Malicious Office Documents Could Open Doors into Your Network

Severity: High

12 April, 2011

Summary:

• These vulnerabilities affect: Most current versions of Microsoft Office, and the components that ship with it
• How an attacker exploits it: Typically by enticing one of your users to open a malicious Office document
• Impact: In the worst case, an attacker executes code on your user’s computer, gaining complete control of it
• What to do: Install Microsoft Office updates as soon as possible, or let Microsoft’s automatic update do it for you

Exposure:

As part of today’s Patch Day, Microsoft released two security bulletins describing eleven vulnerabilities found in Excel and other components that ship with most current versions of Microsoft Office for Windows and Mac.

Though the eleven vulnerabilities differ technically, and affect different Office components, they result in the same problem. If an attacker can entice one of your users into downloading and opening a maliciously crafted Office document, he can exploit any of these vulnerabilities to execute code on a victim’s computer, usually inheriting that user’s level of privileges and permissions. If your user has local administrative privileges, the attacker gains full control of the user’s machine.

According to Microsoft’s bulletins, an attacker can exploit these flaws using many different types of Office documents. In one bulletin, Microsoft specifically states Excel documents are vulnerable. However, they also mention any “Office files” in their other alert. Therefore, we recommend you beware of all unexpected Office documents.

If you’d like to learn more about each individual flaw, drill into the “Vulnerability Details” section of the security bulletins listed below:

• MS11-021: Nine Excel Code Execution Vulnerabilities, rated Important
• MS11-023: Two Office Code Execution Vulnerabilities, rated Important

Solution Path

Microsoft has released patches for Office to correct all of these vulnerabilities. You should download, test, and deploy the appropriate patches throughout your network immediately, or let the Microsoft Automatic Update feature do it for you.

MS11-021:

Excel update for:

• Office XP w/SP3
• Office 2003 w/SP3
• Office 2007 w/SP2
• Office 2010 32-bit
• Office 2010 64-bit

• Office 2004  for Mac
• Office 2008  for Mac
• Office for Mac 2011
• Open XML File Format Converter for Mac

• Excel Viewer
• Office Compatibility Pack for Word, Excel, and PowerPoint 2007 File Formats

MS11-023:

• Office XP w/SP3
• Office 2003 w/SP3
• Office 2007 w/SP2

• Office 2004  for Mac
• Office 2008  for Mac
• Open XML File Format Converter for Mac

For All WatchGuard Users:

While you can configure certain WatchGuard Firebox models to block Microsoft Office documents, some organizations need to allow them in order to conduct business. Therefore, these patches are your best recourse.

If you want to block Office documents, follow the links below for video instructions on using your Firebox proxy’s content blocking features by file extensions. Some of the file extensions you’d want to block include, .DOC, .XLS, .PPT, and many more (including the newer Office extensions that end with “X”). Keep in mind, blocking files by extension blocks both malicious and legitimate documents.

• Firebox X Edge running 10.x
  • How do I block files with the FTP proxy?
  • How do I block files with the HTTP proxy?
  • How do I block files with the POP3 proxy?
  • How do I block files with the SMTP proxy

• Firebox X Core and X Peak running Fireware 10.x
  • How do I block files with the FTP proxy?
  • How do I block files with the HTTP proxy?
  • How do I block files with the POP3 proxy?
  • How do I block files with the SMTP proxy?

Status:

Microsoft has released Office updates to fix these vulnerabilities.

References:

• MS Security Bulletin MS11-021
• MS Security Bulletin MS11-023

This alert was researched and written by Corey Nachreiner, CISSP.

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