Microsoft 365 Security & Incident Response: Mapping Cloud Attacks with MITRE ATT&CK® for CISOs

In today’s cloud-centric landscape, Microsoft 365 stands as the productivity cornerstone for countless organizations. While its extensive capabilities empower modern workplaces, they also present a broad and attractive attack surface for malicious actors. A single compromised Microsoft 365 account can swiftly escalate into a significant security incident, leading to data breaches, financial fraud, and widespread operational disruption.

For security professionals, a deep understanding of the anatomy of these attacks is crucial for building a resilient defense. This is where the MITRE ATT&CK® framework becomes an indispensable ally. The framework provides a globally accessible knowledge base of adversary tactics and techniques derived from real-world observations, creating a common language for cybersecurity professionals to dissect and neutralize threats.[1]

By mapping the common tactics and techniques used in Microsoft 365 account compromises to this framework, organizations can gain a clearer understanding of the threats they face, identify defensive gaps, and align their security controls to counter specific adversary behaviors.[2][3]

This blog post will explore how prevalent attack techniques in a compromised Microsoft 365 environment map to the MITRE ATT&CK framework and, crucially, provide actionable steps to mitigate each threat.

The Initial Breach: Gaining a Foothold (Tactic: Initial Access)

An attacker’s first objective is to breach the perimeter and gain initial access to a user’s account. In the Microsoft 365 ecosystem, this is frequently accomplished through social engineering and the exploitation of weak authentication mechanisms.

Technique: Phishing (T1566)

Phishing remains a dominant initial access vector, where attackers use deceptive emails to trick users into revealing their credentials on fake login pages or opening malicious attachments.[4] This encompasses several sub-techniques:

T1566.001 – Spearphishing Attachment: Targeted emails with malicious attachments.
T1566.002 – Spearphishing Link: Emails containing links to malicious websites designed to harvest credentials.

Steps to Mitigate Phishing:

Enable Multi-Factor Authentication (MFA): MFA is one of the most effective controls to prevent account compromise, adding a critical layer of security beyond just a password.[5] It is highly recommended to use phishing-resistant MFA methods like FIDO2 security keys.[6]
Implement Advanced Email Security: Leverage Microsoft Defender for Office 365 (formerly Advanced Threat Protection) to utilize features like Safe Links and Safe Attachments.[7] These tools scan links and attachments in real-time to block malicious content.[7]
Configure Anti-Phishing Policies: Within Microsoft 365, create and customize anti-phishing policies to detect and block spoofing and impersonation attempts.[8][9]
Deploy Email Authentication Protocols: Configure Sender Policy Framework (SPF), DomainKeys Identified Mail (DKIM), and Domain-based Message Authentication, Reporting, and Conformance (DMARC) to validate email senders and prevent spoofing.[10][11]
User Education: Conduct regular security awareness training and phishing simulations to teach employees how to recognize and report suspicious emails.[5][12]

Technique: Brute Force (T1110)

Attackers may attempt to guess passwords through various brute-force methods, especially against accounts not protected by MFA.

T1110.003 – Password Spraying: A common and effective technique where an attacker uses a single, commonly used password against many accounts.

Steps to Mitigate Brute Force Attacks:

Enforce Strong Password Policies: Require complex passwords with a minimum length and prohibit common or easily guessable passwords.[13]
Implement Account Lockout Policies: Use features like Microsoft Entra ID’s Smart Lockout, which locks an account for a period after a set number of failed sign-in attempts.[14]
Block Legacy Authentication: Legacy protocols like POP3, IMAP, and SMTP often do not support modern authentication methods like MFA, making them a prime target.[15] Use Conditional Access policies to block these older protocols.[13][15]
Monitor for Suspicious Logins: Utilize security tools to monitor for unusual sign-in activity, such as attempts from unfamiliar locations or a high number of failed logins, and configure alerts for such events.[5][16]

Expanding and Exploiting: The Post-Compromise Playbook

Once inside, an attacker’s focus shifts to escalating privileges, moving laterally, exfiltrating data, and using the compromised account for further malicious acts.

