Simply: what is Cross-Site Scripting (XSS) ?

Cross-Site Scripting (XSS) is a hacking attack technique that cybercriminals use to inject malicious scripts into websites or applications considered safe.

These often underestimated attacks represent a serious threat to user data security because, by exploiting XSS vulnerabilities, cybercriminals can access sensitive information.

Detecting and preventing XSS attacks are therefore crucial to protect your website or application and ensure the security of your users.

Worried about Cross-Site Scripting attacks? Want to know what you can do to avoid falling victim? Rest assured, there are effective ways to protect yourself.

In this article, Debugbar will explain XSS attacks in a very simple way and give you the keys to identify and counter them. Ready to start the battle against Cross-Site Scripting attacks? Let’s go!

What is a Cross-Site Scripting (XSS) attack?

An XSS attack, for Cross-Site Scripting, is a security flaw in a website or application that allows a hacker to inject and run malicious code on the user’s browser (client-side).

To make it simple:

• A hacker finds a way to inject malicious code, usually in JavaScript, into a webpage or an application that other people will use.
• This code is then executed on the client-side, often without the user realizing it.
• The script can then perform various malicious actions, such as steal personal information, modify the web page content, or redirect the user to another site.

As you can guess, this type of attack can be particularly formidable because it exploits the user’s trust in the visited website.

In fact, users often have no idea that their browser is running these malicious scripts.

Moreover, they can take various forms which makes them particularly insidious and dangerous.

What are the main types of XSS attacks?

There are mainly three types of XSS attacks:

• Reflected XSS attacks,
• Stored XSS attacks,
• And Document Object Model-based XSS attacks.

Understanding how each one works is the first step to protect your website from these threats.

Reflected XSS attack

A reflected XSS attack occurs when a hacker convinces a user to click on a link containing a malicious script. When the user clicks on this link, the script is sent to the web server and returned to his browser where it is launched.

To explain this simply:

1. The attacker sends you (via email, dialog box, etc.) a link that seems legitimate, but which actually contains a secret code (malicious script) hidden inside.
2. When you click on this link, your browser sends the link, including the secret code, to the server of the website or app you are visiting.
3. The web server, not realizing that the link contains malicious code, returns a modified web page now including this code.
4. As soon as the web page appears on your screen, the secret code executes.

The script can then:

• Access everything you type (like your passwords or credit card details) and send it to the attacker.
• Change what you see on the page to show you false information or advertisements.
• Automatically redirect you to another site which could be dangerous.

In short, the attacker does not need to modify the website or application itself; he simply uses the link to get the malicious code into your browser. This is why it is very important not to click on suspicious or unknown links.

Stored XSS attacks

Also known as persistent XSS, this type of attack happens when the malicious script is permanently stored on the victim server.

Wondering how this could happen? Well, simply:

1. A hacker writes a comment on a site or forum, but instead of a simple message, he will input a malicious script in it.
2. This comment with the hidden script is saved on the web server in the website’s database. It thus becomes a permanent part of the site.
3. Every time someone visits the site page to read the comments, the malicious script automatically launches in their browser.

This attack is particularly dangerous because, unlike reflected XSS attacks which target one person at a time, a stored XSS attack can affect everyone who visits the concerned page.

And the worst part is that the stored XSS attack is really insidious because the malicious script stays on the site until it is detected and removed, potentially affecting thousands of users without either they or the site administrator realizing it immediately.

The Document Object Model-based XSS attack

A Document Object Model (DOM)-based Cross-Site Scripting attack happens when a hacker manipulates a web page’s structure in your browser (the DOM) to execute malicious code (usually it’s a JavaScript one). 

Such manipulation often exploits flaws in the page’s script, modifying the page’s behavior undesirably without directly affecting the web server.

To put it simply, imagine that our webpage is like a bulletin board loaded with various information.

Under regular circumstances, only those managing the site can change what’s displayed on this board. Nevertheless, with a DOM-based XSS attack, it’s as if someone stealthily approaches from behind and covertly sticks a new note on the board.

This note might seem like the other things usually displayed, but in reality, it hides instructions that can do undesired things, such as opening a new window or redirecting you to a different website.

The DOM-based XSS attack happens within your browser, when you access the page: the website itself isn’t altered, but this hidden “note” modifies how the page displays and behaves in your browser.

Through this, the attacker can accomplish several harmful actions directly on your computer:

• Run malicious scripts to steal your personal information or install spyware.
• Manipulate your actions by luring you to click links or buttons that trigger malicious actions.
• Create fake dialog boxes to ask you to input sensitive information that is then stolen.

And, you might have guessed, whatever the attacks launched are, the aftermath could be catastrophic…

What are the risks of an XSS attack?

XSS attacks can cause severe damage, both to users and businesses owning the compromised websites / blogs / applications.

