How do I know if my business network has been compromised?

Look for behaviour that has changed: new outbound destinations, connections at regular intervals, sign-ins at odd hours, one machine connecting to many others, and unusual outbound volume. Each can be checked in firewall, DNS and directory logs. Two or more together are worth an investigation; continuous network monitoring watches for all of them every day, not only on the day you look.

This answer also appears, with more context, here: Is someone inside your network?

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