Updating your Mac to the latest version of macOS might not mean it’s impervious to firmware attacks.

According to a new security report, a shocking percentage of Macs are still vulnerable to critical firmware exploits despite running the latest version of macOS. Making matters worse, most users are completely unaware that their firmware isn’t up-to-date.

In the latest paper published by Duo Security, researchers analyzed more than 73,000 real-world Mac systems and found that 4.2 percent of Macs were running on incorrect EFI versions relative to their model and macOS version.

EFI (Extensible Firmware Interface) is the software that runs on a computer motherboard. It’s used to take a Mac from the bootup process into macOS. Usually, your EFI is updated when you install a new macOS update. The problem is sometimes the EFI update can fail. But your Mac never tells you that there was a problem with the EFI update so most users are completely unaware that it failed.

The percentage of Macs with incorrect EFIs varied widely in the study. If you have the late 2015 21.5-inch iMac, it’s the most likely to be vulnerable with 43% of systems found to be running incorrect EFI versions. Three versions of the late 2016 13-inch MacBook Pro also had high vulnerability rates with 35% to 25% of machines being affected.

What can you do?

Unfortunately, some Macs that are running the wrong firmware are out of luck and can’t be updated to the proper EFI anymore. But there’s still hope for most machines.

Security Duo recommends using its new tools to check if you’re running the latest version of EFI for your system. If your machine is supported by macOS 10.12.6, update to it now. It will give you the latest EFI versions and patch up any other known software security issues too.