How to avoid the WannaCrypt virus if you run Windows on a Mac

WannaCrypt may be exclusively a problem for Windows users, but
the worm/virus combination could hit a Mac user with a Boot
Camp partition or Windows virtual machines in VMware Fusion,
Parallels, or other software. If you fit that bill and haven’t
booted your Windows system since mid-March or you didn’t
receive or install Microsoft’s vital security update (MS17-010)
released at that time, read on.

It’s critical that you don’t start up a Windows XP or later
installation that’s unpatched and let it connect to the
Internet unless you’re absolutely sure you have the SMB
file-sharing service disabled or firewall or network-monitoring
software installed that will block any attempt from an outside

Also, if you use Windows XP or a few later releases of Windows
that are past Microsoft’s end of support since mid-March, you
wouldn’t have received the security updates that Microsoft was
reserving only for corporate subscribers
until last Friday
. At that point, they made these updates
generally available. If you booted any of those systems between
mid-March and Friday, you’re unprotected as well.

If your Mac is on a network that uses NAT and DHCP to provide
private IP addresses, which is most home networks and most
small-office ones, and your router isn’t set up to connect the
SMB file service from outside the local private network to your
computer (whether Boot Camp or a VM), then the WannaCrypt worm
can only attack your system from other computers on the same
network. If they’re already patched or there are no other
Windows instances of any kind, you can boot up the system,
disable SMBv1, and apply the patches.

If you don’t want to take that chance or you have a system that
can be reached from the greater Internet directly through
whatever method (a routable IP or router port mapping to your
Mac), you should disable networking on your computer before
restarting into Boot Camp or launching a VM. This is easy with
ethernet, but if you’re using Wi-Fi for your Windows instance,
you need to unplug your network from the Internet.

After booting, disable SMBv1. This prevents the worm from
reaching your computer, no matter where it is. Microsoft offers
instructions for Windows 7 and later
at this support note
. If you have a Windows XP system, the
process requires directly editing the registry, and you will
want to install firewall software to prevent incoming
connections to SMB (port 445) before proceeding. The firewall
approach is a good additional method for any Windows instance.

Once you’ve either disabled SMBv1 or have a firewall in place,
you can enable network access and install all the patches
required for your release, including MS17-010.

In some cases, you no longer need SMBv1, already known to be
problematic, and can leave it disabled. If for legacy reasons
you have to re-enable it, make sure you have both networking
monitoring and firewall software (separately or a single app)
that prevents unwanted and unexpected SMB access.

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