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MSFT Updates Outlook Ban List for Downloads

Microsoft did some Outlook tweaking recently. They realized that the list of the file extensions that the program should refuse to download needed some updating.

Larry Loeb

September 30, 2019

2 Min Read

Microsoft did some Outlook tweaking recently. They realized that the list of the file extensions that the program should refuse to download needed some updating.

Now, there will be consequences from this. For example, an enterprise (especially) has to make sure that the OwaMailboxPolicy objects changes are handled correctly for each user should they want enterprise-wide download exclusion.

In any case, MSFT says that the newly blocked file types are rarely used, so most organizations will not be affected by the change. However, if your users are using previously approved OMPs while sending and receiving the affected attachments, MSFT says that they will no longer will able to download them.

But, there is a way around this. If you want a particular file type to be allowed for a user, you can add that file type to the AllowedFileTypes property of your users' OMP objects. A file extension will not be added to a user's BlockedFileTypes list if that extension is already present in the AllowedFileTypes list. \r\nThe results of the list twiddling this time around is fairly long.\r\n From the Python scripting language, we find some obsolete file types, including ".py", ".pyc", ".pyo", ".pyw", ".pyz", ".pyzw."\r\nThe PowerShell scripting language -- an enabler of much filleless malware coding -- also gets trimmed. The ".ps1", ".ps1xml", ".ps2", ".ps2xml", ".psc1", ".psc2", ".psd1", ".psdm1", ".cdxml" and ".pssc" extensions are on the darklist.

The ".appref-ms" extension used by Windows ClickOnce gets hit as does the Microsoft Data Access Components (MDAC) extension ".udl."\r\nWindows sandbox gives up ".wsb" to the effort.

Digital certificates aren’t forgotten in the hullabaloo. The ".cer", ".crt" and ".der" extensions used by some are now gone.

The ".jar" and ".jnlp" extensions to be cut are used by the Java programming language quite extensively. Java is so powerful that allowing it unrestrained program access could prove seriously problematical from a security standpoint. Yet, there may be libraries in use by production code which assume they will be able to get to files containing that extension. That's a potential quagmire for an enterprise, especially if it relies on open sourced software.

From the Department of "You're Too Lame to Patch, So We Did It for You" came some extensions that MSFT explained this way. "While the associated vulnerabilities have been patched (for years, in most cases)," they said in the advisory, "they are being blocked for the benefit of organizations that might still have older versions of the application software in use." Lame-o.

The list of the miscellaneous ones is :".appcontent-ms", ".settingcontent-ms", ".cnt", ".hpj", ".website", ".webpnp", ".mcf", ".printerexport", ".pl", ".theme", ".vbp", ".xbap", ".xll", ".xnk", ".msu", ".diagcab" and ".grp".\r\n

— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.

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About the Author(s)

Larry Loeb

Blogger, Informationweek

Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek. He has written a book on the Secure Electronic Transaction Internet protocol. His latest book has the commercially obligatory title of Hack Proofing XML. He's been online since uucp "bang" addressing (where the world existed relative to !decvax), serving as editor of the Macintosh Exchange on BIX and the VARBusiness Exchange. His first Mac had 128 KB of memory, which was a big step up from his first 1130, which had 4 KB, as did his first 1401. You can e-mail him at [email protected].

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