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

• Exploit Database
• 15 阅读

• 作者： Adi Cohen
  日期： 2012-07-12
• 类别：

  • dos
  平台：

  • windows
• 来源：https://www.exploit-db.com/exploits/19777/

• 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