Jake Williams, who heads cyber security consultancy Rendition Infosec, said in a series of tweets that he was appalled at how many infosec analysts were willing to throw Kaspersky under a bus, given the lack of "substance of evidence".
The Wall Street Journal, citing "people with knowledge of the matter", reported that the exfiltration of data had taken place in 2015 but came to light in the Western spring of 2016.
Williams (below, right) has in the past called on the FBI to either provide proof to the public that Kaspersky Lab products are unsafe for use or keep mum. This was after reports emerged of the agency claiming to companies that Kaspersky was spying on behalf of Russia.
"Then replace 'Russian hackers' with 'US hackers'. Consider if this report was in the Russian media. Would you demand evidence?"
Williams, a certified SANS instructor, added: "If you answer 'no' then you're not being honest or your analytical skills suck. If you answer yes, don't you dare jump on Kaspersky."
US moves against Kaspersky Lab have come in the wake of allegations that Russia interfered in the 2016 US presidential elections.
Last month, the US Department of Homeland Security issued an order to all government agencies, giving them 90 days to stop using products from Kaspersky Lab.
Williams, who has also pulled up Kaspersky Lab in the past when he felt the company was misleading people over this debate, said: "Kaspersky might be everything DHS claims. They might not be. You don't know, and that's the point. The US has used propaganda before.
Piling on Kaspersky right now is like taking a high school math exam: you may get the right answer, but no credit unless you show your work.— Jake Williams (@MalwareJake) 5 October 2017
"We are definitely in a cyber conflict with Russia. Could this be propaganda? I don't know. But it's consistent with what I'd expect. When we refuse to demand evidence, substance, we relinquish the moral high ground for future cases like this ones where we might be harmed."
He added: "If we get real data later that implicates Kaspersky, I'll grab a pitchfork and light a torch myself. Until then, I'm standing down. FTR, I've dealt with personally damaging, unfounded claims before. They've completely changed the way I process this sort of thing."