Cross-site scripting (XSS) allows an attacker to inject malicious code into a website, which is then executed by the victim's browser. An XSS attack occurs when an attacker injects malicious code into a website's stylesheet, which specifies a webpage's layout and formatting.
There are several ways that an attacker can inject malicious code into a stylesheet.
For example, consider a website with a stylesheet called style.css that specifies the layout and formatting for the website's pages. An attacker could modify the stylesheet to include a malicious script like this:
body {background-color: #ffffff;}/* malicious code injected by attacker */script {alert("XSS attack!");}
When a victim visits the website, their browser will execute the malicious script, causing a pop-up window with the "XSS attack!".
An attacker could create a new stylesheet that contains the malicious code and host it on their server. They could then use various tactics, such as social engineering or exploiting a different vulnerability, to convince the victim to visit a webpage that includes the malicious stylesheet.
For example, the attacker could create a stylesheet called malicious.css that contains the following code:
/* malicious code injected by attacker */script {alert("XSS attack!");}
They could then create a webpage that includes the malicious stylesheet like this:
<html><head><link rel="stylesheet" href="http://attacker.com/malicious.css"></head><body><p>Welcome to the website!</p></body></html>
When the victim visits the webpage, their browser will execute the malicious script in the stylesheet, causing a pop-up window with the "XSS attack!".
An attacker could also inject the malicious code directly into the HTML code of a webpage, either by modifying an existing website or by creating a new one. When the victim's browser renders the webpage, it will execute the malicious code in the stylesheet.
For example, the attacker could create a webpage with the following HTML code:
<html><head><style>/* malicious code injected by attacker */script {alert("XSS attack!");}</style></head><body><p>Welcome to the website!</p></body></html>
When the victim visits the webpage, their browser will execute the malicious script in the stylesheet, causing a pop-up window with the "XSS attack!".
Note: To prevent XSS attacks through stylesheets, software developers or engineers must ensure that the CSS codes are properly validated and sanitized before they are used on the website. Regularly scanning the website for suspicious or malicious code is also good.