Microsoft 365 gives businesses access to email, files, collaboration tools and cloud applications from almost anywhere. That flexibility is valuable, but it also makes user accounts attractive targets. If an attacker gains access to one account, they may be able to read email, impersonate an employee, create forwarding rules, reach shared documents or use the account to target customers and suppliers.

The good news is that reducing account compromise risk does not always require a large security project. A small number of well-planned controls can remove common weaknesses and make suspicious activity easier to identify. This guide explains practical steps that growing businesses can take to strengthen Microsoft 365 account security.

Why Microsoft 365 accounts are targeted

Microsoft 365 accounts often contain far more than a mailbox. Depending on the licences and services in use, the same identity may provide access to Teams, SharePoint, OneDrive, business applications and administrative portals. An attacker who steals a password may therefore gain access to a broad set of information and services.

Compromise does not always begin with a highly technical attack. Common routes include convincing phishing messages, reused passwords exposed by another service, fraudulent multifactor authentication prompts, malicious consent requests and older sign-in methods that do not support modern security controls.

Security should therefore be built around identity, not only around devices. The aim is to make stolen credentials less useful, restrict the actions available to an attacker and create clear evidence when sign-in behaviour is unusual.

1. Require multifactor authentication for every user

Multifactor authentication, usually shortened to MFA, requires an additional verification method as well as a password. Microsoft describes MFA as an important way to improve account security because a stolen password alone is no longer sufficient to complete a normal sign-in.

Every active user should be covered, including administrators, shared operational accounts and people who rarely sign in. Leaving a small number of accounts outside the policy can create an easy route around the protection.

Businesses using Microsoft Entra ID Free can use security defaults to apply a useful baseline. Organisations with Microsoft Entra ID P1 or P2 can use Conditional Access for more detailed control. Legacy per-user MFA is not the preferred modern approach and should not be confused with security defaults or Conditional Access.

2. Move towards phishing-resistant sign-in methods

Traditional MFA is much stronger than password-only authentication, but not every verification method offers the same protection. SMS codes, one-time codes and simple approval prompts can still be captured or manipulated by convincing phishing campaigns.

Microsoft recommends phishing-resistant methods such as passkeys and FIDO2 security keys, Windows Hello for Business and certificate-based authentication. These methods use cryptographic checks that are tied to the legitimate service, making it much harder for a fraudulent sign-in page to capture something reusable.

A practical rollout can begin with administrators and other high-impact roles before extending to the wider workforce. Users need suitable devices, clear enrolment instructions and a tested recovery process. Stronger authentication should be introduced carefully so that the business does not lock itself out.

3. Protect administrator accounts separately

Administrator accounts can change security settings, create users, reset authentication methods and access sensitive services. They should receive stronger protection than ordinary day-to-day accounts.

• Use a separate account for administrative work rather than giving the everyday mailbox permanent administrator rights.
• Assign only the roles needed for the task instead of using Global Administrator by default.
• Require phishing-resistant MFA for privileged roles where licensing and operational requirements allow it.
• Maintain emergency access accounts and test the documented recovery procedure.
• Review privileged role assignments regularly and remove access that is no longer required.

Microsoft recommends limiting highly privileged roles and using accounts with the least permissions necessary. Reducing permanent privilege lowers the impact if a normal user account is compromised.

4. Block legacy authentication

Older authentication protocols were designed before modern MFA and Conditional Access controls. If they remain available, an attacker may try to use them with a stolen password to avoid protections applied to modern sign-ins.

Microsoft recommends blocking legacy authentication. Businesses with Conditional Access can create a policy for this purpose, while organisations using security defaults receive protection against legacy authentication as part of that baseline.

Before enforcing a block, review sign-in logs and identify any older applications, multifunction printers or email workflows that still depend on legacy protocols. Replace or reconfigure those services rather than leaving a permanent exception that weakens the whole environment.

5. Use Conditional Access or security defaults properly

Security defaults provide a sensible starting point for smaller organisations that do not need complex exceptions. Conditional Access is more flexible and can consider factors such as the user, role, application, device state, location and sign-in risk.

Conditional Access policies should be planned, documented and tested in report-only mode before enforcement. Common baseline policies include requiring MFA for users and administrators, blocking legacy authentication and protecting access to administrative services.

