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What is Cross-Site Scripting (XSS) and how do you prevent it?

Read our guide to find out what Cross Site Scripting (XSS) is, how it works, why it’s dangerous and how to detect, recover from and prevent it.

cross site scripting

Cross-site scripting, also referred to as XSS, is a prevalent vulnerability that is deeply intertwined with client-side scripting, posing an exceptional challenge to web security mechanisms.

Providing a number of valuable insights, we’ll be covering:

  • What is cross-site scripting and how does it work?
  • The different types of cross-site scripting attacks
  • How can cross-site scripting be used and why can it be dangerous?
  • What is the potential business impact of XSS?
  • Real-life examples of XSS attacks
  • How to detect and test for XSS
  • What to do if you’re a victim of an XSS attack
  • How can XSS be prevented?
  • FAQs
  • Conclusion

What is cross-site scripting and how does it work?

Cross-site scripting is a pervasive and powerful web application vulnerability, taking advantage of the trust users place in websites and the freedom websites have to execute scripts on a user’s browser.

It gives attackers the capability to infiltrate malicious scripts into web pages viewed by unsuspecting users. Bad actors are then able to circumvent access controls, leading to a wide range of unauthorized activities carried out under the guise of the victimized user.

Here’s how it works in short:

Injection point

Every cross-site scripting attack begins with the discovery of an injection point. This point refers to the specific spot within a website’s input fields where an attacker can surreptitiously insert a malicious script.

It’s often found in places where user input is accepted and then reflected back to the user in some form, such as search results, user profiles, comments, etc.

Malicious script injection

After identifying a suitable injection point, the attacker will attempt to inject their own malicious scripts, often disguised or encoded to evade detection, and introduce them into the chosen web application’s input field.

Server-side handling

The injection’s success largely depends on the server’s reaction. Once the server receives the input, it processes the request and responds. If the input is not correctly sanitized, the attacker’s script will also be included in the server’s response along with the application’s legitimate code.

Victim interaction

The next step in the attack occurs at the user’s end.

When the victim interacts with the compromised webpage, they unknowingly trigger the malicious script. Such interaction can encompass a range of actions, from clicking on a specific link or button, submitting a form, or simply loading the webpage.

Once executed the script will interact with the application in the current user’s context potentially allowing an attacker to access the application as the legitimate user would.

The different types of cross-site scripting attacks

While the basic concept of cross-site scripting remains the same, the manner in which the attack is carried out varies. Each approach brings unique implications and challenges for both the attacker and the victim.

Let’s look at some common types of cross-site scripting attacks:

Stored XSS

Also known as persistent XSS, the stored cross-site scripting attack is characterized by the malicious script’s permanent presence on the target server. It becomes part of the website and is served to users whenever they access a specific webpage or function.

Given its persistent nature, this type of cross-site scripting is particularly dangerous as it can affect multiple users without requiring individual attacks.

Reflected XSS

Unlike stored cross-site scripting, reflected XSS doesn’t involve the permanent inclusion of the script in the target website. Instead, the malicious script is embedded within a URL.

The script is executed when a user clicks on this manipulated link. Due to its reliance on user interaction, phishing is often employed to lure victims into initiating the attack.

DOM-based XSS

DOM-based cross-site scripting, or Document Object Model-based XSS, is a more advanced form of cross-site scripting.

Here, the attack targets the DOM (the programming interface for web documents) and manipulates it to execute the malicious script. In this case, the server may not be aware of the attack at all as it happens entirely on the client side.

Blind XSS

In a blind XSS attack, malicious scripts are injected without immediate feedback. Attackers rely on unsuspecting users or systems to trigger the execution of these scripts, making it challenging to detect and mitigate the attack.

Self XSS

Self XSS is reflected XSS coupled with social engineering. In a self XSS attack, a user will typically be instructed to insert a script into the developer console within their web browser. Once the script is inserted it operates in much the same way it would in the reflected XSS attack.

How can cross-site scripting be used and why can it be dangerous?

Cross-site scripting is not a threat to be taken lightly. Its potential to cause harm is vast, ranging from mild annoyances to significant threats to personal and organizational security.

The versatility of cross-site scripting is precisely what makes it so dangerous. Some possible uses of cross-site scripting include the following:

Cookie theft and session hijacking

In a cross-site scripting attack, attackers can steal session cookies, allowing them to impersonate the user and hijack their session. This means they can carry out actions on the website as if they were the user themselves, which can lead to unauthorized access to sensitive data or functionality.

Credential theft

Another major threat of cross-site scripting is credential theft. By injecting malicious scripts, attackers can create fake login screens or otherwise trick users into providing their login credentials. This can lead to a breach of personal and financial information, with potentially severe consequences.

Defacement or content manipulation

Cross-site scripting can also enable attackers to modify the content of a website, changing its appearance or the information it displays. This can lead to a spread of misinformation, manipulation of user perception, or damage to the brand’s reputation.

Phishing attacks

Phishing can be significantly amplified with cross-site scripting. By manipulating websites to make their fraudulent attempts appear more legitimate, attackers can greatly increase their chances of success.

Drive-by downloads

Some cross-site scripting attacks involve forcing a download onto the user’s device without their knowledge or consent. Such drive-by downloads often involve malicious software, such as viruses, computer worms, ransomware, or spyware, leading to further compromise of the user’s system.

Browser exploitation

Certain XSS attacks target the browser itself, exploiting vulnerabilities to execute harmful scripts or even gain control over the browser. Such control can then be used for a variety of malicious activities, including, but certainly not limited to, data theft, system compromise, and botnet formation.

The spread of worms and malware

By exploiting cross-site scripting vulnerabilities, attackers can spread worms and other types of malware to other systems. Each compromised system can potentially propagate the attack further, allowing the bad actor to extend the reach and impact of their assault significantly.

What is the potential business impact of XSS?

When a business succumbs to an XSS attack, the effects can be severe and wide-ranging. Depending on the attack specifics, an organization may face any or indeed all of the following: 

Financial loss

The most immediate and noticeable outcome of an XSS attack can often be financial. This can occur directly, like in cases of fraud or theft executed using hijacked accounts, or indirectly, due to costs associated with remediation, damage control, and potential fines or legal settlements.

Reputational damage

Customers and stakeholders invest a lot of trust in organizations, and a security breach can critically damage that trust. Awareness that an organization has been a victim of an XSS attack can lead to a loss of confidence among both clients and the public, damaging its reputation.

Customer trust and loyalty

Similarly, customers directly affected by an XSS attack – for example, by having their account hijacked or their data stolen – may lose trust in the business and become less loyal. This could lead to a loss of customers, negatively impacting a company’s bottom line.

Regulatory compliance issues

Data protection and privacy regulations like GDPR and CCPA impose strict requirements on organizations to safeguard user data. A cross-site scripting attack resulting in a data breach can lead to non-compliance with these regulations, and the business may face heavy fines, legal action, or other penalties.

Intellectual property

A commonly overlooked consequence of XSS attacks is the potential for intellectual property theft. If a bad actor gains access to sensitive information, they can steal or leak this data, potentially leading to competitive disadvantage or other types of harm.

Legal and compliance risks

In addition to the regulatory risks mentioned earlier, businesses may also face other legal and compliance risks following a cross-site scripting attack.

For example, they might be held accountable for any harm suffered by customers as a result of the attack. They may also face scrutiny or penalties from industry bodies or certification authorities.

Real-life examples of XSS attacks

One notable example of a high-profile cross-site scripting attack was the Samy worm, which spread rapidly through the popular social media platform MySpace in 2005.

The attacker exploited a stored XSS vulnerability to create a worm that replicated itself on users’ profiles. In less than 24 hours, the worm had spread to more than one million profiles, making it one of the fastest spreading viruses of all time.

Another example involved the social networking site Facebook. In 2013, a Palestinian security researcher discovered a significant XSS vulnerability that could allow an attacker to post anything on anyone’s timeline.

When his reports were initially ignored, he used the vulnerability to post a message on Facebook CEO Mark Zuckerberg’s timeline to demonstrate the severity of the issue.

How to detect and test for XSS

Detecting XSS vulnerabilities is a significant step in protecting against cross-site scripting attacks. This involves a mix of manual code reviews, automated testing and, most importantly, fostering a security culture within the development team.

Here are some techniques used to detect XSS vulnerabilities:

Manual code review

A manual review of the code is one of the most straightforward yet effective methods of detecting potential XSS vulnerabilities. By closely examining the code, especially in areas that handle user input or involve dynamic content, developers can spot areas where inadequate input sanitization or output encoding might allow a cross-site scripting attack to succeed.

Input fuzzing

Input fuzzing, also known as fuzzing, is a technique that involves supplying unexpected or random data to the system and observing its reaction.

The goal is to trigger an error or exception that might reveal a potential vulnerability. In the context of detecting XSS, input fuzzing may involve submitting scripts or special characters in input fields and checking whether they’re correctly sanitized or encoded in the response.

Output inspection

This method involves inspecting the data sent from the server to the client’s browser. The goal is to identify whether any user input is reflected back without proper sanitization or encoding, which could lead to potential XSS vulnerabilities.

Security scanners

Security scanners are automated tools used to scan applications for known vulnerabilities. They play an important part in detecting XSS by checking for conditions known to lead to these attacks.

Static code analysis

Static code analysis involves analyzing the source code without executing the program. This analysis can detect potential XSS vulnerabilities by identifying dangerous coding practices, such as using unsanitized user input within dynamically generated HTML.

Browser-based testing

Browsers can offer certain tools and modes for security testing. With their help, developers can monitor network activity, inspect HTML elements, and debug JavaScript, which can help identify XSS vulnerabilities.

Penetration testing

Penetration testing an authorized simulated attack on a system designed to uncover weaknesses. In the context of XSS, penetration testers try to exploit these vulnerabilities to test the system’s defenses.

