Changes to File Types Blocked in Outlook on the web
Published Sep 25 2019 07:04 AM 142K Views

Updates:

  • May 2, 2022: We will also be blocking .application, .bgi, .img, .iso and .cab starting with end of May 2022
  • March 6, 2020: We will also be blocking .apk and .appx beginning April 2020.

We will soon be adding several additional file extensions to the BlockedFileTypes property of existing OwaMailboxPolicy objects. This change will prevent Outlook on the web users from downloading attachments that have those file extensions.

Why are we making this change?

We’re always evaluating ways to improve security for our customers, and so we took the time to audit the existing blocked file list and update it to better reflect the file types we see as risks today.

How does this affect me?

The newly blocked file types are rarely used, so most organizations will not be affected by the change. However, if your users are sending and receiving affected attachments, they will report that they are no longer able to download them.

How can I prepare for this change?

If your organization requires that users be able to download attachment of these types from OWA, you should first ensure that our organization's operating systems and application software are up-to-date (in the case files that are opened by application software) or ensure that your users are familiar with the risks associated with the file types (in the case of files that are interpreted by scripting software). 

If you want a particular file type to be allowed, you can add that file type to the AllowedFileTypes property of your users' OwaMailboxPolicy objects.

Note: In order to minimize disruption from this change, we will not add a file extension to a policy's BlockedFileTypes list if that extension is already present in the AllowedFileTypes list. 

To add a file extension to the AllowedFileTypes list:

 

$policy = Get-OwaMailboxPolicy [policy name]
$allowedFileTypes = $policy.AllowedFileTypes
$allowedFileTypes.Add(".foo")
Set-OwaMailboxPolicy $policy -AllowedFileTypes $allowedFileTypes

 

Or if you want (brought to you as part of the service that tries to teach you clever shortcuts) you can use:

 

Set-OwaMailboxPolicy -Identity "<Policy Name>" -AllowedFileTypes @{Add=".foo"}

 

If you wish to remove any extension from BlockedFileTypes list in the future:

 

$policy = Get-OwaMailboxPolicy [policy name]
$blockedFileTypes = $policy.BlockedFileTypes
$blockedFileTypes.Remove(".foo")
Set-OwaMailboxPolicy $policy -BlockedFileTypes $blockedFileTypes

 

Again, or:

 

Set-OwaMailboxPolicy -Identity "<Policy Name>" -BlockedFileTypes @{Remove=".foo"}

 

In order to determine which OwaMailboxPolicy object governs a particular user's Outlook on the Web experience, run the following commands:

 

$casMailbox = Get-CasMailbox [email address]
$casMailbox.OwaMailboxPolicy

 

What file extensions will be added to the BlockedFileTypes list with this change?

  • Extensions used by the Python scripting language: ".py", ".pyc", ".pyo", ".pyw", ".pyz", ".pyzw"
  • Extensions used by the PowerShell scripting language: ".ps1", ".ps1xml", ".ps2", ".ps2xml", ".psc1", ".psc2", ".psd1", ".psdm1", “.cdxml”, “.pssc”
  • Extension used by Windows ClickOnce: “.appref-ms”
  • Extension used by Microsoft Data Access Components (MDAC): “.udl”
  • Extension used by the Windows sandbox: “.wsb”
  • Extensions used for digital certificates: ".cer", ".crt", ".der"
  • Extensions used by the Java programming language: ".jar", ".jnlp"
  • Extensions used by various applications. While the associated vulnerabilities have been patched (for years, in most cases), they are being blocked for the benefit of organizations that might still have older versions of the application software in use: ".appcontent-ms", ".settingcontent-ms", ".cnt", ".hpj", ".website", ".webpnp", ".mcf", ".printerexport", ".pl", ".theme", ".vbp", ".xbap", ".xll", ".xnk", ".msu", ".diagcab", ".grp"

Security of our customers' data is our utmost priority, and we hope our customers will understand and appreciate this change. Change can be disruptive, so we hope the information here explains what we’re doing and why.

The Exchange Team

40 Comments
Copper Contributor

Very meaningful change. Regarding the short version of adding a file type, should it not be Set-OwaMailboxPolicy -Identity "" -BlockedFileTypes @{Add=".foo"} with Add instead of Remove?

