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Keep Yourself Aware and Protected! A Detailed Guide to Cross-Site Scripting (XSS)

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PUREVPNPrivacy & SecurityKeep Yourself Aware and Protected! A Detailed Guide to Cross-Site Scripting (XSS)

Hackers strategically plan different web attacks, and cross-site scripting is one of the attacks they use. SXX attacks emerged in the late 1990s, and they are still active. 

With these, hackers take advantage of the vulnerabilities in websites and exploit them due to multiple flaws in input sanitization. 

According to the reports, in 2021, more than 60% of websites are susceptible to XSS attacks, and 30% of web attacks eventually result from XSS attacks. 


What is Cross-Site Scripting?

In this type of attack, the attacker uses the visitor’s browser to attack the website. Once they access the website, attackers steal sensitive information, modify search results using spammy keywords, store illegal data, and route visitors to dangerous sites. 

It all happens when the attacker injects malicious JavaScript into the website. The script gets activated and then executed when a visitor visits the website. 

Cross-Site Scripting

Types of Cross-Site Scripting

Cross-site scripting is of three types:

Reflected XSS

Reflected XSS is commonly known as Non-Persistent XSS. In this type, attackers attack the user’s browser. Most websites are vulnerable to reflected XSS. 

It occurs when the input gets reflected on the website, and the attacker successfully executes his malicious script, resulting in a case of Reflected XSS. 

For example:

You visit a blog website and search for your desired blog by typing it in the search bar. This input will be processed on the website and generate results. 

If your input gets reflected and the website has not performed input sanitization on this website, it is vulnerable to XSS. The attacker will inject malicious code into the search bar. 

<script>alert(“You have been hacked!”)</script> 

This malicious code will get executed when you click the blog link, and an alert box will appear that says 

‘You have been hacked!”

However, if the website has taken appropriate precautions against XSS, it will be free from vulnerabilities. This way, you can identify reflected XSS on a website. 

Stored XSS

Stored XSS is also referred to as Persistent XSS. In this type, the attacker’s input gets permanently stored in the website database. 

For Example: 

If you shop online, you will know that the website asks you to write reviews under products. This information gets stored on the website’s server. The attacker executes the malicious input in one of the fields on the website. 

It could be crafted into a malicious review text or ratings. Every visitor will face the execution of this payload when they unknowingly interact with the infected webpage. This enables attackers to steal the personal information of the users as output. 

Difference Between Reflected XSS and Stored XSS

In reflected XSS, the malicious input is not stored on the website. Instead, it is reflected to the user in response to their request. 

However, in Stored XSS, the malicious input gets stored in the webpage, and every time a user visits the website, they become a victim of this attack. 

Hence, Stored XSS is very dangerous as it can target multiple users simultaneously. 

DOM (Document Object Model) Based XSS

The Document Object Model has two key components: the Source and Sink. User’s input gets stored in the Source component, and their input gets executed in the Sink Component. 

If an attacker enters malicious code in the source and the Sink successfully executes it, this results in a Document Object Model based XSS. 

What Malicious Activities Attackers Can Perform with XSS?

stored cross-site scripting


Attackers can inflict various harm to users and websites with cross-site scripting:

Stealing sensitive information (Cookies)

By injecting infectious code into the website, attackers can steal sensitive information. Cookies contain your data that gets stored in the computer. 

The browser and server are notified that you’re logged in through cookies. If these cookies get stolen by the attacker, he can log in to your account and gain complete control over it. 

Attackers inject code into websites specially designed to steal the user’s cookies. Every time a user visits the website, the malicious code gets activated, sending the user’s cookies to the attacker as output. 

Directing Users to Malicious Websites

Take the example mentioned above of the reflected XSS. The user looks for something in the search engine and clicks on the results infected with malicious scripts. 

Users are directed to a malicious website when they click on the contaminated results. 

Once the user is in the attacker’s chosen location, they become vulnerable to information loss and computer-based attacks.

Injecting Malware (Trojans)

As mentioned above, users are directed to malicious websites that contain irrelevant and misleading content with XSS attacks. 

You can also download negative information from the same websites into their systems. 

Trojan is the most commonly found malware attackers use to steal your information. Trojan runs silently in the background without anybody’s knowledge. 

Attacks can then exploit the system, extract sensitive data, and even spy on the victim. 

Changing the Content of the Website

With XSS, attackers can add and delete elements from the website. Attackers exploit the vulnerabilities in the website and inject malicious JavaScript code into the page. 

Using this technique, they can change the text and image of the website. 

Another way to change the website’s content is to inject HTML code into the page. 

Via this, not only do attackers change the layout of the website, but they can also add new elements to the page, such as malicious links or images. 

When the users click on a manipulated image or option, they get directed to something offensive or misleading. This practice is commonly observed on online movie streaming platforms.

Gaining Control over Personal Information

A website is never concerned about who uses it as long as the hosting is active. Hackers who manipulate the website’s vulnerabilities access user information as output. 

After stealing the sensitive data, attackers use the login credentials to access the user’s account and take control of it.