Technique: Business Email Compromise (BEC) (T1534 – Internal Spearphishing)

A primary goal for attackers is often financial fraud through Business Email Compromise (BEC). After gaining mailbox access, they can monitor communications to understand business operations and then send fraudulent emails to redirect payments or exfiltrate sensitive data.[5]

Steps to Mitigate BEC:

Enable MFA: As with initial access, MFA is a critical control that can prevent the account takeover necessary for most BEC attacks.[11]
Configure Impersonation Protection: Use Microsoft Defender for Office 365 to set up anti-phishing policies that specifically protect high-value users (like executives) and key external domains from impersonation.[17]
Establish Payment Verification Processes: Implement strict, out-of-band procedures for verifying any requests for changes in payment details or for high-value transactions. This might involve a phone call to a known contact.[11]
Monitor for Suspicious Email Forwarding: Attackers often set up auto-forwarding rules to monitor communications discreetly. Configure alert policies to notify security teams of such activity.[17]
Train Employees: Educate employees, especially those in finance and HR, on BEC tactics and the importance of adhering to payment verification procedures.[10]

Technique: Illicit Consent Grants (T1528 – Steal Application Access Token)

A more sophisticated technique is OAuth consent phishing. Attackers trick users into granting a malicious third-party application access to their Microsoft 365 data. This bypasses the need to steal credentials, as the attacker gains access via a legitimate token granted by the user.[18][19]

Steps to Mitigate Illicit Consent Grants:

Configure Application Consent Policies: In the Microsoft Entra admin center, configure user consent settings to restrict which applications users can authorize.[20] Options include disallowing user consent entirely or only allowing consent for apps from verified publishers.[19]
Educate Users: Train users to scrutinize the permissions requested by applications during the consent process and to be wary of unexpected requests.[20][21]
Leverage Microsoft Defender for Cloud Apps: Use Defender for Cloud Apps to monitor OAuth apps connected to your environment. It can help detect and alert on malicious or risky applications.[19]
Regularly Audit OAuth Applications: Periodically review the permissions of applications that have been granted access to your environment and revoke any that are unnecessary or suspicious.[20]

Technique: Email Hiding Rules (T1564.008 – Email Hiding Rules)

To maintain persistence and evade detection, attackers often create inbox rules within a compromised mailbox. These rules can automatically delete incoming security alerts, forward sensitive emails to an external account, or move messages to obscure folders like “RSS Feeds.”[22][23]

Steps to Mitigate Malicious Inbox Rules:

Regularly Audit Mailbox Rules: Both administrators and users should periodically check for any unfamiliar or suspicious inbox rules in Outlook on the web and the Exchange admin center.[22]
Configure Alerts for Suspicious Rules: Use security solutions to create alerts that trigger when a new forwarding rule to an external domain is created or when a rule is configured with keywords often associated with malicious activity (e.g., “invoice,” “password”).[24]
Disable or Remove Malicious Rules: If a malicious rule is found, it should be immediately disabled or deleted.[25][26]
Investigate the Root Cause: The discovery of a malicious rule indicates an account compromise. Immediately reset the user’s password, revoke their active sessions, and investigate the initial point of entry.[24][25]

Conclusion: Building a Threat-Informed Defense

The threat to Microsoft 365 environments is persistent and multifaceted. By mapping attacker behaviors to the MITRE ATT&CK® framework, security teams can transition from a reactive posture to a proactive, threat-informed defense.[27] This structured approach enables organizations to:

Identify and Prioritize Defensive Gaps: Compare existing security controls against the specific techniques used by adversaries to pinpoint vulnerabilities.
Enhance Threat Hunting: Use the framework to develop hypotheses about potential attacker activity and proactively search for indicators of compromise.
Streamline Incident Response: During an incident, the framework provides a clear model to understand an attacker’s actions, ensuring a comprehensive and effective response.[28]

Microsoft’s own security suite, including Microsoft Sentinel and Microsoft Defender XDR, is increasingly aligned with the MITRE ATT&CK framework, providing powerful tools to help organizations operationalize this intelligence.[1] By combining these advanced tools with a foundational understanding of attacker TTPs, organizations can build a more resilient and effective defense for their critical cloud productivity platform.