This was my main question as well. I read the report, but I'm still not clear on what the browser vulnerability is. This clearly seems like something browser vendors should be able to patch, correct?

The article does say "Both samples use a COM hijack for persistence, though make use of different DLLs." So does this only affect Windows machines? I didn't have time to read the linked 2021 report on Candiru, so maybe more details are there, but for such a detailed write up it would be helpful if the article was more explicit about which browser vulnerability was exploited. The step-by-step info graphic in the article literally skips over how the infection actually occurs.