The following is tip #12 from "20 Tips on securing Outlook in 20 minutes," excerpted from a chapter in Paul Robichaux's book, Secure Messaging with Microsoft Exchange Server 2003 © 2004, published by Microsoft Press. Return to the main page for more tips on this topic.
You might choose not to deploy the public folder that applies settings to your Outlook clients (although by not doing so you're skipping a valuable security feature).
If you don't, then Outlook 2003 will still apply the Level 1 and Level 2 restrictions discussed earlier, but with a twist: each user can customize his or her own copy of Outlook to control the Level 1 and Level 2 lists. The trick is to add a new string value named Level1Remove to the HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security key. The extensions you add here (separated by semicolons if there's more than one) are removed from the list of blocked Level 1 attachments, so creating a value of exe; pl would allow executables and Perl scripts to be saved to disk instead of blocking them completely.
Actually, the extensions you specified are demoted from Level 1 to Level 2; they're not unblocked completely. End users cannot demote file types from Level 2 to being unprotected; only administrators can do so.
If you want to add a new file type to the Level 1 list, you can do so by creating a new string value named Level1Add beneath the HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security key.
Of course, it is more likely that you'll want to prevent users from customizing their own security settings. The easiest way to do this is to add a new REG_DWORD value named DisallowAttachmentCustomization to the Outlook key at HKCU\Software\Policies\Microsoft\Office\11.0\Outlook. When this value is present, Outlook will ignore the Level1Add and Level1Remove keys mentioned earlier.
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About the author: Paul Robichaux is a partner at 3sharp LLC, author of several books on Exchange, Windows, and security, a Microsoft MVP for Exchange Server and a frequent speaker and presenter at IT industry conferences. He's written software for everyone from the U.S. National Security Agency to scientists flying their experiments aboard the Space Shuttle, fixed helicopters in the desert and spent way too much time playing video games.