Avito: CSS injection in avito.ru via IE11

ID H1:276747
Type hackerone
Reporter hussain_0x3c
Modified 2019-12-12T09:33:57


Hi Team Security @avito

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


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



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.


Malicious CSS

Best Regards Hussain Adnan