There just need to be one, with a good write up of the evidence and the evidence be better than "they used the same tools" or "their IP is in Russia". The linked article does not detail them.
I have seen US TLAs blame China on really laughable evidences (and fail to do it properly on the one undeniable attack they did on GitHub)
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There are a ton of security experts who have indeed dug deeper, and came to the conclusion that it was Russia.
There just need to be one, with a good write up of the evidence and the evidence be better than "they used the same tools" or "their IP is in Russia". The linked article does not detail them.
I have seen US TLAs blame China on really laughable evidences (and fail to do it properly on the one undeniable attack they did on GitHub)