Security audits

Security audits involve a comprehensive examination of system security measures. An auditor reviews the system’s user access controls, risk management protocol, and security policies to identify and scan for vulnerabilities, including XSS.

What to do if you’re a victim of an XSS attack

If you have experienced an XSS attack, taking the following steps can help you control the damage and prevent further exploits.

Disconnect from the internet

This halts any ongoing communication between your device and the attacker’s server, helping to prevent the spread of the attack.

Update and enable security software and updates

Ensure that your antivirus software and firewall are updated to the latest versions to guard against further attacks. Enabling automatic updates can help protect your system from known vulnerabilities.

Change passwords and update credentials

Modify all your passwords and other credentials to prevent the attacker from accessing your accounts.

Clear browser cache and cookies

This helps remove any potentially harmful data stored by the attacker.

Monitor account activity

Keep an eye on your accounts for any unauthorized activities, indicating that they might have been compromised.

Report the incident

Inform the website or service where the attack occurred and report to law enforcement agencies if necessary.

Locate the vulnerable code and patch vulnerabilities

Identify and rectify the part of the code that enabled the XSS attack, thus preventing future attacks.

Remove malicious content and backdoors

Ensure your system is thoroughly cleaned to remove any harmful content and close any backdoors the attacker might have installed for future access.

Seek support from the professionals

If you’re unsure about managing the aftermath of the attack, consider consulting with a cybersecurity professional.

How can XSS be prevented?

Given the potential impact of an attack, preventing cross-site scripting is a critical aspect of web application security. This section provides several preventative measures and best practices to help guard against XSS attacks. 

Input validation and sanitization

Ensure that all user inputs are validated and sanitized before they are processed. This can help prevent malicious scripts from being executed.

Train and maintain awareness

Staff training is essential to maintaining a secure environment. Regular training can help employees recognize and prevent potential attacks.

Don’t trust any user input

Always treat user input as potentially malicious, and handle it appropriately.

Output encoding

Make sure any user-supplied data included in dynamic content is properly encoded to prevent it from being interpreted as code.

Limited user-generated HTML

Limit the use of user-generated HTML to reduce the risk of script injection.

Set the HTTPOnly flag

By setting the HTTPOnly flag for cookies, you can prevent them from being accessed by client-side scripts.

Use a content security policy

A Content Security Policy (CSP) can help prevent XSS attacks by limiting the domains from which scripts can be loaded.

Scan regularly for vulnerabilities

Regular scans can help identify and fix vulnerabilities before they can be exploited.

Setup a WAF

A Web Application Firewall (WAF) can help protect against XSS attacks by filtering and monitoring HTTP traffic.

Keep security updated

Regularly update all software and systems to ensure that you’re protected against known vulnerabilities.

Secure coding practices

Following secure coding practices, such as using parameterized queries, can help prevent many types of attacks, including XSS.


How does XSS differ from other web vulnerabilities?

XSS primarily exploits the trust a user has in a website, allowing an attacker to execute scripts in the user’s browser. This contrasts with many other web vulnerabilities, which exploit flaws in the server’s software.

What is client-side code?

Client-side code is code that runs on the user’s machine, rather than on the web server. This includes languages like JavaScript, which is often used in XSS attacks.

Is client-side input validation enough to prevent XSS attacks?

No, client-side input validation is not enough to prevent XSS attacks. An attacker can bypass client-side validation, so it’s crucial to also validate and sanitize inputs on the server side.

How common are XSS attacks?

XSS attacks are among the most common web application vulnerabilities. They appear frequently in the OWASP Top 10 – a list of the most critical web application security risks.

What’s the difference between XSS and SQL injection?

Cross-site scripting targets the users of a website by injecting malicious scripts into web pages viewed by them, while SQL injection targets the website’s database by injecting malicious SQL queries.

What’s the difference between XSS and CSRF?

While both XSS and Cross Site Request Forgery (CSRF) are web security vulnerabilities, they differ in nature. XSS allows an attacker to inject malicious scripts into a webpage viewed by a user, while CSRF tricks the victim into submitting a malicious request on the attacker’s behalf.

Can XSS detection be automated? 

Yes, several automated tools exist for detecting XSS vulnerabilities, including security scanners and static code analysis tools.


Understanding and mitigating cybersecurity threats, such as cross-site scripting, is paramount. As XSS continues to be a significant risk factor, adopting robust security practices, thorough testing mechanisms and continuous education are crucial in mitigating these threats.

At CovertSwarm, we are experts in continuously exposing and helping fix flaws in even the most complex systems. We believe in a proactive approach to security and, through our simulated digital cyber attacks, we can catch XSS vulnerabilities before they can be exploited by threat actors.

Secure your defenses. Choose CovertSwarm. 

Partner with our Swarm of ethical hackers to ensure your cybersecurity stance keeps pace with the bad actors. Contact us for more information about XSS attacks.