Malicious Office Documents Could Open Doors into Your Network

Corey Nachreiner

13 years ago

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:

MS11-023:

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.