Cross-Site Scripting: What You Need to Know

Understanding Cross-Site Scripting

Cross-site scripting or XSS attacks happen when malicious scripts are injected into web applications. As a user, XSS attacks trick you into clicking a link or loading a page that runs the rogue hand.

Also known as XSS, Cross-Site Script Injection, Cross-Site Script Attack, Web Scripting Attack, Client-Side Code Injection, Cross-Site Content Hijacking, Script Injection Attack

Cross-Site Code Injection and Browser Script Injection

There are a few types of XSS attacks to watch out for. Stored XSS is when the malicious code is permanently stored on the target website. Reflected XSS bounces the code off the web app to the victim. DOM-based XSS manipulates the website's structure, while blind XSS fools you into entering hidden input.

The scripts are usually designed to steal your data, install malware, or redirect you to another page. To avoid becoming a victim, avoid unsolicited messages, links, and pop-ups. Never enter personal information on an unverified site. Use strong, unique passwords and two-factor authentication when available.

Staying alert and proactively protecting your accounts and devices is the best defense against XSS attacks. Knowledge is power, so understanding the threats empowers you to surf smarter and safer.

Types of Cross-Site Scripting Attacks

The two most common types of XSS attacks are stored and reflected. With stored XSS, malicious scripts are permanently stored on the target server, like in a database. When victims access the stored data, the script executes. Reflected XSS involves malicious code in a URL or form that is reflected off the web server and sent to the victim.

When the link or form is clicked, the script runs. These attacks can steal cookies, compromise accounts, and spread malware.

To avoid becoming a victim, prevent unsolicited links and forms, especially from untrusted sources. Use updated browsers with XSS filters and turn off scripting languages like JavaScript when possible. Developers must properly validate, encode, and sanitize all user inputs and outputs to block XSS attacks.

Staying vigilant and securing systems are the best defenses against these dangerous exploits.

Attack Vectors

Cross-site scripting (XSS) attacks occur when malicious scripts are injected into vulnerable web applications. As a user, you need to be aware of the various ways XSS attacks can happen.

The most common vectors for XSS are:

Injecting malicious code into user input fields on web forms that are then displayed on the page. 

For example, a hacker inserts JavaScript into a comment field displayed to all users.

Exploiting vulnerable web applications that don’t properly sanitize user input before displaying it on the page. 

For example, a hacker enters JavaScript into a search field and executes it when the results page loads.

Crafting malicious links or iframes that load malicious code into a vulnerable web page. For example, a hacker sends you a link that appears legitimate but contains JavaScript in the URL that gets executed when you click it.

Hiding malicious code in image tags, video embeds, or other rich content types. 

For example, a hacker uploads an image with JavaScript in the image name or embed code.

The bottom line is never click links, download attachments, or enter information on websites you don’t fully trust. XSS attacks can have serious security consequences, so browse carefully and be alert for anything suspicious.

Real-World Examples

Real-world examples of XSS attacks abound, and how much damage they can do is frightening. Hackers have used XSS to steal personal data, install malware, and gain unauthorized access to systems.

In 2010, the Indian airline SpiceJet was hit by an XSS attack that redirected passengers to a defaced page. The hackers left a message threatening to reveal stolen credit card data unless SpiceJet paid a ransom.

In 2017, Wendy’s fast food chain had an XSS vulnerability on its customer feedback form. The flaw lets hackers access customer data and install a keylogger to steal credit card numbers. Over 1000 stores and 5.5 million payment cards were impacted.

These sobering examples highlight why XSS remains a dangerous threat and why constant vigilance and patching are important.

Preventive Measures

To prevent XSS attacks, you can take a few important measures.

\n\n###Validate and Encode User Input

Never trust user input. Validate and encode all input from users before displaying it on a page. This includes data from forms, URLs, cookies, etc. Encoding converts characters into their HTML entity equivalents, preventing browsers from executing them as code.

\n\n###Use the Same-Origin Policy

The same-origin policy prevents scripts on one site from accessing information on another site. This limits the potential damage from XSS attacks. However, there are ways for attackers to get around this, so encoding user input is still critical.

