How Secure Is Microsoft 365 for Businesses? 5 Security Tips
Asking how secure Microsoft 365 is tends to produce reassuring answers. Microsoft invests heavily in the platform, and the built-in protections are substantial. But that confidence can mask a real problem. Security in the cloud operates under a shared-responsibility model. Microsoft protects the infrastructure. Your organisation is responsible for its configuration and management.
The gap between those two things is where most incidents begin. A member of staff clicks a link in a routine-looking email, credentials are compromised, and phishing protection was never activated. This is not unusual, and it rarely stems from a failure in the platform itself. Misconfigured settings and unmanaged permissions are among the most common causes of Microsoft 365 security incidents.
This article explains where those risks arise and provides five steps to address them.
What Microsoft 365 Protects & What It Does Not
Microsoft 365 includes a solid set of built-in protections. Every subscription covers encryption for data in transit and at rest, identity and access management, and threat detection through Microsoft Defender. The platform is designed and maintained to a high standard, and Microsoft is responsible for keeping that infrastructure secure.
What the platform does not control is how your organisation uses it. National Cyber Security Centre's (NCSC) cloud security principles are clear on this point: cloud services should be secure by design and by default, but customers remain responsible for managing their own users, access controls, and configuration. Principle 9 states that providers should make tools available for secure user management, but the responsibility for implementing those controls lies with you [1].
Most Microsoft 365 security problems do not originate in the platform itself. They come from misconfigured settings, unmanaged user permissions, and security features that were never switched on.
For a broader look at why cyber security matters for smaller organisations, our guide to Why Cyber Security is Important for Small Businesses sets out the most common threats and their business impact.
Security Gaps Most Microsoft 365 Users Never Think to Check
Several security risks appear consistently across businesses of all sizes, and none is the result of a technical failure in the platform. They come from settings that were never configured, permissions that quietly expanded, and controls that were simply never switched on:
- Multi-factor authentication is disabled or not enforced across all user accounts.
- Users have more access than their roles require, or accounts are shared among people.
- Phishing protection has not been configured beyond the default state.
- SharePoint and OneDrive sharing permissions have expanded without regular review.
- There is no monitoring or alerting in place for suspicious account activity.
The five steps below cover each one directly.
1. Enable Multi-Factor Authentication
Multi-factor authentication (MFA) is one of the most effective controls available and one of the most commonly left switched off. It requires users to verify their identity through a second method before gaining access.
A Microsoft research study into commercial accounts found that MFA reduced the risk of compromise by 99.22% across the full population studied, with over 99.99% of MFA-enabled accounts remaining secure throughout. Even where credentials had already been leaked, MFA blocked 98.56% of attacks [2].
It should be enforced across every user account, with particular priority given to administrators.
2. Use Microsoft Defender for Email Protection
Microsoft Defender includes email security features that go well beyond basic spam filtering. Safe Links checks URLs in emails and Office documents at the point of click, and Safe Attachments scans files before they reach the recipient. Both reduce the risk of phishing and malware reaching your users, which matters most in businesses handling a high volume of external correspondence.
3. Apply Conditional Access Policies
Conditional access allows you to set rules that govern when and how users can sign in. Access can be restricted based on location, device, or the assessed risk level of a sign-in attempt. An unfamiliar login from an unexpected location can be automatically blocked or challenged, without relying on a user to notice something is wrong.
4. Secure SharePoint and OneDrive Permissions
Sharing settings in SharePoint and OneDrive default to a level of openness that supports collaboration but poses risks if left unmanaged. Anyone with a shared link can access files without authentication. Reviewing external sharing permissions, removing redundant links, and limiting access to named individuals rather than broad groups are steps that reduce your exposure without disrupting how people work.
5. Set Up Data Loss Prevention Policies
Data loss prevention (DLP) policies define how sensitive information is handled within your Microsoft 365 environment. They detect when financial data, personal information, or confidential material is being shared outside the organisation and either block the action or trigger an alert.
For businesses with General Data Protection Regulation (GDPR) obligations, the Information Commissioner's Office's (ICO) guidance on data security requires appropriate technical measures to ensure the confidentiality, integrity, and availability of personal data, and that data can only be accessed or disclosed by those authorised to do so. DLP policies meet both requirements directly. The ICO also notes that measures in place at the time of a breach will be considered when determining any administrative fine.
Is Your Microsoft 365 Environment as Secure as You Think?
Configuring Microsoft 365 correctly at setup is a solid starting point, but it is not a permanent solution. Threats change, staff join and leave, and permissions quietly expand. The businesses most at risk are not those without Microsoft 365. They are the ones using it daily without realising their settings no longer reflect best practice.
Ongoing security means reviewing access controls, monitoring for unusual activity, and keeping pace with new vulnerabilities. Our experts cover all of this:
- Regular access reviews keep user permissions up to date and appropriate.
- Proactive monitoring catches unusual activity before it causes disruption.
- Periodic security audits ensure your configuration stays aligned with best practices.
Treken is a Microsoft Partner providing IT support and managed services to SMEs across Dorset, Hampshire, and Wiltshire. From initial Microsoft 365 setup and configuration to ongoing monitoring and security reviews, the team ensures your environment is properly configured and stays that way. A complete security posture also means having a reliable recovery plan in place. Our Backups & Disaster Recovery service ensures your data can be restored quickly if something does go wrong.
Call 01202 612333 or arrange a consultation to review your current Microsoft 365 setup.
