What Is an XSS Attack? Definition, Types, Prevention

Maybe you’ve been thinking about what XSS attacks are. Cross-Site Scripting, also referred to as an XSS attack, is a sort of injection that gets malicious scripts into otherwise benign and trusted websites.

How do XXS attacks take place?

XSS attacks happen when an attacker uses an online application to send malicious code, usually within the form of a browser-side script, to a distinct end-user. Unfortunately, the vulnerabilities that allow these attacks to succeed are widespread and occur anywhere an online application uses input from a user within the output it generates without validating or encoding it.

An attacker can use XSS to send a malicious script to an unsuspecting user, the user’s browser has no way to know that the script mustn’t be trusted and will execute the script.

Thinking the script in question came from a trusted source, the malicious script can now access freely any cookies, session tokens, or other sensitive data the browser has previously retained, or perhaps rewrite the content of an HTML page.

The malicious content can often take the shape of a JavaScript segment, but might also include HTML, Flash, or other specific code that the browser may execute.

The XSS attack possibilities are almost limitless, but usually include transmitting private data, like cookies or other session information, to the attacker, redirecting the victim to web content controlled by the attacker, or performing other malicious operations on the user’s machine.

Types of XSS attacks

These attacks may be broken into three main categories: stored, reflected and DOM Based XSS with the foremost common ones being stored and also the reflected attacks.

Stored Attack or Persistent XSS

These are defined when the injected script is permanently stored on the target servers, like in a database, in a message forum, visitor log, or comment field. The victim then retrieves the malicious script from the server when it requests the stored information.

Reflected attacks

They get their names from the action taken by the server, during this case the injected script is reflected off the web server, like in an error message, search result, or any other response that features some or all of the input sent to the server as a part of the request. they’re delivered to victims through another route, as in an e-mail message, or on another website.

When a user is tricked into clicking on a malicious link, submitting a specially crafted form, or perhaps just browsing to a malicious site, the injected code travels to the vulnerable website which reflects the attack to the user’s browser. The browser then executes the code because it came from a “trusted” server. Reflected XSS is also sometimes referred to as Non-Persistent or Type-II XSS.

DOM-based XSS attacks

The least common type of XXS attacks acts when an application contains some client-side JavaScript that’s processing data from an untrusted source in an unsafe way, usually by writing the information back to the DOM.

How to stay safe?

It’s been proved that to effectively prevent XSS vulnerabilities it’s necessary to involve a number of the subsequent measures:

Filter input upon arrival

Ideally, you ought to filter input on arrival, meaning that the purpose where user input is received you ought to filter as strictly as possible supported what’s expected or valid input.

Encode your data on output

You can encode the output to stop it from being interpreted as active content, but depending on the output context, this might require applying combinations of HTML, URL, JavaScript, and CSS encoding.

Use only appropriate response headers

To prevent XSS in HTTP responses not intended to contain any HTML or JavaScript, you should use Content-Type and X-Content-Type-Options headers and make sure browsers decode the responses within the way you intend them to.

Have a Content Security Policy in place

As a final line of defense, you can use Content Security Policy to scale back the severity of any XSS vulnerabilities that also occur.

Dora Tudor

Cyber Security Enthusiast

Dora is a digital marketing specialist within Heimdal™ Security. She is a content creator at heart - always curious about technology and passionate about finding out everything there is to know about cybersecurity.