The Australian Signals Directorate (ASD), the sigint outfit renowned for its “don't be stupid” guide to infosec, has published its latest guidelines for e-mail admins.
E-mail being what it is, its Malicious Email Mitigation Strategies carries a fair amount of detail, but the basics are easy: treat attachments like live grenades; don't accept attachments you can't scan; and please reject Microsoft Office macros.
There's a simple reason the focus is on stopping badware getting into users' inboxes in the form of attachments: users are going to click on attachments or links no matter how many times you tell them not to.
While the advice won't surprise infosec insiders, it's a very handy checklist to make sure your security admins have ticked all the boxes.
Top of the list isn't, as you might expect, “run all attachments through antivirus”; rather, if you can, the ASD reckons your mail systems should change the attachment's file type. For example, incoming Word attachments could land in the user's inbox as PDFs, meaning that users can view a document without risking a macro virus coming from the far end.
Incoming file types should be explicitly whitelisted, the ASD reckons, and the mail server should quarantine encrypted attachments, password-protected zips, and the like (with whitelisting only if there's a legitimate reason to encrypt).
Sender verification – Sender Policy Framework (SPF) or Domain Keys Identified Mail (DKIM) – should be supplemented with Domain-based Message Authentication, Reporting and Conformance (DMARC). ®