Microsoft Internet Explorer 9 / SharePoint / Lync - toStaticHTML HTML Sanitizing Bypass (MS12-037/MS12-039/MS12-050)

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



*HTML Sanitizing Bypass -

*CVE-2012-1858<http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2012-1858>



Original advisory -

http://blog.watchfire.com/wfblog/2012/07/tostatichtml-the-second-encounter-cve-2012-1858-html-sanitizing-information-disclosure-introduction-t.html



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<http://blog.watchfire.com/wfblog/2011/07/tostatichtml-html-sanitizing-bypass.html>

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