OCS ADVISORY NUMBER:
2010-108

DATE(S) ISSUED:
12/21/2010

SUBJECT:
Vulnerability in Internet Explorer Could Allow Remote Code Execution

OVERVIEW:

A vulnerability has been discovered in Microsoft's web browser, Internet Explorer, which could allow an attacker to take complete control of an affected system. Exploitation may occur if a user visits or is redirected to a web page which is specifically crafted to take advantage of the vulnerability. Successful exploitation of this vulnerability could result in an attacker gaining the same privileges as the logged on user. Depending on the privileges associated with the user, an attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. Failed exploit attempts may result in a denial-of-service condition.

It should be noted that there is currently no patch available for this vulnerability and a working exploit is available which results in remote code execution.  The exploit has been tested with Internet Explorer 7 & 8 on a Windows XP SP3 platform and confirmed to result in remote code execution.

SYSTEMS AFFECTED:

• Internet Explorer 6
• Internet Explorer 7
• Internet Explorer 8

RISK:
Government:

• Large and medium government entities: High
• Small government entities: High

Businesses:

• Large and medium business entities: High
• Small business entities: High

Home users: High

DESCRIPTION: 
A vulnerability has been discovered in Microsoft's web browser, Internet Explorer, which could allow an attacker to take complete control of an affected system. This vulnerability occurs when rendering a web page containing a reference to a Cascading Style Sheet (CSS) file with "@import" rules. This may result in a use-after-free condition in the mshtml.dll library. A use-after-free condition occurs when an application de-allocates a memory block and then later attempts to access that de-allocated space.

Exploitation may occur if a user visits or is redirected to a web page which is specifically crafted to take advantage of these vulnerabilities. Successful exploitation of the vulnerability could result in an attacker gaining the same privileges as the logged on user. Depending on the privileges associated with the user, an attacker could then install programs; view, change, or delete data; or create new accounts with full user rights. Failed exploit attempts may result in a denial-of-service condition.

It should be noted that there is currently no patch available for this vulnerability and a working exploit is available which results in remote code execution.  The exploit has been tested with Internet Explorer 7 & 8 on a Windows XP SP3 platform and confirmed to result in remote code execution.

RECOMMENDATIONS:
We recommend the following actions be taken:

• Set Internet and Local Intranet security zone settings to block ActiveX controls and Active Scripting.
• Run all software as a non-privileged user (one without administrative privileges) to diminish the effects of a successful attack.
• Read e-mail in plain-text format.
• Inform and educate users regarding the threats posed by hypertext links contained in e-mails or attachments especially from untrusted sources.
• If you have an alternate browser deployed, consider using it until this vulnerability is remediated.

REFERENCES:
US-CERT:
http://www.kb.cert.org/vuls/id/634956

Secunia:
http://secunia.com/advisories/42510

Threat Post:
http://threatpost.com/en_us/blogs/new-remotely-exploitable-bug-found-internet-explorer-121010

Security Focus:
http://www.securityfocus.com/bid/45246

Metasploit:
https://www.metasploit.com/redmine/projects/framework/repository/entry/modules/exploits/windows/browser/ms11_xxx_ie_css_import.rb