Klijnsma passed away on 6 January, according to a post by his wife, Nady, on his Twitter feed on 10 January.
Former Virus Bulletin editor Martijn Grooten said he had first encountered Yonathan seven years ago, "when I was doing some research on a subject he had also worked on. When I later gave a presentation on the same subject, he messaged me to say he had really enjoyed my talk. It meant a lot to receive this compliment from him".
iTWire has reported on a number of Yonathan's research efforts; he covered the compromise of e-commerce sites, with his focus being on credit card skimmers that are together known as Magecart.
"As he was due to receive chemotherapy and radiation treatment, we had a few honest conversations in which he told me about the physical and mental challenges he was facing," he wrote.
The statement issued by Yonathan's widow, Nady.
"The treatment seemed to have its desired effect and as he was recovering Yonathan was already talking about submitting to BlackHat. But then there was 'bad bad news', as he told me in April: the cancer had returned, was now in his lungs and was deemed incurable.
"After he explained a new experimental treatment he was going to receive, we had a beautiful conversation about life and death – a conversation that will stay with me forever."