In this blog post, we will discuss how to bypass HTML encoding for Cross-Site Scripting (XSS) attacks. HTML
encoding is a common technique used by developers to prevent XSS attacks, but as a security researcher or
ethical hacker, you may need to know how to bypass it to test the security of a web application.
Understanding HTML Encoding
HTML encoding is the process of converting special characters, such as <,
>, and &, into their corresponding HTML entities, like
<, >, and & respectively. This
preventing XSS attacks.
Bypassing HTML Encoding
To bypass HTML encoding, an attacker can use various techniques that exploit specific vulnerabilities or
weaknesses in the web application. Some of these techniques include:
Using different character sets: If a web application does not properly validate user
input or specify a consistent character set, an attacker can use different character sets to bypass
HTML encoding. For example, using UTF-7 instead of UTF-8 can allow an attacker to execute an XSS attack
In UTF-7, this could be encoded as:
interpret HTML entities as actual characters, such as
example, if a web application uses
innerHTMLto display user input, an attacker can
bypass HTML encoding like this:
document.getElementById(‘output’).innerHTML = ‘<script>alert('XSS')</script>’;
This will cause the browser to execute the injected
Using alternative syntax: Sometimes, web applications only encode specific characters,
allowing an attacker to use alternative syntax to bypass HTML encoding. For example, if a web
application only encodes < and > characters, an attacker can use
the following syntax to execute an XSS attack:
Bypassing HTML encoding for XSS attacks is an important skill for security researchers and ethical hackers.
Understanding the different techniques and their underlying vulnerabilities can help you identify and
exploit weaknesses in web applications, ultimately making them more secure. Always remember to use these
techniques responsibly and only for testing purposes.