Cross Site Scripting (XSS)

A few days ago, my friend was trying to log into her bank account digitally when a link on the computer appeared asking her to click on it. The link looked pretty legit and promised to give out some of the best financial tips for the upcoming year. 

I did not let her click on it, although she could really use some sound financial advice since she got laid off not more than a week ago (more on that later). 

Why? Well, clicking on that link could lose her all her money, in the blink of an eye. It was an inevitable cross-site scripting trope, one of the deadliest cyber attacks to exist today. 

If this sounds familiar, read on to learn what cross-site scripting, also known as XSS, entails.

What is Cross-Site Scripting?

Cross-site scripting is a cyber attack that entices users to click on a particular link on their device. This link, once clicked, executes a malicious script, enabling the attacker to steal the user’s active session cookie. 

Popularly known as XSS, this attack, if successful, allows the attacker to steal all of the victim’s data from the application. Now, imagine the victim user has subscribed to certain exclusive benefits on the application. Using his access, the attacker, posed as the user himself can manipulate all the data the user account has access to. 

Cross-Site Scripting Types

As one unanimous modus operandi is easily noticeable, a cross-site scripting attack can take over many forms. They can appear on your device out of the blue and not just as direct links. These attacks are usually a result of vulnerable applications that require an immediate security updation to protect their database. Here are a few types of XSS attacks:–

1. Reflected XSS

This is the most common type of XSS attack where the user, during their session with the application receives a link on their system. The attacker executes the reflected XSS by sending the link to the application in the form of an HTTP request. 

This request is overlooked by the vulnerable application and not checked for any malicious intent. Now, the unsuspecting user receives the link on their device and clicks on it to give the attacker complete access to their data.

2. Stored XSS

Stored XSS occurs when the user does not come in contact with an active link from the attacker’s end but rather a stored redirect on the server’s end. Also known as persistent or second-order XSS, they are usually posed unceremoniously in the form of a comment or any other user-entered detail. 

Unlike reflected XSS where the payload is immediately returned in the response, stored XSS saves the injected scripts into the database, file, or any other storage medium. When the user browses this page, the section with the malicious script is loaded and retrieved to undergo execution. 

Many websites, social media platforms, and applications encourage their visitors to leave opinions, feedback, or contact details. These applications are a core hub for stored XSS attacks. With stored XSS attacks, attackers reach a large number of users, deploying a single resource over time.

3. DOM-based XSS

Now, stored and reflected XSS manipulates the server to draw victims. However, DOM-based XSS does not need to interact with the server to execute itself. In this scenario, the attacker sends a malicious payload to the client-side Javascript code. 

Once this code is processed, the Document Object Model (DOM) undergoes modification to suit the attacker’s intent. Following this, the attacker can now steal cookies, and manipulate any information on the website.

How Cross-Site Scripting (XSS) Works

So now that you know what XSS is, why not understand how it works? Understanding the attack flow and the role of user input and injection points helps developers implement effective security measures to prevent XSS vulnerabilities. 

Let me show you an overview of the attack flow:

1. User input

The first step of an XSS attack begins with a user submitting malicious input. This could be a script containing JavaScript code entered into a vulnerable form field or URL parameter within the web application.

2. Injection points

Now, the injected malicious script is stored or reflected in the application's response, depending on the type of XSS as I have discussed earlier. These points or junctures where the script is injected are known as the injection points. These could be anything ranging from search boxes to HTTP headers or even URL query strings.

3. Execution

When other users interact with the affected web page or resource, the browser interprets the injected script as legitimate code. The unsuspecting user and the browser come together to execute this code within the context of the vulnerable website.

4. Attacker's objectives

There is no one malicious intent fueling these attackers. Once they have control over your data and unlimited access to it, they can do anything including stealing sensitive information such as cookies or tokens or imitating the user to perform unauthorized actions. This can deface websites, spread malware to other visitors through the user’s account, or misuse any sensitive data for financial benefits.

Impact of XSS Attacks

A cross-site scripting attack poses serious threats to users and organizations that can not only affect business but cause irreversible damage to the site’s reputation. Following are a few impacts of an XSS attack.

Consequences for users:

1. Data exposure

XSS exploits result in the exposure of sensitive user data, including passwords and financial information. This data can be used for identity theft, fraud, or unauthorized access to accounts. As Dwight Schrute said, “Identity theft is not a joke, Jim. Millions of families are affected every year!” 

2. Financial losses

Users may incur financial losses owing to cross-site scripting attacks. This is bound to happen once the financial information of the victim is accessible by the attacker who can then carry out unauthorized transactions or fraudulent activities.

3. Privacy violations

XSS attacks compromise user privacy by exposing personal information to unauthorized parties. In the age where data privacy is causing quite the hubbub, this unauthorized access leads to targeted advertising, privacy infringements, and invasive profiling.

Consequences for organizations:

1. Reputational impact 

Starting at where we left off for users, companies can suffer significant reputational damage due to security breaches caused by XSS vulnerabilities. This can lead to customer churn, negative publicity, and diminished brand loyalty for the business.

2. Financial implications 

XSS attacks can also result in financial losses. Once victims file a case with the cybercrimes department, the company will have to bear the brunt of legal fees, remediation costs, regulatory fines, and potential lawsuits, to name a few.

3. Legal and regulatory compliance

Talking about legal expenses, companies will face legal and regulatory repercussions for failing to protect user data adequately. This will not just include a financial landslide but also follow up with sanctions and bans for non-compliance with data protection laws.

