A zeroday vulnerability has been published today affecting the Microsoft Office Web Components ActiveX controls. The flaw exists in the controls used by IE to display Excel spreadsheets. Exploitation requires enticing the target user to navigate to a malicious web page, although this can be largely automated. It is reported that exploitation of this flaw resulting in process flow diversion is fairly reliable and the vulnerability is being actively exploited in the wild.

The affected ActiveX controls are not installed by default on any Windows platform, however they do come bundled with the popular MS Office application suite and some select server applications. This makes for a very large base of affected users.

The CLSIDs of the affected controls are listed:

0002E541-0000-0000-C000-000000000046 0002E559-0000-0000-C000-000000000046

The affected controls can also be instantiated through scripting:

OWC10.Spreadsheet OWC11.Spreadsheet

Microsoft has listed workarounds to prevent possible exploitation in a security bulletin. This vulnerability has been assigned a CVE id of CVE-2009-1136.

SonicWALL has written and released four IPS signatures that will detect and block generic attack attempts and two GAV signatures that detect specific exploits.

The following IPS signatures have been released to address this vulnerability:

• 1014 – MS Office Web Components ActiveX Instantiation 1
• 1016 – MS Office Web Components ActiveX Instantiation 2
• 1023 – MS Office Web Components ActiveX Instantiation 3
• 1024 – MS Office Web Components ActiveX Instantiation 4

The following GAV signatures have been released to address this vulnerability:

• ScriptUE
• OWCref.A