What is cross-site scripting (XSS)?

XSS features consistently among the Top3 vulnerabilities detected on websites. There’s a reason why cross-site scripting is so popular with fraudsters and other cybercriminals: not only is it a widely available attack surface, but a single XSS vulnerability compromises the entire domain on which it occurs.

How does cross-site scripting (XSS) work?

There are three types of XSS attack

Cross-Site Scripting (XSS) attacks involve the injection of malicious scripts into otherwise trusted websites. XSS attacks happen when an attacker uses a web application to send malicious code, usually in the form of a browser-side script, to a different end user. Cross-site scripting is one of the most widely used types of attack by cybercriminals.

There are three types of cross-site scripting attack:

XSS Reflected: A non-persistent XSS attack, where a malicious script is reflected back to the user by the end server. This attack is typically exploited via a link that is sent to the victim. The link embeds a malicious script, which is not sanitized by the end server, and is sent back to the end user’s browser session where it executes.

 

XSS Persistent: The malicious script gets stored on the server side, usually in a DB, as legitimate content. The attack typically takes place on web app areas where users submit input, through which the malicious script is injected. With Persistent XSS, the end server stores the malicious script. Because of this, the attack executes whenever the script is fetched from the server.

XSS DOM: Malicious script is executed on the client as a result of modification of the DOM dynamically. Unlike Reflected and Persistent XSS, there’s no server-side vulnerability with DOM XSS. The attack typically is delivered via links where an input element in the link is modified to include the malicious script. This input becomes part of dynamic code execution on the client side, and gets executed.

Who does XSS target?

Cross-site scripting targets every industry

The secret to success for many XSS attacks is our tendency to click without thinking, particularly when it's a link or a message from a trusted brand, product, business or known website. The fact that a single website vulnerability gives attackers access to the entire domain, and it's easy to understand the popularity of XSS attacks.

Magecart groups often use XSS as a method of attack. Examples of cross-site scripting attacks include:

  • In April 2020, researchers revealed a massive growth in XSS attacks on Wordpress websites, mainly targeting plug-ins. These allowed attackers to change a site's home URL to re-direct visitors to malicious or malvertising sites. 
  • In early 2020, the WhatsApp platform was found to have gaps in its Content Security Policy (CSP) that enabled XSS attacks capable of sending harmful code to end users via harmless-looking messages. 

 

How to prevent XSS attacks

Traditional ways of preventing XSS

One traditional way to prevent cross-site scripting attacks is to prevent users from posting HTML markup on websites. This isn't always possible, so a classic approach was to filter out potentially dangerous tags and JavaScript and create a whitelist of "safe" tags, but this is very difficult to implement securely. 

 

Many modern websites try to prevent XSS attacks by using a template engine. If not implemented correctly, this method can expose the website to server-side template injection, which can be as serious - or worse - than the cross-site scripting.

Automating XSS prevention with Tala

Stop cross-site scripting attacks.

Tala detects and prevents both Reflected and Persistent XSS by analyzing the app, creating a list of all legitimate scripts (inline and non-inline), and whitelisting them through CSP’s ‘script-src directive. The reflected script will not be allowed to run because it won’t be part of the whitelist. Tala uses nonces to protect against malicious inline JavaScript or tampering. Any inline JavaScript that Tala determines is malicious will not be certified with a nonce.

Tala detects all DOM XSS sinks an application might be using (which lead to a DOM XSS attack).

Protect your website from client-side attacks with Tala Security. Tala prevents cross-site scripting attacks by combining the power of standards-based security with patented analytics and automation to deliver rich insights into the code running on your website and safeguard it from attacks. Tala’s Web Application Runtime Protection (WARP) eliminates client-side vulnerabilities that lead to browser session attacks and data theft. Learn more about how Tala protects websites with WARP

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Is your website safe from XSS attacks?

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