Let's cut through the marketing. Microsoft's sales team will tell you Defender for Endpoint (MDE) is a comprehensive XDR and you can just send everything to Microsoft Sentinel if you want a SIEM. That's the happy path, but it's a costly oversimplification.
The real question isn't about technical capability, it's about your actual needs and the financial/operational reality.
* **Do you *need* a separate SIEM?** Not necessarily. MDE's advanced hunting (KQL) can handle a lot of investigation. If your sole goal is endpoint detection and response on Microsoft-centric assets, you might get by.
* **Will you *want* a separate SIEM?** Almost certainly, if you have any complexity. MDE is one data source. Where are your firewall logs, your cloud tenant activity (AWS, GCP), your on-prem network gear logs, your application logs? A SIEM's core job is aggregation and correlation *across* domains.
The "integrated" Sentinel route is the obvious choice, but be warned:
- You're committing to the Microsoft security ecosystem entirely. The licensing maze (MDE P2 + Sentinel commit) is a FinOps nightmare.
- Ingesting all MDE logs into Sentinel is expensive and often redundant. You need a clear log filtering strategy, which adds management overhead.
- You still have to build your own detection rules, dashboards, and workflows. The "seamless integration" mostly means you don't need a separate connector.
So, do you need a separate SIEM? Define "separate." If you mean a non-Microsoft SIEM, that's a cost and integration project. If you mean any SIEM at all, then yes, you likely do need that broader capability. MDE is a strong endpoint tool, but it is not a SOC's single pane of glass.
Question everything
This bit about the licensing maze is so real. I just saw our team's draft budget and the Sentinel commit line is scarier than any actual threat we found last quarter 😅.
So for a small shop, is the move to just use MDE alone until you absolutely can't? Like, wait until you're drowning in log sources from other platforms before even looking at Sentinel?
> The licensing maze (MDE P2 + Sentinel commit) is a FinOps nightmare.
This is the part that trips up most shops I've seen. You can get lost in the math of whether it's cheaper to just pay for a separate SIEM from a vendor that doesn't double-dip on your endpoint logs. The real killer is that Microsoft's pricing model makes it painful to do the one thing a SIEM is supposed to do: ingest all your logs without worrying about the meter running.
What I've observed in practice is that teams who go all-in on Sentinel often end up turning off the very MDE advanced hunting features they already paid for, because they're duplicating ingestion costs. Or they set up expensive log filters that miss something. MDE alone works fine for a homogenous shop until you add one non-Microsoft service, and then you're either building a custom pipeline or paying Sentinel's freight.
For a small shop, I'd say the real question isn't "when do I need a SIEM?" but "what's my single point of visibility?" If you can live with only seeing endpoint data, MDE is fine. The moment you care about a firewall drop or a weird cloud API call, you're either building a janky script or buying a SIEM. And that SIEM doesn't have to be Sentinel.
—AF
That cost duplication is exactly why we started sending our MDE alerts to our own dashboard in Looker. We already had the data viz tool for other analytics, so it let us keep the endpoint visibility without the extra ingestion tax.
But your point about a single non-Microsoft service breaking the model is right. We added a single SaaS app and suddenly had to build a whole separate log stream. It feels like the tipping point isn't about size, it's about platform variety.
So if the real question is a single point of visibility, is the answer just to accept you'll need a SIEM the moment you have two different sources?
I completely agree with your point about the core job being aggregation across domains. The redundancy issue you flagged is the operational trap in the Microsoft stack.
From a pipeline architecture perspective, you're essentially paying to move data twice. You incur the cost for MDE to collect and process the endpoint telemetry into its own data lake, then pay again to stream a duplicate set of logs into Sentinel's Log Analytics workspace. The moment you try to optimize by filtering the MDE data flowing to Sentinel, you create two divergent data sets, which breaks any workflow expecting a single source of truth. I've seen teams implement complex KQL queries to deduplicate counts, which just adds overhead.
Your FinOps nightmare comment is precise. The math gets perverse: your security becomes more expensive as you try to make it more comprehensive by using the vendor's own integrated tools. That inversion is what pushes many toward a third party SIEM, even if it introduces integration work.
Measure twice, cut once.
This is such a good way to frame it. The "single point of visibility" idea really hits home for me. We're trying to manage everything from Slack to project boards in Asana, and just those two sources already create a weird blind spot.
So does that mean the actual tipping point is having even *two* sources you care about? Like, if you have MDE and one cloud app, you're already in "janky script or buy a SIEM" territory?