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Exchange 2013 - Problem with some attachments

Posted on 2016-09-30
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Last Modified: 2016-10-03
Hello ,


We have an Exchange 2013 infrastucture . 2 Edge servers , 2 CAS and 2 MBX . Infront of all this we use an antispam cloud service. Filtering is disabled in Edge Servers.

I have a strange problem with incoming attachments from a bank. Everyday they send statement files as a zip file. But exchange remove the attachment and placed a txt file . So the file name is statement.zip and exchange rename it to statement.zip.txt . In OWA also i see a message that says "  An attachment was removed from this email message
because the attachment may pose a security risk "

Is there a way to bypass this ? I have also whitelist the user in our cloud antispam platform but the problem continues .

Thanks
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Question by:Anestis Psomas
10 Comments
 
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Accepted Solution

by:
Kimputer earned 500 total points
ID: 41823287
Either totally disable Exchange 2013 Attachment filter function, or selectively only disable it for zip files. See here for the steps: https://technet.microsoft.com/en-us/library/aa997139(v=exchg.150).aspx
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Author Comment

by:Anestis Psomas
ID: 41823297
Thanks for the info but zip files are not included in the attachment filter list.
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LVL 35

Expert Comment

by:Kimputer
ID: 41823314
You will know for sure if you totally disable the filter, then receive the email again.
If the zip arrives normally, it's REALLY still being filtered (albeit not named simply zip, but under somthing else). Please try that first.
If after you've disabled the whole filter, and it's still coming in as zip, we know we don't have to check this filter config at all.
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Author Comment

by:Anestis Psomas
ID: 41823319
Thanks Kimputer ,

I must disable filtering only in Edge Servers ? Or also run commands in CAS and MBX Servers ?

Thanks
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LVL 35

Expert Comment

by:Kimputer
ID: 41823325
The function is only for Edge servers. So do it twice if you have 2 Edge servers. Don't forget the restart of the Transport service. Obviously, try to keep the testing as short as possible, disable filter, restart service, send email, check results, enable filter (because the filter DOES have its useful functions).
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LVL 63

Expert Comment

by:Simon Butler (Sembee)
ID: 41824059
How do you know it is Exchange removing the attachment? I am pretty sure that Exchange doesn't remove the attachment with that text (unless you are paraphrasing from another language). I would be looking at your external filtering service.

Although if you are using an external filter, why bother with Edge servers as well?
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LVL 14

Expert Comment

by:Justin Yeung
ID: 41824361
Exchange do have a feature called Antimalware however it doesn't strip out the attachment and replacing with txt file.

Antimalware scans the email when it hits the "Transport", by default rule it deletes the entire email but I don't believe it has the ability to remove just the attachment without other Microsoft product, forefront protection

It should be an issue from your cloud spam filter which probably being stripped at that level.
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Author Comment

by:Anestis Psomas
ID: 41826075
Hello all ,


I disable the attachment filter scanning and now the attachment received withouy any problem. Funny is that zip files are not in the attachment filter list. Any ideas why this is happening ?

Sembee you are right about the edge servers . They have no meaning now that we have a Cloud Service for antispam filtering . We keep them because we want to be sure we are ok first with the cloud antispam filtering and we will remove them later.

Thanks
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LVL 35

Expert Comment

by:Kimputer
ID: 41826095
So now that you know where to look (Edge server, Attachment filter), you can enable it, and try to find which rule is causing it. Probably one of the MIME types is taking care of the zips, under another name.
If you really can't find it, you probably have to disable application/octet-stream (though obviously, you'll start letting in a lot of things now, even .exe files!!!!). So either

disabled octet-stream, you don't mind .exe files (unsafe!)
disabled octet-stream, but you'll replace it with your own filters, including .exe etc etc
enabled, live with the fact zip files come in as txt files
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Author Comment

by:Anestis Psomas
ID: 41826162
I will leave it disabled because im using a Cloud Antispam Solution so i filter attachments there.

Thanks a lot for your help kimpuer !!
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