How to find XSS Vulnerability in a Website 

You can use multiple methods to identify XSS vulnerabilities in a website. A few are mentioned below.

image tag

Automated Application Testing

SXX vulnerabilities can be identified using testing methods. Scanner applications are convenient to use to generate results instantly. 

Static Application Security Testing (SAST)

SAST ensures that the websites are running without vulnerabilities. Static Application Security Testing is employed before running the website to detect potential weaknesses. It is a fast and efficient application. 

You can examine the codes early during the website’s development process instead of waiting until the program runs in a testing environment to check for problems. 

Lastly, SAST tools also monitor whether the website complies with coding standards and best practices.

Dynamic Application Security Testing (DAST) 

DAST practically interacts with the website to recognize security flaws in it. This method can be applied once the website is fully operational. 

Otherwise, it would be complex for the application to detect vulnerabilities by analyzing the source code only. 

DAST tools target the website under inspection by sending malicious input. It can be SQL injection attacks or cross-site scripting attacks. 

If the website executes the malicious input, DAST detects it and identifies vulnerabilities in the system.

Interactive Application Security Testing (IAST)

IAST is an amalgamation of both SAST and DAST. It examines codes early during the website’s development process while also interacting with the application functionality.

Manually Find Vulnerabilities

Manual code reviews identify vulnerabilities by finding the root cause of the problem. Automated tools need the contextual understanding that humans possess. Hence, it is essential to test for security flaws in a website manually. There are multiple ways to identify vulnerabilities manually.

Identify Code that Outputs User Input

Codes that do not undergo proper sanitization are susceptible to vulnerabilities. To identify the compromised code, follow the instructions below.

  1. Press Ctrl + U to view the page source.
  2. Identify the codes that give output and the user input. To test if this code is vulnerable, inject code “onmouseover= alert(‘hello’);” to see if the information gets executed.
  3. You will see a pop-up alert if the code is vulnerable to XSS. 
  4. If the code filters for <> characters, you can use the script{alert(‘hello’). This script uses HTML entities to represent the < and > characters, allowing it to pass the filter. 

Check that the Output is Encoded

Ensure that every user input is encoded before it is displayed on the web page by applying proper output encoding. 

This can be done using the HtmlEncode() method for HTML output and the UrlEncode() function for URL strings.

There are multiple sources for user input. They include query strings, form fields, cookies, HTTP headers, and databases. 

The data they enter on the website gets stored in the database. By encoding the input data, it will no longer be interpreted as executable code by the browser. 

Here is an example of how website developers can encode user input before displaying it on a web page:

$username = $_POST[‘username’];

// Encode the username before displaying it on the page.

$username = HtmlEncode($username);

echo “Hello, $username!”;



How to Prevent XSS Attacks

Through XSS attacks, attackers can compromise the display of the website. They can also exploit vulnerabilities to steal sensitive information from site visitors and impersonate them. 

Therefore, it is pivotal to take protective measures against these attacks. 

The following section covers how to protect your site visitors from XSS attacks:

Install Web Application Firewall

Firewalls create resistance between the system and external forces. It can protect your site visitors from adverse consequences. 

By installing firewalls, you protect your users against online malicious attacks as they closely monitor data distribution to prevent accidental penetration and exposure against third parties. 

Firewalls have stringent rules to filter requests that contain suspicious data. 

Update all Plugins and Themes

Security vulnerabilities are often discovered in plugins and themes. To fix the vulnerability, developers release security patches so the websites are not at risk. 

Once the patches are released, users who discover the vulnerabilities tell the public about them so the website owners can take steps to protect their websites. 

Hence, it suggested installing plugins and themes from trusted developers and updating them. 

Install prevent XSS vulnerability:

Prevent XSS vulnerability plugins block commonly found parameters that attackers use in XSS attacks. 

For instance, hackers use exclamation marks or opening round brackets when they add malicious code to the website. You can add the parameters to the plugin and prevent XSS attacks.

For example, You can specify to block or remove certain entities in the URL. 

  • Less than sign < 
  • Greater than sign >
  • Vertical Bar | 

Update Your Browser

Updating your browser is crucial for protecting against cross-site scripting attacks because newer versions often include security features and patches for vulnerabilities. 

XSS attacks involve injecting malicious scripts into websites, which can then be executed in the browsers. 

Modern browsers regularly release updates to address known vulnerabilities that provide resistance to such attacks.

Use a VPN

Having a reliable VPN is always a good idea to enjoy base-level security. VPN provides anonymity, masks your IP address, and offers encryption that enhances your chances of not being affected by cyber attacks, including cross-site scripting.

Prevention is Better than Cure 

Modern-day attackers have become highly skilled in the attacks they execute. With knowledge and advancements, these attacks become more and more sophisticated and powerful. 

Fortunately, we have some preventive strategies to help us stay on guard. Whenever you are online, make sure you stay alert. At the same time, educate yourself about the preventive strategies against these attacks. 


Anas Hasan


October 18, 2023


8 months ago

Anas Hassan is a tech geek and cybersecurity enthusiast. He has a vast experience in the field of digital transformation industry. When Anas isn’t blogging, he watches the football games.

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