The 7 Deadly Sins of a Microsoft Sentinel Deployment

Alright, class.

You've read the guides, you've watched the videos, and you've bravely deployed Microsoft Sentinel. You're feeling pretty good. But a SIEM deployment is like a video game: it's easy to start, but the difference between a casual player and a pro comes down to avoiding the classic, rookie mistakes.

Over the years, I've seen a lot of Sentinel deployments. Some are beautiful, efficient, threat-slaying machines. Others... not so much. They're often bloated, expensive, and as noisy as a vuvuzela in a library. The difference almost always comes down to a few key decisions made right at the beginning.

Today, we're going to talk about the seven deadly sins of a Sentinel deployment. These are the most common, most expensive, and most soul-crushing mistakes I see in the wild. Consider this your cheat sheet to getting it right the first time.

Sin #1: The "Workspace Sprawl" - A Flawed Foundation

The Mistake: In a fit of organisational excitement, you create separate Sentinel workspaces for your servers, another for your cloud apps, and maybe a third for your network gear. It seems logical on a spreadsheet.

Why It's a Trap: Sentinel's superpower is correlation. It's about seeing a weird sign-in from Entra ID, connecting it to a strange process on a server, and linking that to a firewall alert from Palo Alto. When your data is fragmented across multiple workspaces, you shatter this ability. You've created data silos, complicated your management, and made your analysts' lives miserable as they try to hunt across three different screens. On top of that, you're now managing three sets of rules, three sets of playbooks, and three different bills.

The Professor's Fix: Unless you have a hardcore, legally-mandated data residency requirement (e.g., "EU data cannot leave EU data centres"), you should have one, and only one, Sentinel workspace. For multi-tenant scenarios (like MSSPs), the correct tool is Azure Lighthouse, not a jumble of workspaces (and worse - switching the tenants!) One workspace provides a single pane of glass, enables total correlation, and simplifies your entire operation.

Sin #2: The "Log Everything" Fallacy - Drinking from the Firehose

The Mistake: You open the Data Connectors blade and go on a clicking spree. "Yes, I want all the logs! Every last one!" You connect everything, from your most critical domain controllers to the verbose diagnostic logs from a forgotten dev server.

Why It's a Trap: This is the single most expensive mistake you can make. More logs do not equal more security. They often just equal more cost and more noise. You end up paying a fortune to ingest terabytes of data that you have zero detection rules for. A classic example is enabling verbose logging on end-user workstations when you already have Defender for Endpoint. You're now paying to ingest redundant, low-value noise that your EDR is already handling better.

The Professor's Fix: Be a surgeon, not a lumberjack.

1. Prioritise the "Free" Stuff: Start with the "low-hanging fruit." Enable the native Microsoft connectors first: Entra ID, Microsoft 365, and the Defender suite. The ingestion of many of these is free and provides the richest context (especially with the E5 license)
2. Justify Every Source: For every other data source, ask two questions: "What specific threat does this help me detect?" and "Can I filter this at the source?" Never ingest unfiltered firewall logs. Choose the structured CEF format over raw Syslog whenever possible to save yourself a parsing nightmare later.

Sin #3: The "Set and Forget" Analytics Rule - Cultivating Noise

The Mistake: You go to the Content Hub, install a solution, and enable all 50 of its analytic rules. You lean back, proud of your new detection coverage. Two days later, your incident queue has 3,000 alerts, and your analysts are threatening to unionise.

Why It's a Trap: Out-of-the-box rules are templates, not gospel. They are designed to be generic. Your environment is specific. Failing to tune these rules is the primary cause of alert fatigue, which is the number one killer of SOC morale and effectiveness. You're training your team to ignore the alerts screen.

The Professor's Fix: Treat your rules like a garden that needs constant tending.

