Cross-Site Scripting – Safeguarding Your Website from XSS Vulnerabilities

By Collin Weekly
By Collin Weekly

Cross-site scripting attacks (XSS attacks) are a type of cyberattack where attackers insert harmful code into safe websites. They do this by finding weaknesses in a website and sending harmful code to the users who visit it. Instead of attacking the website itself, they target the people using the website.

These attacks happen when website owners don’t properly check the content they get from users or untrusted sources. Because of this problem, XSS is one of the top 10 security worries for websites, according to cybersecurity experts. This makes it a pretty big issue since many organizations depend on websites to interact with customers.

We’ve gathered information about cross-site scripting, types of cross-site scripting, and preventative measures to avoid your organization getting attacked with cross-site scripting attacks.

What is Cross-Site Scripting?

Cross-Site Scripting (XSS) occurs when attackers trick web applications into sending malicious code to users’ browsers. It happens when the application displays data from untrusted sources without proper validation. XSS can enable attackers to execute code in users’ browsers, potentially accessing sensitive data like session cookies. These attacks target users and exploit their trust in the affected website.

How a Website Becomes Vulnerable to Cross-Site Scripting

When websites and web-based applications let users input information in login pages, search boxes, comment fields, sign-up forms, or forms asking for name, address, phone, and credit card numbers, they come at the risk of cross-site scripting attacks. It’s hard to find a website today that doesn’t have at least one of these features.

To protect against XSS attacks, a web application should never trust user input without checking it first. That means the application should only accept the right kind of input based on what’s expected, like five numbers for a zip code or 16 numbers for a credit card. It should not accept any input that includes special characters used in the code. An XSS attack happens when a website doesn’t validate the input and then sends it back to the user.

This failure to validate input lets attackers insert harmful scripts into the website’s code. It’s similar to SQL injection attacks, but in XSS attacks, the harmful code runs in the user’s browser instead of targeting the website’s database.

What are the Three Types of Cross-Site Scripting Attacks?

In the early days, two main types of XSS were recognized: Stored XSS and Reflected XSS. Later, in 2005, a third type called DOM-based XSS was identified by Amit Klein. These three types of XSS are defined as follows:

Reflected XSS (Type I) 

In a reflected cross-site scripting attack, a vulnerable website accepts malicious data (a harmful script) sent by the target’s web browser to harm the target. This type of attack is also known as “non-persistent” because the malicious script is not stored on the vulnerable server.

For example, an attacker might create a URL with a small malicious script as a search query parameter for a vulnerable website’s search page:

http://vulnerable-website.com/search?search_term=”<script>(bad things happen here)</script>”

The attacker then tricks targets into clicking this URL, perhaps by emailing it or posting it on a public website. When the target clicks the link, the vulnerable website thinks the query parameter “search_term” is a valid search term for which the user wants to search. However, the value is a harmful script.

Since the website doesn’t properly check the search term value, the malicious script gets injected into the webpage the target’s browser loads and executes, causing harm.

Persistent XSS (Type II) 

The harmful script is stored on the vulnerable server in a persistent cross-site scripting attack. Unlike a reflected attack, where the user sends the malicious script, a vulnerable website or web app user can be attacked during normal interactions with the site.

For instance, an attacker could post a message on a forum hosted by a vulnerable website. Instead of a regular post, this message contains the attacker’s harmful script. When a user opens and reads this forum post, their web browser loads and runs the harmful script.

The key difference between reflected and persistent cross-site scripting attacks is that persistent XSS targets all users of a vulnerable site or app, not just a specific individual who clicks a malicious link.

DOM-Based XSS (Type 0) 

Another type of cross-site scripting attack is DOM-based XSS, where the vulnerability lies in the client-side scripts the website or app provides visitors. Unlike reflected and persistent XSS attacks, the site or app doesn’t send the harmful script directly to the target’s browser. Instead, in a DOM-based XSS attack, the site’s client-side scripts deliver the malicious script to the target’s browser. Like a reflected attack, the malicious script is not stored on the vulnerable server.

For example, consider the same scenario as the reflected cross-site scripting attack. The attacker creates a URL with a malicious script as the “search_term” and shares it with potential targets.

When a target clicks the URL, their browser loads the site’s search page and the vulnerable client-side processing scripts. Although the “search_term” is still sent as a query parameter to the site’s backend for processing, the site itself doesn’t create the webpage with the injected malicious script.

Instead, the site’s vulnerable client-side scripts are designed to dynamically substitute the search term value (i.e., the malicious script) directly in the target’s rendered search page in the target’s browser. As a result, the target’s browser loads and executes the attacker’s script.

DOM-based cross-site scripting attacks demonstrate that XSS vulnerabilities are not limited to server-side software; they can also exist in client-side scripts provided to visitors.

