Malicious Office Documents Could Open Doors into Your Network

Severity: High

12 April, 2011


  • 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


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.


Excel update for:


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.


Microsoft has released Office updates to fix these vulnerabilities.


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

About Corey Nachreiner

Corey Nachreiner has been with WatchGuard since 1999 and has since written more than a thousand concise security alerts and easily-understood educational articles for WatchGuard users. His security training videos have generated hundreds of letters of praise from thankful customers and accumulated more than 100,000 views on YouTube and Google Video. A Certified Information Systems Security Professional (CISSP), Corey speaks internationally and is often quoted by other online sources, including C|NET, eWeek, and Slashdot. Corey enjoys "modding" any technical gizmo he can get his hands on, and considers himself a hacker in the old sense of the word.


  1. Malicious Office Documents Could Open Doors into Your Network | microreksa - April 12, 2011

    […] Malicious Office Documents Could Open Doors into Your Network […]

  2. Malicious Office Documents Could Open Doors into Your Network | Interwork Blog - July 14, 2011

    […] more here. Posted in […]

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s


Get every new post delivered to your Inbox.

Join 8,241 other followers

%d bloggers like this: