The hackers, which US intelligence agencies have said likely have Russian origins, used a backdoored update for SolarWinds Orion software to target a small number of Mimecast customers. Exploiting the Sunburst malware sneaked into the update, the attackers first gained access to part of the Mimecast production-grid environment. They then accessed a Mimecast-issued certificate that some customers use to authenticate various Microsoft 365 Exchange web services.
Tapping Microsoft 365 connections
Working with Microsoft, which first discovered the breach and reported it to Mimecast, company investigators found that the threat actors then used the certificate to “connect to a low single-digit number of our mutual customers’ M365 tenants from non-Mimecast IP address ranges.”
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