The main risks from this type of attack include the following:

• Data loss: Cybercriminals can use scripts to steal sensitive information such as passwords, credit card numbers, or other personal data.
• Session hijacking: By accessing cookies, pirates can impersonate users and take over their session.
• Backdoor creation: Attackers can exploit an XSS flaw to create a backdoor on a website or application, allowing them controlled and persistent access.
• Spreading Malware: Attackers can force the user’s browser to download and run malicious software.
• Phishing: Pirates can make the user believe they are interacting with a trusted site, whereas in reality, they are redirected to a malicious site controlled by the attacker.

The consequences of a successful attack could then be devastating for a business:

• User trust loss,
• reputation damage,
• legal procedures and fines in case of data protection laws violation.

Everything you’ve just read worries you? Want to know if your site is well-protected against such an attack? Let’s find out right now if your site has any vulnerabilities!

How to determine if your website is vulnerable to an XSS attack?

To find out if your website is an easy prey, you can do 3 things :

• Run a scanner.
• Conduct manual tests.
• Perform a code audit.

Let’s delve into these a bit more.

Vulnerability scanner

A vulnerability scanner is an automated tool that scours your website for potential security flaws.

It checks whether scripts can be injected into various parts of your site, including forms, cookies, and comment fields.

To use it, you simply need to enter your site’s URL and run the scanner.

It will then analyze all pages and provide a detailed report of weak areas found.

However, keep in mind no automated tool is perfect and there may be false positives or negatives. So it’s always best to also conduct manual tests to confirm the results.

Manual testing

Manual testing is another effective method for detecting XSS vulnerabilities.

This typically involves inserting scripts into different locations of your site to see whether they are executed or not.

To conduct a manual test:

1. Select a section of your site where users can enter text (say a contact form) ;
2. Try to input a simple script there.

For example, you can try entering the following script: alert(‘Test’).

If an alert window pops up with the word “Test” when you access this page, it means your site is vulnerable to XSS attacks.

Code audit

This method involves scrutinizing the source code of your site for sections of code that could be exploited by attackers to inject malicious scripts.

Carrying out a code audit can be a daunting task, especially for a big site, but it can also uncover vulnerabilities that might have been missed by automated scanners.

A code audit requires some skill and a good understanding of web programming languages (like JavaScript). Therefore, it’s better to hire a professional.

Note: The sections of code to check first are:

• Those handling user inputs,
• Cookies,
• And those generating HTML content.

Did you perform all the tests and your site appears vulnerable? Do you want to bolster its security? Then let’s see how you can protect against XSS attacks.

How to protect against an XSS attack?

Defending your website from XSS attacks requires a multilayered approach. But there are 4 things you absolutely must do for protection:

1. Learn to recognize an XSS attack.
2. Maintain ultra secure code.
3. Get trained.
4. Utilize security tools.

Learn to recognize the threat: What does an XSS attack look like, some examples

XSS attacks can take many forms and manifestations. The first thing to do is to know them to better recognize and guard against them.

Here are a few typical examples of XSS attacks…

Malicious script in a comment

The attacker will input a malicious script into a message on a blog or forum.

Whenever a user views this comment, the script launches in their browser, jeopardizing their session. This type of attack is a classic example of stored XSS.

Script injection into a search field

An attacker can insert a harmful script into a search bar on a website. When a user performs a search, the script runs and can steal sensitive information.

Script hidden in an image

A cybercriminal can also hide a malicious script in an image’s source code. When the image is displayed on the website, the script runs in the user’s browser.

Phishing via malicious links

Attackers can send malicious links via email or through social media. When the user clicks on the link, they are redirected to a compromised site where XSS scripts run to steal their credentials.

Script in a contact form

Hackers can also insert a harmful script into a contact form on a site.

When the user fills out and submits the form, the script runs, potentially compromising their data.

These examples highlight just how varied and sophisticated XSS attacks can be. It’s therefore crucial to always remain vigilant and take preventative measures to protect your site.

Adopt secure coding practices

On any site, secure code is the first line of defense against XSS attacks.

To ensure your code is secure you should:

• Ensure input validation: Never trust data supplied by users. Make sure it’s in the expected format before using it.

Escape special characters (like <, >, &, ‘, “, and /) from user inputs. This is crucial for any data that will be reinserted into the webpage.
For example, if you’re expecting a phone number, only accept digits and reject any other character.

• Use Secure Frameworks: Employ frameworks that automatically escape XSS vulnerabilities, like ReactJS for front-end or Ruby on Rails for back-end.
• Use whitelisting: Only allow safe and known input by implementing a whitelist of permitted values.
• Use output encoding and implement a Content Security Policy (CSP): When you display data supplied by the user, make sure to encode it properly. Implement CSP headers to restrict how resources such as JavaScript can be executed on your pages. This can prevent XSS attacks even if a vulnerability is present.

