CSS injection in avito.ru via IE11

How it was found

Hi Team Security @avito

I discovered CSS Injection on avito.ru in form search via IE11

Description

CSS injection vulnerabilities arise when an application imports a style sheet from a user-supplied URL, or embeds user input in CSS blocks without adequate escaping. They are closely related to cross-site scripting (XSS) vulnerabilities but often trickier to exploit.

Being able to inject arbitrary CSS into the victim's browser may enable various attacks, including :

• Executing arbitrary JavaScript using IE's expression() function.
• Using CSS selectors to read parts of the HTML source, which may include sensitive data such as anti-CSRF tokens.
• Capturing any sensitive data within the URL query string by making a further style sheet import to a URL on the attacker's domain, and monitoring the incoming Referer header.

Affected URL

https://www.avito.ru/rossiya/nedvizhimost?s='><b/style=position:fixed;top:0;left:0;font-size:200px>XSS<!--

Proof of Concept

{F228726}

Remediation

Ensure that user input is adequately escaped before embedding it in CSS blocks, and consider using a whitelist to prevent loading of arbitrary style sheets.

References

Malicious CSS

Best Regards Hussain Adnan