The security of corporate email accounts has become one of the main targets for cybercriminals. In Microsoft 365 environments, just a single compromised credential can lead to the theft of confidential information, financial fraud, lateral access to other cloud services, and the exfiltration of SharePoint and OneDrive documents.
One of the biggest problems is that many modern attacks don't cause obvious symptoms. The attacker doesn't need to break the system: it's enough to gain legitimate access through phishing, session cookie theft, infostealer-type malware, or MFA fatigue attacks.
How to commit a Microsoft 365 account
Main techniques for attacking corporate accounts
The main methods include classic phishing, session token theft, password spraying, and malicious OAuth consent.
Session token theft is especially dangerous because it lets the attacker get in even when MFA is turned on.
Check suspicious sign-ins in Microsoft 365
Microsoft Entra ID allows you to review the Sign-in Logs from https://entra.microsoft.com.
The most important indicators to check are:
- Anomalous countries
- Suspicious User Agents
- Impossible Travel
- Use of legacy protocols like IMAP and POP3
How to analyze Microsoft 365 audit logs
Critical Events in Microsoft Purview
At Microsoft Purview Compliance Portal it's possible to review critical events such as:
- New-InboxRule
- Set-InboxRule
- MailItemsAccessed
- Consent to application
These events allow you to identify persistence, mass reading of emails, and suspicious OAuth apps.
Detect malicious rules in Outlook
Review of suspicious mail rules
Attackers usually create rules to hide financial emails, delete security alerts, or forward messages to external accounts.
The rules can be reviewed from Outlook Web or via PowerShell:
Get-InboxRule -Mailbox user@company.com
How to spot suspicious OAuth apps
OAuth apps can keep persistent access to the tenant without needing to know the user's password.
It should be checked:
- unknown apps
- excessive permissions
- apps without a verified publisher
Check registered devices in Microsoft 365
Corporate devices and unauthorized access
Registered devices can show persistence through unknown phones or equipment.
It's recommended to check:
- Unknown Android/iPhone
- old devices
- Non-corporate Windows
Signs of suspicious activity in Outlook and Microsoft 365
Some common IOCs include:
- mass reading of emails
- creation of hidden folders
- MFA changes
- multiple refresh tokens
- suspicious external forwarding
How to respond to a compromised account in Microsoft 365
Steps for responding to security incidents
Recommended steps:
- Revoke active sessions
- Reset password
- Check MFA
- Remove malicious rules
- Revoke OAuth permissions
- Check external forwarding
PowerShell Example:
Revoke-AzureADUserAllRefreshToken -ObjectId user@company.com
Hardening and monitoring recommendations
- Disable legacy authentication
- Activate Conditional Access
- Enable Defender for Office 365
- Enable audit logs
- Integrate SIEM like Microsoft Sentinel or Splunk
How to protect Microsoft 365 corporate accounts
Most attacks in Microsoft 365 aim for silent persistence and access to corporate email. A modern security strategy should combine strong MFA, continuous monitoring, getting rid of legacy protocols, and log analysis.
ABD Consulting: Experts in Microsoft 365 Security and Protection
At ABD Consulting and IT Solutions We help companies protect their Microsoft 365 environments by spotting potential vulnerabilities and strengthening the security of their corporate accounts.
Our services include:
- Microsoft 365 and Microsoft Entra ID security audits.
- Review of multi-factor authentication (MFA) settings.
- Analysis of activity logs and suspicious logins.
- Implementation of conditional access policies (Conditional Access).
- Advanced email protection with Microsoft Defender for Office 365.
- Monitoring and responding to security incidents.
- Implementation of SIEM solutions like Microsoft Sentinel for proactive threat detection.
The reality is that many current attacks aren’t aimed at blocking systems, but rather at quietly accessing email accounts, documents, and corporate data. That’s why having a strategy for continuous monitoring and a proper Microsoft 365 setup is key to reducing risks and ensuring business continuity.
If you want to know the security status of your Microsoft 365 environment or need help implementing advanced protection measures, At ABD Consulting we can help you evaluate, protect, and monitor your cloud infrastructure.
Contact our team and find out how to boost Microsoft 365 security before a threat becomes a problem for your organization.