Additional technical information that describes the Microsoft Internet Explorer Cascading Style Sheets remote code execution vulnerability is available.
Description
Microsoft Internet Explorer versions 6 and 7 contain a vulnerability that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition or execute arbitrary code.
The vulnerability is due to an error by the Microsoft HTML Viewer. The attacker could exploit this vulnerability by enticing a user to visit a malicious website. Visiting the site could cause Internet Explorer to become unresponsive or possibly execute arbitrary code in the context of the current user.
Proof-of-concept code is publicly available.
Microsoft has confirmed this vulnerability in a security bulletin and released software updates that correct it.
Warning Indicators
Microsoft Internet Explorer versions 6 and 7 are vulnerable.
IntelliShield Analysis
An attacker cannot exploit this vulnerability directly and must rely on social engineering tactics to entice a user to view a malicious site. Attackers may provide links to users via e-mail messages or post links on public websites.
If an exploit is successful, the attacker could execute arbitrary code on the system with the privileges of the user. On systems where users run applications as the Administrator account, the attacker could execute code that could result in complete system compromise. Systems that limit user privileges may have a reduced impact as the result of an exploit because any executed code would run in a restricted security context.
Initial reports from sources indicate that the publicly released proof-of-concept code is not very reliable. Attempted exploits may fail, preventing exploitation. However, reports caution that a fully functional, more reliable version could be constructed in the future.
The Cisco Applied Intelligence team has created an Applied Mitigation Bulletin to address vulnerabilities that Microsoft disclosed in the December 2009 security bulletin release. This Cisco bulletin, which assists administrators in identifying or mitigating these vulnerabilities using Cisco devices, is available at the following link: Cisco Applied Mitigation Bulletin: Microsoft Security Bulletin Release for December 2009
Vendor Announcements
Microsoft has released a security bulletin at the following link: MS09-072
US-CERT has released a vulnerability note at the following link: VU#515749
Impact
An unauthenticated, remote attacker could exploit this vulnerability to cause a DoS condition or execute arbitrary code on the vulnerable system with the privileges of the user. If the user holds elevated privileges, the attacker could execute code that could result in complete system compromise.
Technical Information
This vulnerability is due to unsafe creation of memory buffer objects. The CDispNode::SetExpandedClipRect function improperly calculates the size of input supplied from other functions, which could trigger memory corruption when input is copied to memory.
An unauthenticated, remote attacker could exploit the vulnerability by convincing a user to view a malicious web page designed to submit malicious input to Internet Explorer. Successful exploitation could allow the attacker to execute arbitrary code with the privileges of the user.
Safeguards
Administrators are advised to apply the appropriate updates.
Administrators may consider configuring Internet Explorer to prompt users before running Active Scripting or ActiveX controls by setting the Internet and Local Intranet security zone settings to High. Alternately, administrators could disable Active Scripting and ActiveX controls in these security zones.
Users are advised to run applications with the least necessary privileges.
Users should verify that unsolicited links are safe to follow.
Users are advised not to visit websites or follow links that have suspicious characteristics or cannot be verified as safe.
Administrators may consider disabling JavaScript in the vulnerable applications.
Administrators are advised to implement an intrusion prevention system (IPS) or intrusion detection system (IDS) to help detect and prevent attacks that attempt to exploit this vulnerability.
Patches/Software
Microsoft customers can obtain updates directly by using the links in the security bulletin. These updates are also distributed by Windows automatic update features and available on the Windows Update website. Microsoft Windows Server Update Services (WSUS), Systems Management Server, and System Center Configuration Manager can assist administrators in deploying software updates.
Signatures
Cisco Systems Cisco Intrusion Prevention System (IPS) 6.0
Version 4, December 8, 2009, 2:22 PM: Microsoft has released a security bulletin and updated software to address the Cascading Style Sheets remote code execution vulnerability in Internet Explorer. US Cert has also issued a vulnerability note.
Version 3, November 24, 2009, 2:40 PM: Cisco has released an Applied Mitigation Bulletin to address the Cascading Style Sheets remote code execution vulnerability in Internet Explorer.
Version 2, November 24, 2009, 10:36 AM: Microsoft has released a security advisory to address the Cascading Style Sheets remote code execution vulnerability in Internet Explorer.
Version 1, November 22, 2009, 1:51 PM: Microsoft Internet Explorer contains a vulnerability that could allow an unauthenticated, remote attacker to cause a denial of service condition or execute arbitrary code. Updates are not available.
Home Edition Base, SP1, SP2, SP3 | Professional Edition Base, SP1, SP2, SP3 | Professional x64 (AMD/EM64T) Base, SP2
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