Noting this, the security firm Sophos said in a detailed paper released on Wednesday that unpatched Windows PCs were still providing a welcome for WannaCry which was crafted using the leaked NSA exploit, EternalBlue.
Sophos researchers Peter Mackenzie, Fraser Howard and Anton Kalinin said that more than 12,000 variants of what is arguably the most famous ransomware had been discovered.
WannaCry was stopped in its tracks when British security researcher Marcus Hutchins registered a domain he had found in its source code; the malware had been programmed to check this domain, and continue spreading if it could not access the domain.
They also found that about 97% of the Windows PCs that came under attack were running Windows 7, adding "so this is not just a story about forgotten Windows XP devices".
WannaCry has three elements – the worm part which enables it to spread without any vector, the kill switch, and the ransomware part which is a zipped archive.
Many of the variants that were encountered by the Sophos researchers had the kill switch disabled and skipped over to the last part of the ransomware. But as the zipped archive had been corrupted, the expected function did not run to completion, something mentioned by British security researcher Kevin Beaumont.
Mackenzie, Howard and Kalinin said that this behaviour had been noted as early as two days after the initial breakout — 12 May 2017 — with French researcher Matt Suiche confirming this from a sample that had been uploaded to the virus database VirusTotal by Kaspersky staffers.
"The MD5 hash of the file uploaded to VirusTotal, which doesn’t have a kill switch and doesn’t encrypt files, is none other than the exact same file we now see causing the highest number of WannaCry detections: MD5: d724d8cc6420f06e8a48752f0da11c66," the Sophos team wrote.
"It is number one on the of unique file variants list provided earlier, causing 29% of all WannaCry detections in our data.
"Even more amazing is that the top three files on our list are all variants of this same file. The other two files contain the same corrupt archive; the only difference is in how the kill switch has been removed."