Introduction to Cross-site Scripting
Cross-site scripting, also known as XSS, is a code injection attack that allows hackers to inject malicious code into legitimate websites. It is a client-side code injection attack that is triggered as soon as the victim loads the website, which executes the malicious code.
XSS is one of the major security concerns today that compromises user data when interacting with an unsafe web page or application. Users accepting data from untrusted sources like comments, messages, forums, etc. are most vulnerable to XSS attacks.
It’s important to note that cross-site scripting attacks are most common in JavaScript since it’s a popular scripting language.
The Working Mechanism of XSS Attacks
A hacker planning for a cross-site scripting attack first figures out a way to inject malicious code into the user’s web page. Once the malicious JavaScript code is injected, the attacker waits for the user to visit the page.
As soon as the user accesses the page, the client-side code is executed, leading to data vulnerability. Further, the attacker might gain access to user cookies, along with other sensitive user information, compromising data security and integrity altogether.
3 Major Cross-site Scripting Types
Cross-site scripting is primarily categorized into three distinct types, as mentioned below.
- Reflected XSS
- DOM-based XSS
- Stored XSS
Reflected XSS is the simplest and most common of all cross-site scripting attacks. In this, the malicious client-side code is added at the end of the HTTP URL. The attacker makes the victim load this URL with malicious code, which leads to cyber vulnerability.
To entice or trick the user into clicking the vulnerable link, the attacker uses social engineering techniques or phishing emails.
Compared to Reflected XSS, DOM-based cross-site scripting is a more advanced attack. This occurs when the attack payload is executed by altering the DOM environment in the user’s browser. In DOM-based attacks, the malicious code never reaches the server. It means this type of XSS attack is advanced in a way that it’s not very easy for Web Application Firewalls (WAFs) to detect and tackle.
Arguably the most havoc-causing cross-site scripting attack is the Stored XSS attack. Here, the user’s browser or application receives data from untrusted sources through mediums like comments, blogs, visitor logs, and more.
When the victim opens the web page or application, the payload is permanently stored in the browser as an HTML code.
How to Prevent Cross-site Scripting Attacks?
To keep yourself safe from falling victim to an XSS attack, you must adhere to some proven cross-site scripting prevention techniques as given below.
- Input Validation
- Output Encoding
- HTML Sanitization
- Choose Secure Frameworks
A common way to prevent unwanted payloads from entering your application and compromising sensitive data is input validation. It means all inputs coming to the app are carefully validated so that only those inputs that meet the set criteria are accepted.
Output encoding is an effective way to prevent XSS attacks that involves encoding each message properly before it’s rendered in HTML or JavaScript. Make sure to follow the correct encoding method for different outputs; otherwise, the attacker could easily inject the code.
HTML sanitization is the process of cleaning out HTML codes to eliminate any possible risk or dangerous elements on the web page. This safeguards your website from harmful cross-site scripting attacks and strips away data from untrusted sources.
There are many web development frameworks on the market, such as ReactJS, that automatically take care of security vulnerabilities like cross-site scripting attacks. They have built-in sanitizing inputs for you to leverage to prevent XSS from attacking your app.