toStaticHTML: The Second Encounter (CVE-2012-1858)

HTML Sanitizing Bypass - CVE-2012-1858

Introduction

The toStaticHTML component, which is found in Internet Explorer > 8, SharePoint and Lync is used to sanitize HTML fragments from dynamic and potentially malicious content.

If an attacker is able to break the filtering mechanism and pass malicious code through this function, he/she may be able to perform HTML injection based attacks (i.e. XSS).

It has been a year since the first encounter was published, we've now returned with a new bypass method.

Vulnerability

An attacker is able to create a specially formed CSS that will overcome toStaticHTML's security logic; therefore, after passing the specially crafted CSS string through the toStaticHTML function, it will contain an expression that triggers a JavaScript call.

The following JavaScript code demonstrates the vulnerability:

<script>document.write(toStaticHTML("<style>div{font-family:rgb('0,0,0)'''}foo');color=expression(alert(1));{}</style><div>POC</div>"))</script>

In this case the function's return value would be JavaScript executable:

<style>div{font-family:rgb('0,0,0)''';}foo');color=expression(alert(1));{;}</style><div>POC</div>

 

The reason this code bypasses the filter engine is due to two reasons:

1. The filtering engine allows the string "expression(" to exists in "non-dangerous" locations within the CSS.
2. A bug in Internet Explorer's CSS parsing engine doesn't properly terminate strings that are opened inside brackets and closed outside of them.

When combining these two factors the attacker is able to "confuse" the filtering mechanism into "thinking" that a string is open when in fact it is terminated and vice versa. With this ability the attacker can trick the filtering mechanism into entering a state of the selector context which is considered safer where in fact the code is just a new declaration of the same selector, thus breaking the state machine and bypassing the filter.

Impact

Every application that relies on the toStaticHTML component to sanitize user supplied data had probably been vulnerable to XSS.

Remediation

Microsoft has issued several updates to address this vulnerability.

MS12-037 - http://technet.microsoft.com/en-us/security/bulletin/ms12-037

MS12-039 - http://technet.microsoft.com/en-us/security/bulletin/ms12-039

MS12-050 - http://technet.microsoft.com/en-us/security/bulletin/MS12-050

 

Discovered by - Adi Cohen, IBM Application Security Research

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