1. Start Small: Enable a core set of high-confidence rules first.
2. Tune Aggressively: Is a rule firing for legitimate admin activity? Create an automation rule to auto-close those specific instances, or add an exclusion to the KQL.
3. Have a Process: A mature SOC has a formal use case development lifecycle. You identify a risk, ensure you have the data, develop and test a new rule, and create a response playbook for it. Don't forget to leverage the built-in UEBA engine; it provides invaluable context that traditional rules miss.
4. Correlate: When creating analytic rules, why not join Sign-in Logs with the IdentityInfo table to get more context about who the user is? Or build a use anomaly detection? Always ask yourself about how SOC will investigate this specific alert and what information you can surface to help them

Sin #4: The "Legacy Agent" Tech Debt - A Self-Inflicted Wound

The Mistake: You need to get logs from your servers, so you deploy the agent you've always used: the Microsoft Monitoring Agent (MMA).

Why It's a Trap: The MMA is a ghost. It's deprecated and should not be used at all. Starting a new deployment with it is like building a new house with asbestos. You're creating a massive technical debt problem that you'll have to pay off with a painful, rushed migration project later.

The Professor's Fix: Use the Azure Monitor Agent (AMA) for all new deployments. Period. It's more efficient, more secure, and most importantly, it uses Data Collection Rules (DCRs). DCRs are your best friend for cost control. They allow you to do granular, server-side filtering of noisy logs (like Windows Security Events) before they ever get sent to Sentinel, saving you a fortune in ingestion costs.

Sin #5: The "Hot Storage for Everything" Budget Fire

The Mistake: An auditor tells you that you need to keep your firewall logs for seven years. You set your Log Analytics Workspace retention to 2,555 days and call it a day, ignoring the eye-watering cost projections.

Why It's a Trap: You've just signed up to pay the premium "hot" analytics price for data you will almost never query. You're paying for Ferrari-level access speeds for data that's just sitting in a garage, collecting dust and costing you a fortune.

The Professor's Fix: Implement a tiered storage strategy from day one.

• Hot (Analytics Tier): Keep the last 90 days of data for active hunting and real-time alerting.
• Cold (Azure Data Lake): Use the new, managed Data Lake integration for everything else. It's ridiculously cheap for long-term storage, and you can still run KQL jobs against it when that auditor comes knocking.

Sin #6: The "Manual Toil" Mindset - Ignoring Your Robot Army

The Mistake: An alert fires. The analyst copies the user's UPN, pivots to Entra ID to check their risk level, copies the IP, pivots to a TI tool to check its reputation, and then manually creates a ticket in ServiceNow. This process takes 15 minutes.

Why It's a Trap: You are wasting your most expensive resource: your analyst's brain. Sentinel has a powerful SOAR engine built in called Logic Apps. If a task is repetitive and predictable, a robot should be doing it.

The Professor's Fix: Identify your top 3 most common, repetitive analyst tasks. Spend some time building a Logic App playbook for each. Automating that initial enrichment process can turn a 15-minute triage into a 2-minute one, freeing up your team to work on actual, complex investigations that require human intelligence.

Sin #7: The "Lone Wolf" Deployment - The Political Blind Spot

The Mistake: You, the brilliant Sentinel engineer, plan the entire deployment in isolation. You have a perfect list of all the data you need. Then, you start asking for it. The firewall team says they're busy for six weeks. The SaaS application owner doesn't know how to configure an API. Your project grinds to a halt.

Why It's a Trap: A SIEM deployment is a political and organisational challenge, not just a technical one. The logs you need are owned by other teams. You need their buy-in, their expertise, and their time. Many modern SaaS apps don't have a simple connector; they require a custom one built with an API, which means you need developer resources.

The Professor's Fix: Build a coalition.

1. Identify the System Owners: Find the subject matter experts for your top 5 critical log sources before the project even starts.
2. Bring Them In: Make them part of the planning meetings. Their involvement is a project dependency.
3. Secure Resources: If you need custom connectors, get a commitment from a developer or a DevOps team early on. A little bit of stakeholder management at the beginning will save you months of frustration and delay.
4. If you have any third party connectors you want to bring in, like Salesforce, Google Workspace or Slack - check if there is a way to deploy them, if there is already a data connector for it, or if it's an API call? Are you able to bring those logs by yourself, or do you need additional help from your DevOps team? Planning ahead will help you massively.

Avoiding these seven sins won't just save you money; it will result in a Sentinel deployment that is efficient, effective, and a genuine asset to your security program.

Class dismissed.