Microsoft 365 Security Review: What to Check

Most Microsoft 365 breaches do not start with a dramatic technical failure. They start with a basic gap: legacy authentication left switched on, multifactor authentication applied inconsistently, or sharing settings that made sense two years ago but no longer fit the way the business works. A proper Microsoft 365 security review is less about ticking boxes and more about finding the quiet weaknesses that attackers look for first.

For growing organisations, that matters because Microsoft 365 often sits at the centre of day-to-day operations. Email, files, Teams chat, identity, mobile access, and collaboration with third parties all converge in one place. If the platform is not configured with care, one weak policy can affect the whole environment.

Why a Microsoft 365 security review matters

Many businesses assume Microsoft has already handled security because the platform is cloud-based. Microsoft does secure the underlying service, but customers are still responsible for how identities are managed, how data is shared, what devices can connect, and which policies are enforced. That shared responsibility is where risk often creeps in.

The challenge is not just technical. Security settings in Microsoft 365 affect productivity, supplier collaboration, hybrid working, and user experience. If controls are too loose, you increase exposure. If they are too restrictive, staff find workarounds. A good review balances protection with how your business actually operates.

This is also why a one-off setup is rarely enough. Businesses change. People join and leave. Acquisitions happen. Teams begin sharing more externally. New Microsoft features are switched on. What looked acceptable last year may now be a genuine risk.

What a Microsoft 365 security review should cover

A useful review starts with identity. In most cases, identity is the front door to the whole platform. If an attacker compromises one account with broad access, they may not need anything more sophisticated.

Identity and access controls

The first area to inspect is multifactor authentication. It should be enforced consistently, especially for administrators, senior leaders, finance users, and anyone handling sensitive data. If MFA is optional or only partly deployed, that is a priority issue.

Legacy authentication should also be disabled wherever possible. Older protocols can bypass modern security controls and remain a common entry point for password-based attacks. They often linger because of old devices, printers, or line-of-business systems, so this part of the review needs a little care rather than blanket assumptions.

Conditional Access is another major control. Done well, it allows access based on risk, location, device compliance, and user role. Done badly, it becomes a patchwork of exceptions nobody fully understands. Reviewing these policies helps you see whether access decisions still reflect your current workforce and risk profile.

Privileged accounts deserve separate attention. Global administrator rights should be limited to a very small number of people, with clear controls around how those accounts are used. If standard user accounts also hold elevated permissions, that increases the impact of a single compromise.

Email protection and phishing resilience

Email remains one of the most common attack routes, so any Microsoft 365 security review should look closely at Exchange Online Protection and Microsoft Defender settings where applicable. The question is not simply whether anti-spam is switched on. It is whether phishing controls, spoof protection, safe links, safe attachments, and mailbox intelligence are configured at the right level for your risk profile.

Domain protection matters too. SPF, DKIM, and DMARC help reduce spoofing and improve trust in legitimate messages. Yet many organisations still have partial or inconsistent deployment. That leaves room for impersonation attacks aimed at finance teams, directors, or suppliers.

It is also worth reviewing mailbox forwarding rules, especially automatic forwarding to external addresses. These rules are frequently abused after account compromise and can quietly leak sensitive information for weeks.

Reviewing data sharing and collaboration risks

Microsoft 365 is designed for collaboration, which is one of its strengths. It is also where security can become messy if governance has not kept pace with growth.

SharePoint, OneDrive and Teams settings

External sharing should be reviewed across SharePoint, OneDrive and Teams. The right level depends on your business model. A company working closely with clients and subcontractors may need broader sharing than a business with tightly controlled internal data. The key is to be deliberate. Open-by-default settings rarely stay proportionate for long.

You should also look at anonymous links, guest access, and default link permissions. If users can share files externally without expiry dates, approval workflows, or visibility over who still has access, data can spread far beyond its intended audience.

Sensitivity labels and data loss prevention can help, but only if they have been mapped sensibly to how the business classifies information. Overcomplicated labelling schemes tend to be ignored. Simple, well-explained controls usually get better adoption.

Retention, audit and visibility

A review should also examine what evidence you can rely on if something goes wrong. Audit logging, alerting, and retention policies are not glamorous topics, but they are critical when investigating suspicious behaviour or demonstrating compliance.

If logs are not enabled correctly, or retention periods are too short, you may lose the visibility needed to understand an incident. Equally, if alerting is generating too much noise, important warnings can be missed. This is an area where practical tuning makes a real difference.

Device and endpoint considerations

Microsoft 365 security is not confined to the cloud portal. The devices connecting to it matter just as much. A well-configured tenant can still be exposed if unmanaged or poorly secured devices have broad access.

Intune and device compliance policies should be reviewed to confirm whether laptops, mobiles, and tablets meet your baseline for encryption, patching, screen lock, and antivirus. The answer varies by organisation. A fully managed estate allows tighter controls. A business with bring-your-own-device arrangements may need a more flexible model. Either way, the risks should be understood rather than inherited by default.

Mobile application management is often overlooked. In businesses where staff read email and access documents on personal phones, app-level protection can help separate company data from personal use without creating unnecessary friction.

Common findings in a Microsoft 365 security review

Across small and mid-sized businesses, the same issues appear again and again. MFA is enabled for some users but not all. Old accounts remain active after leavers have gone. Admin rights are too widely distributed. Sharing settings have grown organically without oversight. Security features included in the licence are available but not fully configured.

Another common issue is assuming that buying a higher Microsoft licence means security is now handled. Better licensing can provide stronger tools, but tools alone do not reduce risk. They still need design, implementation, monitoring, and periodic review.

There is also the question of business fit. Some organisations have inherited policies from a previous IT provider or an internal project that was never finished. Others have copied settings from online recommendations that do not reflect their own users, locations, or compliance needs. Security should be aligned to the business, not bolted on as a generic template.

How often should you review Microsoft 365 security?

For most organisations, an annual review is the minimum. In practice, more frequent checks make sense after major changes such as mergers, migrations, office moves, leadership changes, or a shift in remote working patterns.

You should also trigger a review if there are warning signs: repeated phishing incidents, uncertainty about who has admin access, a lack of confidence in your sharing controls, or concern that the environment has simply evolved without proper oversight. Those are usually signs that risk has drifted ahead of governance.

A good review does not need to create disruption. In many cases, the value comes from clarifying what is already in place, identifying where controls are inconsistent, and prioritising improvements in a sensible order. That gives decision-makers a clearer view of risk without turning the exercise into a large transformation programme.

Turning findings into action

The most useful outcome is not a long technical report that sits unread. It is a prioritised plan. High-risk items should be addressed first, especially around identity, privileged access, and phishing exposure. After that, attention can shift to governance, data protection, and longer-term improvements.

This is where an experienced managed services partner can add value. Independent review matters, but so does practical follow-through. If your internal team is already stretched, having a safe pair of hands to assess the tenant, explain the findings in plain English, and implement changes with minimal disruption can save time and reduce the chance of gaps being missed.

For many businesses, Microsoft 365 is now too central to leave on autopilot. The right security review gives you more than a technical health check. It gives you confidence that the platform supporting your people, data, and operations is working as safely as it should - and that your security settings still match the business you are running today.