Tuesday, September 01, 2009

toolsmith: OffVis 1.0 Beta - Office visualization tool

My monthly toolsmith column in the September 2009 edition of the ISSA Journal features OffVis, a tool for detecting malicious Microsoft Office documents. This tool was created by MSRC's Engineering team, a group that spends a great deal of time looking for ways to detect exploitation of given vulnerabilities, in particular those that are Office-related.
Their efforts led to the creation of OffVis, starting in November 2008. First released in beta to MAPP participants, it has matured into a UI-based tool that analyzes a very specific set of vulnerabilities in order to better help defenders. MSRC Engineering’s work allows them to build detection logic, and then reuse it as part of ongoing analysis efforts.

A typical targeted attack often includes an email sent to an intended victim with a malicious Excel document attached. When the victim opens the Excel document the following sequence might occur. First, it exploits a vulnerability to force Excel to run embedded shellcode. The shellcode then extracts an XOR’d, well-formed XLS file, and an EXE. The XLS opens in Excel, and the extracted EXE is executed which installsa backdoor as a service.9 This actual limited targeted attack resulted in Microsoft releasing KB 94756310 on January 15, 2008. The OffVis Excel parser includes detection logic for CVE-2008-0081,11 the National Vulnerability Database CVE released in accordance with KB 947563. We’ll look at a specific sample exploiting CVE-2008-0081 in Using OffVis.
Stepping through the exploit more specifically might appear as seen in Figure 2.

Figure 2

Typical exploit structure (Figure 3) ensures that everything is included in the document; please note that there can be variations including multiple shellcode stages, multiple Trojans, and obfuscation of both Trojan and the document.

For a much deeper dive into exploit structure, as well as disassembly and debugging techniques, see Bruce Dang’s topical Black Hat Japan 2008 presentation.

Figure 3

The article PDF is here.
Grab OffVis here.
Thanks to Dan, Kevin, Bruce, Robert, and Jonathan for the time and feedback that contributed to this month's article.

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