Yes, WannaCrypt can infect all those machines that still run Windows XP, but because XP is so flaky the zombie boxen are unlikely to have contributed much to the spread of the worm.

That's the conclusion of Kryptos Logic researchers after a couple of weeks trashing crash-test-dummy machines in the laboratory.

The company notes that the infection rate probably got up to as many as 727,000 unique IPs at its height.

Early in the infection, it was assumed – including by El Reg – that unpatched Windows XP systems were part of the problem, especially since it lives on in the systems of early victim, England's National Health Service.

Kryptos (whose roster includes celebrated sink-holer MalwareTech) has bumped its head on this for an almost-comical reason: yes, WannyCrypt could infect XP machines, but the underlying DOUBLEPULSAR payload kept crashing the targets.

It's well down the scrollbar in this extensive analysis of WannaCrypt:

The researchers were running through the infection step-by-step: first, manually execute the WannaCrypt binary on a Windows 2008 Server SP1 machine; second, test propagation via the ETERNALBLUE exploit; and third, send the payload on using DOUBLEPULSAR.

Here's the short version:

Windows XP with Service Pack 2 – No infection

– No infection Windows XP with Service Pack 3 – Random blue-screen of death (BSOD) but no infection

– Random blue-screen of death (BSOD) but no infection Windows 7 64 bit with Service Pack 1 – Infected after multiple attempts

– Infected after multiple attempts Windows Server 2008 with Service Pack 1 – Could not replicate infection, but reported exploited

The WannaCrypt infection process, according to Kryptos. Click to embiggen

Next, the Kryptos chaps went to work on manually backdooring test systems with DOUBLEPULSAR. While that worked just fine on Windows 7 64-bit, SP1, “it was found that both Windows XP hosts kept blue-screening and rebooting without any infection occurring.”

It doesn't mean that XP was mysteriously protected by its love of BSOD – if you got a user to manually execute it locally, it would encrypt the victim's files.

However, Kryptos writes, XP probably didn't contribute much to the number of infected machines, “since the main infection vector was the SME exploit” because “the exploit as implemented in WannaCry does not seem to reliably deploy DOUBLEPULSAR and achieve proper RCE.”

Mind you, getting a cycle of BSODs would spoil anyone's day. ®