Patching these systems is not a straightforward task. Email servers are difficult to maintain, even for security professionals, and many organizations lack the expertise to host their own servers safely. For years, Microsoft been pushing these customers to move to the cloud, where Microsoft can manage security for them. Industry experts said the security incidents could encourage customers to shift to the cloud and be a financial boon for Microsoft.

Because of the broad scope of the attack, many Exchange users are probably compromised, Mr. Adair said. “Even for people who patched this as fast as humanly possible, there’s an extremely high chance that they were already compromised.”

Nicole Perlroth contributed reporting.

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