toStaticHTML HTML Sanitizing Bypass

Type packetstorm
Reporter Adi Cohen
Modified 2012-07-11T00:00:00


                                            `toStaticHTML: The Second Encounter (CVE-2012-1858)  
*HTML Sanitizing Bypass -  
Original advisory -  
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  
published, we've now returned with a new bypass method.  
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:  
In this case the function's return value would be JavaScript executable:  
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  
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.  
Every application that relies on the *toStaticHTML* component to sanitize  
user supplied data had probably been vulnerable to XSS.  
Microsoft has issued several updates to address this vulnerability.  
MS12-037 -  
MS12-039 -  
MS12-050 -