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Early this year, a group of hackers associated with the Chinese government, known as Hafnium, exploited a vulnerability in Microsoft's Exchange Server. The attack allowed them to gain access to over 60,000 servers, including those of major corporations and banks.
This attack is separate from the SolarWinds hack that affected thousands of customers last year through a backdoor vulnerability in the company's software. In that case, a Russian group was able to piggyback on SolarWinds' software, which--when installed via an update on client networks--allowed the hackers to deploy malicious code. In that case, Microsoft worked with Fire Eye to cut off the attack by sink-holing the domain used to receive further instructions.