No, that wasn’t possible. The system was not exposed to any of the old data files or the Internet. We suspect a hidden area on the hard drive in one instance and the system AMI BIOS in another instance.
MSI is now providing system boards with two EUFI on firmware, and then followed up with the ability to restore a EUFI from a USB flash drive. This was done in part to defeat the efforts of malware to infect and/or brick the system board by attacking the BIOS/EUFI.
When we used Malwarebytes and other anti-malware software to repair a couple of FBI ransomware hijackings, we found variants of the malware were systematically digging itself ever deeper into the system as we defeated the earlier efforts. It finally got down to the point where a Dell Optiplex 755 system would no longer enter the firmware setup. These variants were doing things the anti-malware software sources were saying weren’t possible, but they were possible and real.
Well, that’s discouraging. My main computer is a Dell Optiplex 755. I have MSE and have once been hit with the FBI ransomware. I went ahead and did a reformat/reinstall.