the user on the targeted website, executing social engineering attacks or even gaining unauthorized access to sensitive data, such as bank account numbers.

*Cookies are temporary login credentials stored on a user’s computer. For instance, when a user logs into a platform like Facebook, the site assigns them a cookie. Consequently, if the user closes the browser window and revisits Facebook later that day, the cookie automatically authenticates them, eliminating the need to log in again.

What are the different types of cross-site scripting?

There are two primary types of cross-site scripting attacks that are commonly encountered: reflected cross-site scripting and persistent cross-site scripting.

• Reflected Cross-Site Scripting: Reflected cross-site scripting is the most prevalent form of cross-site scripting attack. In this type of attack, malicious code is appended to the URL of a website, often a legitimate and trusted one. When the victim opens the link in their web browser, the injected code within the URL is executed by the browser. Typically, the attacker employs some form of social engineering to deceive the victim into clicking on the link.

For instance, a user might receive an email that appears to be from their bank, prompting them to take action on the bank’s website and providing a link. The link might resemble the following:

http://legitamite-bank.com/index.php?user=

Even though the initial part of the URL seems safe and corresponds to the domain of a trusted website, the injected code appended to the URL can be malicious.

• Persistent Cross-Site Scripting: Persistent cross-site scripting occurs on websites that allow users to post content visible to other users, such as comment forums or social media platforms. If the site fails to properly validate user-generated content, an attacker can insert code that will be executed by other users’ browsers when they load the page. For example, an attacker on an online dating site might include the following text in their profile:

“Hi! My name is Dave, I enjoy long walks on the beach and ”

Any user who visits Dave’s profile will fall victim to Dave’s persistent cross-site scripting attack.

How to prevent cross-site scripting

Mitigating cross-site scripting requires a variety of strategies, as different web applications necessitate varying levels of protection. Here are several protective measures that can be implemented:

• Avoiding HTML in inputs, if possible: A highly effective approach to prevent persistent cross-site scripting attacks is to restrict users from submitting HTML in form inputs. Alternatives like markdown and WYSIWYG editors can allow users to create rich content without relying on HTML.

• Validating inputs: Validation involves implementing rules that restrict users from submitting data that does not meet specific criteria. For instance, an input field requesting the user’s “Last Name” should have validation rules that only permit alphanumeric characters. Validation rules can also reject tags or characters commonly used in cross-site scripting, such as “

• Sanitizing data: Similar to validation, data sanitization occurs after the data has been submitted to the web server but before it is displayed to other users. Various online tools are available to sanitize HTML and remove any malicious code injections.

• Enhancing cookie security: Web applications can employ special rules for handling cookies to mitigate cookie theft through cross-site scripting attacks. Cookies can be tied to specific IP addresses to prevent access by cross-site scripting attackers. Additionally, rules can be established to prevent JavaScript from accessing cookies entirely.

• Implementing WAF rules: A Web Application Firewall (WAF) can be configured to enforce rules that thwart reflected cross-site scripting attacks. These rules employ strategies to block suspicious requests to the server, including cross-site scripting attacks. Toffs WAF, for example, offers straightforward installation and safeguards web applications against cross-site scripting, DDoS attacks, SQL injection, and other common threats.