Outlook Web Access (OWA) is configured "out of the box" to block certain kinds of email attachments. Like the full...
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version of Microsoft Outlook, this attachment blocking works on two tiers.
The first tier of attachment blocking prohibits OWA access to the file entirely; the second tier prohibits the attached file from being accessed unless it's first saved to the client's hard drive or accessible network drive.
The first-tier blocks the usual suspects -- e.g., .EXE, .COM and .BAT file extensions. But it is possible for a file to be registered in both tiers at once by default (e.g., .COM). This means that if a file is removed from the first tier, it'll still be blocked by the second tier, which increases user protection.
The list of files in tier 1 is in the registry under: HKLM\System\CurrentControlSet\Services\MSExchangeWeb\OWA\Level1FileTypes as a REG_SZ entry.
Tier 2 files are in: HKLM\System\CurrentControlSet\Services\MSExchangeWeb\OWA\Level2FileTypes.
In each case, the file lists are stored as a simple comma-separated string and can be edited as needed.
Unless you have a specific reason for unblocking a particular email attachment type, it's best to leave the lists as they are. But it can be useful to know where the lists are in case you need to add a new attachment type to OWA's attachment-blocking lists.
Note that you can always work around this restriction by compressing the file as a .ZIP archive (either with or without password protection). Most clients -- even those without a third-party .ZIP extraction tool handy -- can work with .ZIP files.
About the author: Serdar Yegulalp is editor of Windows Insight, a newsletter devoted to hints, tips, tricks, news and goodies for all flavors of Windows users.
I make it a point to block all email file attachments that can be used to send anything harmful. This especially includes .ZIP files. I get more .ZIP file viruses than any other format these days. Our antivirus (CA E-Trust) strips out all specified email attachments and removes macros from Microsoft Office documents.
Currently, I allow most media file types up to a certain size. I will restrict them only if there is a danger from a new media exploit, such as the one that affected images on unpatched machines a while back.
I ask people to use WinRAR to compress and send me a file or ask them to rename the .ZIP extension to .ZIT. This ensures that email file attachments can't be opened unintentionally. We have not had a virus released in our organization since we have had external email access (1998 on Exchange 5).
It does annoy some people to have to go through a few extra steps to send us a file. But in the end it is worth it to have some peace of mind.
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