Avoid creating a collection of overlapping rules without clear ownership. Each exclusion should have a documented reason, an owner and a review date. At least one tested emergency access route should remain available in case a configuration mistake blocks normal administrators.

6. Review mailbox forwarding and inbox rules

Attackers sometimes create forwarding rules so that they continue receiving messages even after the user changes a password. They may also create inbox rules that hide replies, security warnings or conversations with suppliers.

Review automatic forwarding settings, transport rules and unusual inbox rules. External forwarding should be disabled unless there is a genuine business requirement. Where it is permitted, record who approved it and review it regularly.

After a suspected compromise, changing the password is not enough. Administrators should revoke active sessions, check authentication methods, review registered devices and applications, inspect forwarding and inbox rules, and determine what information may have been accessed.

7. Strengthen email authentication and anti-phishing controls

SPF, DKIM and DMARC help receiving mail systems assess whether messages using your domain are legitimate. They reduce opportunities for attackers to impersonate the organisation’s domain, although they do not stop every lookalike-domain or account-compromise scenario.

Configure these records carefully, monitor DMARC reports and move towards an enforcement policy when legitimate senders have been identified. Microsoft 365 anti-phishing policies and Defender for Office 365 can add spoof intelligence, impersonation protection and additional detection controls depending on the licences in use.

Email authentication should be paired with staff awareness. Employees should know how to check unexpected payment requests, password-reset messages, document-sharing invitations and changes to supplier bank details using a separate trusted channel.

8. Monitor sign-ins and act on meaningful alerts

Security controls are most useful when somebody reviews the evidence they produce. Microsoft Entra sign-in logs can reveal unfamiliar locations, impossible travel patterns, repeated failures, legacy authentication attempts and unexpected applications.

Define who reviews alerts, how quickly they are assessed and what happens when compromise is suspected. High-risk events should not wait for an informal check at the end of the month. The response process should include containment, investigation, recovery and communication.

For smaller businesses, a short documented checklist is more valuable than a complex incident plan nobody can follow. Record the Microsoft 365 tenant details, administrator contacts, support provider, cyber insurance contact and the steps required to revoke sessions and secure an affected account.

9. Keep account ownership and access current

Joiner, mover and leaver processes are an important security control. New users should receive only the access they need. Role changes should trigger an access review, and departing users should be disabled promptly with data ownership transferred appropriately.

Review inactive accounts, guest users, shared mailboxes, application permissions and group memberships. Old accounts and forgotten external access create risk because they attract less attention than active users.

Where possible, keep an accurate inventory of users, licences, devices, privileged roles and business applications. Clear ownership makes regular reviews faster and reduces uncertainty during an incident.

10. Make recovery part of the security plan

Preventive controls reduce risk, but no organisation should assume that compromise is impossible. Recovery procedures should explain how to verify a user’s identity, reset credentials, revoke sessions, replace authentication methods, inspect mailbox changes and restore normal access safely.

Microsoft 365 retention and recovery features are useful, but they are not a complete substitute for an independently managed backup strategy. Decide which Microsoft 365 data must be protected, how long it should be retained and how recovery will be tested.

BitFlux can help with backup and business continuity planning alongside Microsoft 365 security and administration.

A practical Microsoft 365 security checklist

• Require MFA for every active user.
• Introduce phishing-resistant methods, beginning with administrators.
• Separate everyday and privileged administrator accounts.
• Block legacy authentication after checking dependencies.
• Use security defaults or well-designed Conditional Access policies.
• Disable unnecessary external forwarding and review inbox rules.
• Configure SPF, DKIM and DMARC for business domains.
• Review sign-in activity, alerts and risky authentication methods.
• Remove obsolete accounts, access rights and application permissions.
• Document and test the account-compromise response process.

Start with the controls that reduce the most risk

Microsoft 365 security does not improve simply because more settings are enabled. Controls must match the organisation’s licences, devices, applications and working practices. Begin with identity protection, administrator security and legacy authentication, then improve monitoring, email protection and recovery.

BitFlux Ltd provides Microsoft cloud services, cybersecurity support and managed IT support for businesses across Watford and London, with remote support available worldwide.

If you would like an independent review of your Microsoft 365 security settings, contact BitFlux. We normally respond within one business day.

Further reading from Microsoft

• Set up multifactor authentication for Microsoft 365
• Block legacy authentication with Conditional Access
• Microsoft Entra authentication methods overview
• Set up DMARC for Microsoft 365