4. Operational disruptions

Now, once the blow is over and the company is reeling from the loss, dealing with the aftermath of XSS attacks can disrupt normal business operations. The organization will be forced to divert resources away from essential activities toward damage control, forensic investigations, and security upgrades. This inevitably affects the company's scalability and stunts any prospective growth.

5. Competitive disadvantage 

Publicized security incidents will put organizations at a competitive disadvantage, affecting their ability to attract and retain customers compared to more secure competitors.

How to Prevent Cross-Site Scripting

So, is there a way to completely avoid cross-site scripting? Yes, there is. 

Cross-site scripting prevention is a manifold procedure that includes several steps to ensure that your website is not a potential goldmine for attackers. If you want hands-on experience with these prevention mechanisms, head over to upGrad’s CTME Cybersecurity Program, in collaboration with Caltech, and register yourself today. 

However, here’s a brief overview of the preventive ways to tackle cross-site scripting:

1. Input validation

The first thing you ought to do is control what is being entered into your website through input validation routines on server-side code. Ensure that inputs are in expected formats and reject those containing potentially malicious code. This approach helps neutralize harmful characters before reaching vulnerable components, reducing the risk of XSS attacks.

2. Output encoding

Now that you’ve taken care of the inputs encode user-supplied data appropriately based on formats such as HTML, JavaScript, and URLs. Use context-specific encoding functions to neutralize special characters that could be interpreted as executable code by browsers.

3. Content security policy (CSP)

Filter the sources that will be able to load or execute content. This you can do with a strict CSP to define and enforce content rules within your web application.

4. Secure coding practices

Avoid dynamic script generation with user inputs and use libraries or frameworks that handle encoding and escaping automatically. Prioritize secure coding practices and regularly update dependencies to patch known vulnerabilities, reducing the attack surface and strengthening application security.

5. HTTP headers and cookies

Configure HTTP headers and use secure cookies to protect against XSS attacks. Secure HTTP headers help protect cookies from being accessed by malicious scripts, preventing unauthorized access to session tokens.

Case Studies

Let us take a look at some real-life case studies that demonstrate the impact and consequences of XSS attacks and the importance of implementing robust security measures:–

1. MySpace Worm (2005)

In 2005, a notorious XSS attack known as the "Samy" or "MySpace worm" exploited a vulnerability in MySpace's profile system. The attacker, Samy Kamkar, injected malicious JavaScript code into his profile, which replicated itself when other users viewed his page. 

This resulted in a hilarious and harmless prank that sent Samy Kamkar a friend request from the user’s MySpace account. It also displayed a message on the victim’s MySpace profile that said, “But most of all, Samy is my hero.”

2. Yahoo Mail (2015)

In 2014, Yahoo Mail was targeted by an XSS attack that allowed attackers to steal users' email cookies. By exploiting a persistent XSS vulnerability, attackers could execute malicious scripts on Yahoo Mail's web interface and access users' email accounts. 

This incident was brought to notice by Jouko Pynnönen, a researcher from Finland and he bagged a whopping $10,000 for this discovery.

3. Equifax Breach (2017)

Moving on to one of the most horrifying cyberattacks in history, the Equifax data breach in 2017 was initiated through an XSS attack on a vulnerable web application. Attackers exploited an XSS vulnerability to gain access to Equifax's systems and exfiltrate sensitive customer data, including Social Security numbers and credit card details. 

This incident affected and compromised about 147 million Americans along with 15 million British citizens and about 19,000 Canadian citizens. 

Wrapping Up

Having discussed XSS and its prevention techniques in detail, as of 2023, 30% of websites are vulnerable to XSS attacks. This means that with the growing digitization of businesses, the need for skilled cybersecurity experts is through the roof. According to a World Economic Forum survey of global cyber leaders in 2022, 59% of the participants struggle to respond appropriately to a security breach due to the lack of skilled experts on the team. 

Remember my friend Sheena, who got laid off last week? Well, she is already taking steps to upskill herself with upGrad’s Advanced Certificate Program in Cybersecurity, a course with over 300 hiring partners! So, if this sounds like something you would be into, then the opportunity is just a click away. 

Frequently Asked Questions

1. What is cross-site scripting?

Cross-site scripting is a form of cyber attack where a particular user action enables malicious scripts into a website or application.

2. What is an example of XSS attack?

A typical  cross site scripting example is the Uber case when an attacker injected a script such that drivers, when they registered themselves on the app, would have no control over their data. This means that all the private information that they used for the registration was visible to the attacker who could then manipulate the data to his benefit. 

3. What is XSS and how do you prevent it?

XSS, or cross-site scripting is a cyberattack mechanism that arises from a client-side vulnerability. You can prevent XSS by employing strict security measures on the website or application. Before processing a script, you must also verify every HTML input and Javascript element. 

4. What is XSS and CSRF?

XSS stands for Cross-Site Scripting and CSRF stands for Cross-Site Request Forgery, also known as the one-click attack.

5. Why is it called cross-site scripting?

It's called "cross-site" because attackers use scripts from one site to inject into and corrupt another site, thereby compromising security on the latter.

6. Is cross-site scripting still used?

Yes, cross-site scripting is still used and has grown to become one of the biggest cybersecurity threats in the world.

7. Why is cross-site scripting illegal?

Cross-site scripting is illegal because it poses a serious threat to the user’s data privacy and allows the attacker to cause irreparable damage to both the user, and the company.

8. What is the difference between cross-site scripting and CORS?

A cross-site scripting vulnerability injects malicious scripts into web pages, while Cross-Origin Resource Sharing (CORS) controls access to resources across different origins.

9. How many types of cross-site scripting are there?

XSS comes in three types: DOM-based, reflected, and stored. They are differentiated based on where the malicious script is injected and presented to the user before execution.