On a recent engagement, I took a different approach to finding possible malicious files entering the customer's network. Rather than focusing on the e-mail, I looked for any RAR, macro-enabled office documents, and portable executable files (PE) entering the network where no service was specified. Of course, this was done using RSA NetWitness and immediately I found a RAR file which contained a malicious executable. Although, this was a different vector by which it entered the network. It didn't appear to be a link that someone clicked from an e-mail and it wasn't an attachment from an email either. It was from a customer configured, cloud based support site. You can find many customers who use these types of sites.
So here's how I believe this was attempted. A malicious actor goes to the <customer name>.<customer support site>.com site where they open a support ticket for an order they placed. (Of course they probably didn't place an actual order) Then using the support interface, they upload what appears to be an order list. In this instance I found the file name was "OrderList_Xlsx.arj" which is a RAR file and inside was a file called "OrderList.exe" all of which was downloaded by the customer support representative using their admin console to the site.
It's a simple approach. It involves the actor opening a support ticket on the customer's site but whether they are actually doing this or using a script/automation is another question. In this instance, I didn't see it as being targeted towards this customer but maybe they're testing the waters. Without having access to this customers admin console to this service it's hard to determine whether this is happening more frequently because from our perspective, we only see where the employee downloads the file and enters into the customer's network.
I created a quick and easy search to find this type of activity.
alias.host = '<customer name>.<customer support site>.com' && (filetype = 'rar' || filetype = 'windows executable' || ((filetype = 'zip' || filetype contains 'office') && filename contains 'vbaproject.bin') || extension contains 'docm','xlsm','pptm' || content contains 'macro')