@cehrit  You win today's prize for today the 'deliberate' mistake. Well done! I win today's prize for worst copy and paste. 

 

All fixed now, and glad you like the change. 

Iron Contributor

Hi, Greg: When does this change roll out, where? I am guessing Exchange Online first, then the next Exchange Server cumulative updates?

Rob

Rob, changes start rolling out from November in Exchange Online, they will come to on-prem in a future CU. 

Copper Contributor
It appears that ".psd1" is listed twice - is that a duplicate or is there another extension?

@bsommers - you win the second eagle eye prize and I shall hang my head in shame - for the second time. 

 

Thanks for spotting that (there was one other duplicate too which I fixed), all good now I hope. 

Copper Contributor

@Greg Taylor - EXCHANGE How do I manage this for my IT team? They will often be sharing PowerShell and certificates - but don't want to open this up for my whole organisation...

Copper Contributor

Are you aware that .appref-ms / clickonce is used by your own eDiscovery service for Exchange Online? 

 

https://docs.microsoft.com/en-us/exchange/policy-and-compliance/ediscovery/export-results-to-pst?vie...

 

@mr_kitson  - You can use the New-OwaMailboxPolicy and Set-OwaMailboxPolicy cmdlets to create and configure a new policy that meets the needs of your IT staff, and then use the Set-CasMailbox cmdlet to assign that policy to those people.

@lochii - yes, but sending them to someone as a clickable link in an email isn't recommended. If you want to create an exception for that type you can do so using the steps in the article. 

Copper Contributor
Also regarding the first clever shortcut for adding a file type - should it not be -AllowedFileTypes rather than -BlockedFileTypes? Sorry. Thanks for the useful post.
Copper Contributor

Hello!   Why should certificate files be blocked?

Copper Contributor

Oh hey, I've got one: customer's is possessive for a single customer. I think you meant customers'. First few words of the last paragraph.

Copper Contributor

Will these file types be blocked just in Outlook web app or will this also impact the desktop apps?

@chalmerstax - I'm going to fire the person who proof read this blog post given how many small errors are in it. 

@richardmanley3  - sorry, I can't answer this, I just got fired. 

 

(the answer is in the title...) 

Copper Contributor

Thanks for the heads up. Are you able to define "soon" to an expected timeframe?

Copper Contributor

Also, where can I find a list of the file types that are currently blocked?

Copper Contributor

Does this also take effect on Android and iOS applications?

Copper Contributor

@greg Taylor fair comment, it is in the title!  Just checking as I know our service desk will ask :)

Copper Contributor

If certs are blocked, does that mean signed and encrypted emails are no longer accessible via Outlook on the web?

@flinderbahn - no. Certs sent as attachments are blocked. That's not how signed and encrypted emails are made. 

Copper Contributor

@Greg Taylor - EXCHANGEI've always found it more bearable to endure the awkwardness of editing errors being pointed out by using an English/British accent. Enjoy. Cheers.

Will This change be made to only the default policy or will it also be added to any custom policies created?

Copper Contributor

Good Day!

 

Despite checking since the start of November, I do not see the following additional extensions one can pick from when in Exchange Admin Center:  ".psd1", ".psdm1", “.cdxml”, “.pssc”.  MS said, "We will be adding some additional file extensions to the BockedFileTypes list.... This change will roll out in November."  I added other extensions, such as ".psc1", so I believe I am looking in the right place.  I realize November is not over, yet, but I am getting anxious considering this is a holiday week for us USAers.  Anyone know when the new extensions may be made available, or ?  Thank you.

Copper Contributor

@Greg Taylor - EXCHANGE I was checking and found that the file types are added as duplicates in the blockedfiletypes on our default OWAMailboxPolicy. I know this might not affect anything as such, but wanted to put it out there.

Copper Contributor

Where is the list of blocked Extensions please?

Copper Contributor

Admin -- Exchange admin center -- malware filter (under the "protection" category) -- Edit the "default" filter -- click on "settings" -- scroll down a little to see "Common Attachment Types Filter" and specifically the listed "FILE TYPES" made available to

 

Enable, test, and tweak to taste.  Enjoy!

 

I hope this helps.

Copper Contributor

Can these blocked file types be sent in a compressed format (eg zip) or are the zip files inspected and banned too?

 

Thanks.

