Microsoft released patches Tuesday for four critical vulnerabilities Chinese hackers are using in targeted attacks on Exchange Server, SC Media has learned.

On a series of three blog posts to be released Tuesday, Microsoft said targeted hacking from a group operating out of China that the company calls Hafnium, linked together chains of vulnerabilities to garner access.

"We are sharing this information with our customers and the security community to emphasize the critical nature of these vulnerabilities and the importance of patching all affected systems immediately to protect against these exploits and prevent future abuse across the ecosystem," Microsoft noted in the blog post, which was provided to SC Media before release.

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