If I have Microsoft 365, do I need a SIEM?
Not automatically. Start with the security features you already have, the events you need to investigate and who will act on them.
Microsoft’s own guidance says the answer depends on your security requirements and where your data resides. Microsoft 365 already provides security capabilities; an additional SIEM is a decision about the wider monitoring and investigation you need, not a requirement simply because you use the platform.
What problem are you trying to solve?
For some teams, the immediate concern is whether an account is signing in unexpectedly or a mailbox rule has been added. Other teams need to trace an incident across firewalls, endpoints, servers and several cloud applications. Those are different buying requirements.
Start with your existing Microsoft controls
List the Microsoft licences, permissions, audit sources, alert policies and review process you already use. Available features and data vary by subscription and configuration. An extra monitoring tool does not bypass Microsoft licensing or guarantee data that your tenant cannot expose.
When a full SIEM may be appropriate
- You need to investigate across several kinds of systems and security products.
- You need broad log searches and correlation as part of an established security operations process.
- Your organisation has specific monitoring, evidence or retention requirements that a proposed design must meet.
Validate those requirements with the people responsible for them. A purchase should not be based on a blanket claim that every Microsoft 365 organisation needs a SIEM.
When focused Microsoft 365 monitoring may fit
If your existing engineers mainly need account and mailbox events surfaced for review, assess a focused tool against those events. Vigil Alerts monitors supported sign-ins, inbox rules, forwarding and mailbox access changes. It is available self-hosted or on a dedicated hosted installation.
Vigil does not replace a full SIEM, a managed security operations team or every Microsoft security feature. It provides a narrower workflow for the engineers who will carry out the investigation.
Having logs is different from reviewing them
Decide who receives the alert, what they check and what happens outside their working hours. If the gap is unstaffed investigation, a software subscription alone will not fill it. If the gap is a usable review workflow, demonstrate that workflow before choosing a platform.
What should you compare next?
For software scope, read Microsoft 365 monitoring versus SIEM. For operating costs, compare your actual quote. For a wider service, consider whether managed ITDR matches your staffing requirement.
Source checked 18 October 2026: Microsoft guidance: Do I need a SIEM server?. This is general product guidance, not a compliance assessment.