Steel Contributor

Sorry to nitpick but the way the new extensions that are being added is written, it's hard to conclude if those are just a list of extensions, or if they are actually the new ones being added.

"What file extensions will be added to the BlockedFileTypes list with this change?

The following extensions are used by the Python scripting language:"

 

Is it safe to read and interpret that 2nd line (and each of the next sets) like this:

"What file extensions will be added to the BlockedFileTypes list with this change?

The following extensions, which are used by the Python scripting language:"

Copper Contributor

@steve271 had the exact same thought... 

 

Can these blocked file types be sent in a compressed format (eg zip) or are the zip files inspected and banned too?

 

Thanks.

 

Copper Contributor

I also am very interested in the response to @steve271 's question - very relevant.

Copper Contributor

@steve271, most likely MS will scan inside files (as i remember is a normal practice in Outlook, Hotmail, Yahoo and GMail). They even scan a file with unknown extension, so if you change foo.zip to foo.abc it will get scanned anyways (it will read the header and know is a zip file).

 

I usually use 7-Zip and compress with password, encrypting filenames also. It prevent the scanning and consecuent blocking. But even that can be futile if they decide to block a compressed file that cant be scanned.

Copper Contributor

Hi - we have a colleges that have msu.edu in their emails -- will these be blocked, or is it JUST file types with .msu that will be blocked.

Copper Contributor

You keep saying OWA policy, Does this also filter down if you are connecting with normal desktop version of Outlook?

Steel Contributor

@IllusionOfSafety OWA Mailbox Policy applies to OWA only, no other client access type.

 

I guess they even renamed these policies to the newer OWA name "Outlook on the Web", but here's a link to start into on them:

https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/outlook-on-the-web/o...

 

You can do lots of things like prevent downloading attachments, as an example.  You can have multiple policies assigned to different groups of users who have different needs.

Copper Contributor

I think the decision to block digital certificate files is may not be a well-considered one since many Certificate Authorities still send newly-issued certificates via email. This will become an operational problem for IT departments.

Copper Contributor

I wonder the same thing  than Isaacchua: I think the decision to block digital certificate files is may not be a well-considered one since many Certificate Authorities still send newly-issued certificates via email. This will become an operational problem for IT departments. What is gonna be work around on this issue. We can't afford having our services down if certificates need to be renewed and the notifications from email are being blocked.  Any update on this? Else, many business will be facing problems

Brass Contributor

In response to RobertoIT as mentioned in the piece, you can edit the policy to ensure certs are allowed if you think it'll be a problem for your organisation.

Copper Contributor

Who decided that adding python text file (.py extension) was a security best practice?  Now I have to jump through hoops to share a script (that never even runs on a Windows platform) with a colleague. This is the illusion of security..... like putting the Club on your steering wheel but not locking it.

Iron Contributor
When editing the default OWA policy - step by step as described in the article: https://techcommunity.microsoft.com/t5/exchange-team-blog/changes-to-file-types-blocked-in-outlook-o..., Exchange "Cannot process argument transformation on parameter 'Identity'".
 
Any better docs to share or I'm the only one who gets the issue?

$policy = Get-OwaMailboxPolicy OwaMailboxPolicy-Default
$blockedFileTypes = $policy.BlockedFileTypes
$blockedFileTypes.Remove(".mag")
Set-OwaMailboxPolicy $policy -BlockedFileTypes $blockedFileTypes
 
Cannot process argument transformation on parameter 'Identity'. Cannot convert the "OwaMailboxPolicy-Default" value of
type "Deserialized.Microsoft.Exchange.Data.Directory.SystemConfiguration.OwaMailboxPolicy" to type
"Microsoft.Exchange.Configuration.Tasks.MailboxPolicyIdParameter".
    + CategoryInfo          : InvalidData: (:) [Set-OwaMailboxPolicy], ParameterBindin...mationException
    + FullyQualifiedErrorId : ParameterArgumentTransformationError,Set-OwaMailboxPolicy
    + PSComputerName        : outlook.office365.com
Brass Contributor

Hi,

Well...

 

What I see is that this is a default policy. And for all boxes in the organization.

Speaking of course from an online environment.

 

But my doubt is:

I can't create a policy just for some boxes, which would have to receive some messages with attachments with blocked extensions....

Co-Authors
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‎May 02 2022 12:24 PM
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