Description

The library html.parser allows an attacker to bypass any whitelist of HTML tags and attributes that seek to mitigate XSS. This is possible because the application does not correctly parse the HTML comments in the user input.

Vulnerability

This vulnerability occurs because the application does not correctly parse the HTML comments in the user input.

Exploitation

In this scenario a developer parses the HTML entered by the user to validate it with an allowlist of tags and attributes. This is to prevent XSS attacks. In this case we see how we can bypass a security check of this type, thanks to the fact that the parser does not parse the HTML comments properly.

poc.py