What is Cross-site Scripting (XSS)? Definition, Types & Examples

Every time you browse a website, your browser quietly runs scripts that make pages interactive and dynamic. But what if those scripts are malicious? This is where Cross-Site Scripting (XSS) can become a serious Cyber Security issue. XSS attacks allow Hackers to inject harmful code into trusted websites, putting user data, sessions and privacy at risk.

Understanding how these attacks work is essential for Developers and users alike. In this blog, we break down what is Cross-site Scripting, explore its different types, and reveal real‑world examples that show how easily vulnerabilities can be exploited. So read on!

Table of Contents

1) What is Cross-site Scripting (XSS)?

2) How Does Cross-site Scripting Work?

3) Types of Cross-site Scripting Attacks?

4) How to Prevent Cross-site Scripting?

5) How To Find XSS Vulnerabilities?

6) How Attackers Exploit XSS to Cause Damage?

7) What is the Impact of XSS?

8) Example of a Cross-site Scripting Attack

9) Conclusion

What is Cross-site Scripting (XSS)?

Cross-site Scripting (XSS) is a kind of cyberattack in which the attacker injects a malicious code into a legitimate website. When the user visits the affected page, the harmful code runs in their browser. This code is typically embedded in URLs or placed in areas that allow user-generated content. In technical terms, XSS is a client-side code injection attack that targets users interacting with the compromised webpage. Its prevalence can be gauged by the fact that by mid-2025, the Microsoft Security Response Center (MSRC) had addressed more than 970 Cross-site Scripting (XSS) incidents reported since January 2024.

How Does Cross-site Scripting Work?

Cross-site Scripting works by taking advantage of security weaknesses in a website that allows malicious code to be returned to users when they visit a page. When this happens, harmful JavaScript runs inside the user’s browser, allowing the attacker to interfere with the user’s interaction with the website.

Although XSS can involve different scripting languages and technologies such as VBScript, ActiveX, Adobe Flash, or CSS, it most commonly targets JavaScript, which is widely used in web browsing environments. An XSS attack generally occurs in two stages:

1) The first stage involves injecting malicious code, known as a payload, into a webpage. This becomes possible when a website allows user-generated input, such as comments, forms, or search fields, without properly filtering it.

2) The second stage occurs when a victim visits the compromised page containing the malicious payload. Attackers often use Social Engineering or phishing techniques to trick the users into clicking a link or visiting an infected webpage.

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Types of Cross-site Scripting Attacks

The three primary categories of XSS attacks are persistent, reflected and DOM-based XSS. Let's explore them in detail:

1) Persistent XSS

Persistent, also known as stored XSS, occurs when untrusted user input is saved directly on a server. Attackers exploit the vulnerable websites or applications to inject malicious code that becomes stored in the system. When other users later visit the affected page, the stored code automatically runs in their browsers.

2) Reflected XSS

Reflected, or non-persistent XSS, happens when an unverified user input is immediately returned by a web application and displayed in the user’s browser. Attackers usually trick victims into clicking a specially crafted malicious link that sends harmful data to the target website.

3) DOM-based XSS

DOM-based XSS differs from other types because the vulnerability exists in the user’s browser rather than on the server. The malicious script is neither stored nor sent to the server. Instead, it exploits weaknesses in client-side JavaScript that fail to validate or sanitise input properly.

How to Prevent Cross-site Scripting?

The following best practices help reduce application security vulnerabilities that may lead to Cross-site Scripting attacks:

1) Escaping User Input

Escaping user input involves processing the data received by an application before displaying it to users. This ensures that the special characters in the input are not interpreted as executable code by the browser. Instead, these characters are converted into safe formats, preventing them from triggering malicious scripts.

2) Sanitising User Input

Sanitising input removes or modifies potentially harmful characters from user-submitted data. This process converts the unsafe input into a secure format so it cannot be executed as code. It's particularly useful for websites that allow users to include HTML or formatted content.

3) Validating Input

Input validation ensures that only appropriate and expected data is accepted by an application. By restricting certain characters or formats in input fields, websites can prevent malicious scripts from being submitted through forms. This helps protect the website, its database and its users from potential XSS attacks.

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How to Find XSS Vulnerabilities?

Finding XSS vulnerabilities can be challenging because they often depend on the specific type of attack, the way user input is handled and the programming framework or scripting language used. However, many of these vulnerabilities can be identified using web vulnerability scanning tools.

XSS weaknesses may also appear when Developers update the code or integrate new libraries, modules or software components. For this reason, organisations must regularly scan their web applications to detect unusual behaviour or security gaps before they can be exploited.

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How Attackers Exploit XSS to Cause Damage?

Cross-site Scripting attacks often target JavaScript because it can access sensitive information within a user’s browser. For example, JavaScript can read browser cookies, which may contain login details or session data. JavaScript can also send HTTP requests, allowing stolen information to be transmitted to the attacker’s server.

Additionally, client-side scripts may access certain browser APIs that reveal data such as geolocation or other sensitive information. A typical XSS attack usually follows these steps:

1) A victim visits a webpage containing malicious code, which secretly copies the user’s cookies.

2) The script prompts an HTTP request to the attacker’s server with the stolen cookies included.

3) The attacker then uses those cookies to impersonate the victim on the website, potentially carrying out Social Engineering attacks or accessing sensitive information such as personal or financial data.

What is the Impact of XSS?

The impact depends on the type of website or application targeted, the data involved and the level of access of the affected user. These factors typically determine the severity:

1) If sensitive information such as banking details or healthcare records is exposed, the consequences can be significant.

2) The attack becomes more critical when the compromised user has higher access privileges within the application.

3) When users provide sensitive personal or identifiable information, the potential damage from the attack can be substantial.

Example of a Cross-site Scripting Attack

A common example of Cross-site Scripting occurs on websites that allow user comments without properly validating the input. In such cases, an attacker may post a comment containing executable code placed within tags.

These tags instruct the browser to treat the content between them as JavaScript. When other users visit the page and the comment loads, the browser executes the malicious code, making those new users the victims of the attack.

Conclusion

Cross‑site scripting may be invisible to the untrained eye, but its impact is anything but subtle. By understanding XSS, its types, and real‑world examples, you can gain the tools to spot and stop these sneaky attacks. With awareness and strong security practices, you can outsmart malicious scripts and keep our digital experiences safer and more resilient.

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