How Does Cross-Site Scripting Work?

It’s important to understand that the mechanics of cross-site scripting attacks can vary depending on the type of attack used. However, most attacks follow a similar process:


  • The attacker finds a way to insert harmful code into a web page. It’s possible if the website allows users to add content through comments, posts, or contact fields. If the attacker has a specific target, they may use social engineering techniques like phishing or spoofing to lure the user to the affected site. Otherwise, the harmful code remains for any user to stumble upon.


  • The victim visits the website containing the injected code. Their device considers the infected script part of the trusted site’s source code, so it accepts and executes it. Since the code is usually hidden and many internet users don’t understand programming languages like JavaScript, detecting cross-site scripting attacks can be challenging for the average user.

How to Detect Cross-Site Scripting Attack

Detecting and eliminating XSS flaws from a web application can be challenging. The most effective approach is to conduct a thorough security review of the code, examining all areas where input from an HTTP request could potentially find its way into the HTML output. It’s crucial to bear in mind that various HTML tags can be exploited to transmit malicious JavaScript.


While tools like Nessus, Nikto, and others can scan websites for these flaws, they can only provide a basic assessment. If one part of a website is vulnerable to XSS, other issues may probably exist elsewhere within the application. Therefore, comprehensive manual review and vigilant testing are essential to ensure the complete removal of XSS vulnerabilities.

How Can You Avoid Cross-Site Scripting Vulnerabilities?

Aside from the obvious technical security controls like installing updates and patching known web applications and browser vulnerabilities, there are other equally important ways to protect against cross-site scripting attacks.

Input Validation

One crucial measure is input validation. All external input provided by users on websites and web applications should be treated as untrustworthy. Hence, validating this input is essential, ensuring it meets the required criteria, such as having five numeric digits for a U.S. zip code or a properly formatted date.

Organizations might consider denylisting, which specifies disallowed characters, as a way to protect against XSS attacks. However, allowlisting, which specifies the exact allowed input, is more effective because it prevents the possibility of missing unsafe characters, which could happen with deny listing alone.

Escaping & Filtering 

Another defense strategy is escaping and filtering. Character escaping involves identifying potentially unsafe characters (like <, >, &, and others) and converting them into “safe” representations to avoid their interpretation as executable code. Filtering, on the other hand, completely removes these unsafe characters. Both methods are referred to as input sanitization.

Utilizing input validation and sanitization as the first line of defense can significantly reduce the risk of XSS attacks.

HTTPOnly Cookies 

Using HTTPOnly cookies is another helpful tactic. These cookies have an HTTPOnly flag, instructing the browser to allow access to the cookie only by the server and not by JavaScript’s document.cookie API or any other means. This approach can help mitigate reflected XSS attacks by preventing attackers from obtaining a cookie and impersonating a user. However, it does not address the fundamental input validation issue in XSS attacks.

User Awareness Training 

User awareness training is vital because XSS attacks target unsuspecting users. Providing security awareness training helps users develop safe online habits. Some important tips to remember are:

  • Never click on or share unverified links. Instead, manually look up the address or domain name and evaluate the search results. Alternatively, copy and paste the link into Notepad to check the full URL without activating it.
  • Be cautious of suspicious domain names that don’t logically reflect the company name, contain misspellings, or use non-standard characters. When in doubt, manually search for the domain.
  • Don’t blindly trust a website because it displays a padlock, HTTPS as the connection type, or the word “SECURE” in the address bar. Attackers may use encryption to make their malicious websites appear legitimate. Click the padlock to view the website’s certificate details.


Safeguarding your website from XSS vulnerabilities is of utmost importance in today’s cybersecurity landscape. Client-side scripting poses significant risks to web applications, potentially leading to data breaches, unauthorized access, and other malicious actions.

You can fortify your website against XSS attacks by implementing the prevention methods mentioned above. Additionally, conducting thorough code reviews and utilizing reliable scanning tools will aid in identifying and addressing potential flaws.

Prioritizing security measures and staying vigilant is crucial in ensuring a safe online environment for your website and its users. 

Learn how to safeguard your organization from cross-site scripting attacks by starting a conversation with professionals!


How are XSS and CSRF different?

  • Cross-site scripting (XSS) lets a bad actor run their code in a person’s web browser without them knowing.
  • Cross-site request forgery (CSRF) tricks someone into doing things they didn’t mean to do on a website.

A cross-site scripting attack involves sending harmful code or scripts. This malicious code is typically written in client-side languages like JavaScript, HTML, VBScript, and Flash. However, JavaScript and HTML are the most common languages for this attack.

Cross-site scripting vulnerabilities remain a prevalent security issue for online applications. They are often introduced easily but can be challenging to detect and repair, posing an ongoing risk of making their way into production code.

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