Cross-site scripting (XSS): what it is, how to prevent it, and how to fix it

Learn about the dangers of Cross-site Scripting and how it might affect the security of your website's users.

Updated: February 12, 2021
cross-site scripting

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Are you sure your website is really secure against most threats? As technology evolves, hackers get more creative with their invasion techniques. Cross-site Scripting attacks are an example of this.

Concerning your website security, there are several measures you need to take in order to protect it from the actions of hackers and other malicious parties. Depending on how vulnerable your structure is, it might allow others to run unauthorized code within your own website.

That is the basic premise of a Cross-Site Scripting attack. Typically found in web applications, it takes advantage of vulnerable input fields to run code on your website that otherwise would not be possible. This is usually done to steal data from the user through a seemingly secure website.

Below, you will find a list of everything you will learn in this article. Check it out:

Are you ready? Then, read on!

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What is Cross-site Scripting?

Cross-site Scripting, also known as XSS, is a kind of injection attack that involves adding malicious scripts to otherwise safe and trusted websites. By exploiting vulnerabilities in the code and lack of preventive measures, hackers are able to send malicious scripts to affect other users.

This usually works by allowing unauthorized code to run on your website through input entry. That is the case, for instance, of adding HTML code to a search box and being able to run JavaScript code that steals user data.

The attack works because the web browser has no way of knowing whether the code it is running is a legitimate part of the website or was added by a third party through an XSS vulnerability.

What are the Cross-site Scripting types?

There are multiple kinds of XSS attacks, depending on the method used for injecting malicious code. Learning about their differences is important in order to understand how they work and prevent them from happening on your website.

Below are the most common types of Cross-site Scripting attacks.

Persistent XSS (or Stored)

The Stored XSS kind happens when the unauthorized input is stored through interactivity methods, such as a comment field or forum message. Then, the user’s browser reads this data without it being safely rendered.

This attack becomes even more dangerous when considering the new capabilities of HTML5, which involve storing data on the user’s browser local storage.

Non-Persistent XSS (or Reflected)

The Reflected XSS kind happens when the input is returned immediately through any kind of response, such as a search result, error message, or other that includes the input provided. The data is not rendered as safe and is included in the newly loaded page.

Before we begin, it is important to know that several of these initiatives can be automated with the use of security plugins. If you use WordPress, some of them might help close various vulnerabilities that can lead to XSS.


This kind of XSS happens when the unauthorized data flow takes place within the browser and never leaves it.

For instance, it would be the case of injecting code through the page URL or an HTML element. 


This is a form of social hacking. It relies on convincing the user to run malicious code. This might be done through phishing methods and is just as threatening as the other kinds of XSS.

Self-XSS is an example of a vulnerability that requires a fix not only to protect your website against attackers but also users that might have been tricked to run malicious code. 

Mutated XSS (mXSS)

Lastly, we have the mutated XSS. It happens when the injected code is apparently safe but is repurposed by the browser when parsing it.

This is a higher class of XSS as it is harder to detect through regular sanitation and filtering processes.

How to prevent a Cross-site Scripting situation?

Cross-site Scripting attacks can seriously damage your site and your visitors. Because of that, you should put in the appropriate efforts to prevent it from being possible on your content. You might also be vulnerable and not even know it.

Below, you will find the necessary steps to prevent a Cross-site Scripting situation on your site.

Sanitize data

Data entry on your website needs to be properly sanitized to prevent improper inputs that might lead to Cross-site Scripting.

Developers should read the documentation on how WordPress deals with PHP filters, as the goal is to be able to automatically remove undesirable characters from input fields. Applying function calls that sanitize data is within many WordPress common conventions.

Validate data

Data validation helps you make sure that the data you are receiving from the user matches what you need. Fortunately, there is a PHP function for that.

This can be done through filter_var, which accepts validation such as, for example, making sure that an e-mail address is really formatted as an e-mail address. Further options can be read through within PHP documentation.

Use a library

Using the escape exit is the way to prevent user-submitted data from including HTML entities. As such, this can be done through PHP functions such as noHTML.

To further improve your hosting security against Cross-site Scripting, prefer to use a library like HTML Purifier, as it is able to escape entire blocks of HTML in a standards-compliant way.

How to fix the Cross-site Scripting if the attack has already happened?

In case it was not possible to run a real-time threat detection and the Cross-site Scripting attack already happened on your website, it is important to apply the corrections as soon as possible.

Such a discovery should be made through regular vulnerability scans. Finding an opportunity for XSS to run must inspire immediate action to remedy it. Apply the solutions recommended in this article through sanitization and better filtering on your fields so the issue does not happen again.

You should also try to learn more about the impact of the attack. Start by checking your application logs for traces of HTTP requests with malicious scripts on them. In addition, keep an eye out for communication coming from your visitors, as they might have noticed suspicious activity.

After patching the vulnerabilities, the next most important thing you need to do is stay on top of your website security. Make sure to always escape HTML entities on your forms and watch out for the actions of third-party tools. Lastly, force all your visitors to re-authenticate in order to refresh their cookies.

Now that you know about how Cross-site Scripting works and the ways it can threaten your website visitors, make sure to do a security sweep on your settings.

Consider the tips in this article on how to prevent the issue from happening and be quick to resolve it in case it happens. Such a problem might irreparably damage your user’s trust in you, so be mindful of how serious this is.

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