This means you should convert certain special characters into a representation that can be read by the browser but won’t be executed as code. 

• Configure HTTP headers: HTTP headers play a pivotal role in safeguarding web applications from various types of attacks, including Cross-Site Scripting (XSS). Properly configuring these headers can provide an essential layer of security.
  • Use HttpOnly and Secure flags in cookies to prevent access to cookie data through client-side scripts.
  • X-Content-Type-Options: Set to nosniff to prevent browsers from interpreting files as something else than declared by the content type in the HTTP headers.

By limiting what each part of your code can do, you can limit the damage a hacker can cause by exploiting vulnerabilities to inject his own scripts.

Educate and train your teams continuously

Education is another powerful weapon against XSS attacks. Indeed, understanding what an XSS attack is, how it works, and how to avoid it can greatly contribute to reinforcing your site’s security.

You should therefore:

• Train your developers on secure coding best practices.
• Encourage a security culture within your organization, where all stakeholders, from leaders to employees, are aware of threats and know how to act securely.

Use appropriate security tools

Finally, you should use security tools and software capable of protecting your site.

There are numerous tools that can help you protect your site against XSS attacks.

For example :

• Web Application Firewalls (WAF) can detect and block XSS attacks in real time.
• Encoding libraries can help prevent XSS attacks by automatically converting special characters into their safe HTML representation.
• If you’re using WordPress, certain plugins can handle protection. Some of the most well-known include:
  • Sucuri,
  • Wordfence,
  • MalCare,
  • and Web ARX.

As you can see, guarding against an XSS attack demands ongoing effort and constant vigilance.

However, with a good understanding of the issue, safe coding practices, and appropriate security tools, you can greatly reduce the risk to your site and its users.

Despite all your efforts, has your site been affected by an XSS attack? You need to act fast!

What to do when victim of an XSS attack?

If unfortunately, you discover that your site has fallen victim to an XSS attack, it’s important to take immediate action to minimize damage and prevent future attacks.

Identify and isolate the vulnerability

When you spot an XSS attack, the first thing to do is locate the vulnerability that allowed the exploitation.

Here’s how to proceed step by step:

1. Detect the attack: Often, you can be alerted of an ongoing attack by user reports or monitoring systems.

Symptoms of an XSS attack can include :

• A sudden increase in traffic,
• Unexpected changes to your site,
• Or user complaints about strange behavior.
  

2. Identify the source: After confirming the attack, you need to identify exactly where the malicious script was injected. This could be in a comment, a blog post content, or a form field.
3. Isolate the vulnerability: Once you’ve identified the source of the attack, disable this part of your site or put it in maintenance mode. This will stop the malicious script from continuing to affect users while you work on fixing the issue.

This is an urgent and necessary step that not only helps minimize damage but also prevents potentially active attackers from causing more harm.

Analyze and fix the vulnerability

Once the vulnerability is isolated, the next step is to understand how it was exploited and fix it accordingly.

Here’s how:

• Understanding the vulnerability: Study the code linked to the identified flaw and try to understand how and why it was exploited.

For example, if the attack took place through a comment form, the problem might be that your site allows inclusion of HTML code in comments, making it possible to insert malicious scripts.

• Fixing the vulnerability: Once you’ve figured out how the attack took place, make the necessary corrections to your code to prevent it from happening again.

In our earlier scenario, you would need to modify your comment form so that it doesn’t accept HTML code or use an encoding method to render special characters that could be potentially dangerous, harmless.

• Testing your fixes: After making the required changes, test them thoroughly to ensure they solve the problem without creating new ones. You can do this manually or use automated security testing tools.

After fixing the flaw, you should always remain vigilant. Continue to monitor your site for any suspicious activity and malicious content, and ensure that all your other codes are secure against XSS attacks.

XSS attacks: Key takeaways

In conclusion, XSS attacks pose a serious threat to website security. They are diverse, cunning and can have disastrous consequences for both users and website owners.

Here are some important points to remember:

• Understand what an XSS attack is: An XSS attack is a technique used by cyber criminals to inject malicious code into a trusted website.
• Learn to recognize the different types of XSS attacks: There are three main types of XSS attacks – stored, reflected and DOM-based. Each one has its own characteristic and operates in a particular way.
• Be aware of the risks: XSS attacks can lead to a variety of problems such as data theft, identity fraud, distribution of malware, and much more.
• Prevent XSS attacks: Prevention is always better than cure. Make sure to adopt safe coding practices and use tools for detecting and managing vulnerabilities.
• Know what to do in case of an attack: If your site falls victim to an XSS attack, respond quickly and effectively by identifying the flaw, analyzing it, and then fixing it.

Remember: In online security, education and prevention are your best allies.