\n\n###Keep Frameworks and Libraries Up to Date

Use the latest versions of web frameworks, content management systems, and libraries. Older versions often contain XSS vulnerabilities that were later fixed, so updating is important for security.

\n\n###Conduct Penetration Testing

Penetration testing, or pentesting, involves hacking into your systems to uncover and fix vulnerabilities before attackers can exploit them. Pentesting for XSS specifically can help find input validation issues and other weaknesses that could lead to successful XSS attacks.

\n\n###Educate Developers

Developers need to be properly educated on secure coding practices to avoid introducing XSS vulnerabilities in the first place. With education and vigilance, many XSS attacks can be prevented.

Browser Security Mechanisms

To protect against XSS attacks, browsers have developed some built-in security mechanisms. As a user, you should keep your browser current to ensure you have the latest protections.

Browsers use input validation to sanitize untrusted data from forms, URLs, and HTML. They strip out potentially malicious code before it gets processed. Browsers also implement the same origin policy, which prevents scripts from accessing data from other domains. This limits the impact of any XSS attacks.

Some browsers go further by disabling or limiting certain HTML tags, attributes, and scripting functionality. For example, they may disable inline JavaScript, block iframes, or limit CSS styles. These restrictions make XSS attacks much harder to carry out.

Though not foolproof, browser security mechanisms offer a first line of defense. Keeping your browser updated is one of the best ways to help prevent XSS and other web-based threats.

Frequently Asked Questions (FAQs)

What exactly is Cross-Site Scripting (XSS), and why should mobile app developers be concerned about it?

Cross-Site Scripting (XSS) is a web application vulnerability that allows attackers to inject malicious scripts into web pages viewed by other users. It is a concern for mobile app developers because it can lead to data theft, unauthorized access, and the spread of malware, compromising the security and integrity of web applications.

How can mobile app developers prevent XSS vulnerabilities?

Mobile app developers can prevent XSS vulnerabilities by implementing the following best practices:

  • Input validation and data sanitization to ensure that user-supplied data is safe.
  • Output encoding to prevent malicious code execution when displaying data.
  • Implementing a Content Security Policy (CSP) to control resource loading and sources.
  • Using HTTP-Only and Secure flags on cookies to restrict script access.

Regularly updating the app and its components to patch potential vulnerabilities.

What are the different types of XSS attacks, and how do they differ from each other?

There are several types of XSS attacks, including Stored XSS, Reflected XSS, DOM-based XSS, and Blind XSS. Each type varies in how the malicious script is injected and executed, but they all aim to exploit vulnerabilities in web applications.

What are the potential consequences of an XSS attack on a mobile app?

An XSS attack on a mobile app can have severe consequences, including:

  • Theft of sensitive user data (e.g., login credentials, personal information).
  • Session hijacking, enabling attackers to impersonate legitimate users.
  • Financial losses if the app involves financial transactions.
  • Reputation damage to the app and its developers due to security breaches.
What are the types of XSS attacks that can affect mobile apps?

There are three main types of XSS attacks:

  • Stored (Persistent) XSS: Malicious code is permanently stored on the server and served to users whenever specific content is requested.
  • Reflected (Non-Persistent) XSS: Malicious code is reflected back to users through a URL or input field, often using social engineering to trick them into clicking on a malicious link.

DOM-based XSS: The vulnerability arises in the Document Object Model (DOM) environment, where client-side scripts manipulate the DOM based on user input, allowing an attacker to inject malicious code.

Should we consider hiring external security experts to conduct penetration testing and evaluate our application for XSS vulnerabilities?

Engaging external security experts to perform penetration testing can provide an unbiased evaluation of a mobile application's security posture. This can help identify vulnerabilities and weaknesses that internal teams may overlook.


In conclusion, understanding and preventing XSS attacks is crucial for ensuring web application security. Remember to validate and sanitize user inputs, update frameworks regularly, and promote secure coding practices among developers.

For an extra layer of protection against XSS and other security threats, check out Appknox. With Appknox's advanced security solutions, you can fortify your web applications and safeguard your users' data. Take the first step towards a safer web today.