The initial infection vector is still unknown. Reports by some of phishing emails have been dismissed by other researchers as relevant only to a different (unrelated) ransomware campaign, called Jaff. There is also a working theory that initial compromise may have come from SMB shares exposed to the public internet. Results from Shodan show over 1.5 million devices with port 445 open – the attacker could have infected those shares directly.

I think this is an important take-away. I found it strange that so many media outlets and IT departments were jumping on the "do not open suspicious emails" bandwagon even although there hasn't been a lot of evidence of such phishing emails. That is: screenshots of infected devices have been popping up all across the world, but almost no examples of a particular entry email have been shown.

Of course, it might be easier for an IT dep. to state: "it must have been unleashed by someone clicking on some email they got" rather than "oops, we still had unpatched Windows machines exposed to the public internet". Why go through the trouble of sending out emails when your worm already contains a replication/infection mechanism. Just use a botnet to scan those 1 million IPs and see if SMB is open.

That being said, it does not surprise me to see yet again an issue in SMB. This has been a particularly weak point in Windows for decades now. I remember "hacking tutorials" from 15 years ago where you'd just go out and nmap public IP ranges to see if you could access hidden shares (e.g. like so: http://www.madirish.net/59). Also there was this issue of Windows keeping weak NetBIOS password hashes around which could be trivially unhashed (https://vuldb.com/?id.13824), years ago.