I work for an MSP supporting a college that is dealing with recurring Microsoft 365 account compromises. The usual pattern is that a student receives a phishing email, follows a link to a third-party form or fake login page, and enters their Microsoft 365 credentials. The attacker then signs in and may send more phishing messages to other students. We also see cases involving several failed sign-ins followed by a successful one, but Entra ID does not consistently classify the activity as risky, so risk-based Conditional Access policies do not always activate.
The college is strengthening Conditional Access and moving toward a Zero Trust approach. Organization-owned Windows devices are being enrolled in Intune so compliant-device policies can eventually be enforced. However, students also use personal phones, and the college does not want to fully enroll or manage those devices.
We are considering Defender for Office 365 P2 and Safe Links, Intune Mobile Application Management and App Protection Policies for BYOD, Conditional Access requiring protected mobile apps, stronger MFA, compliant-device requirements for school-owned devices, and possibly an MDR or SOC provider for after-hours response.
The main concern is what happens when a student clicks a phishing link outside business hours. For organizations supporting higher education or another large BYOD population, which combination of controls has worked best? Would you prioritize Safe Links and MAM, a third-party email security platform, phishing-resistant MFA or passkeys, an MDR provider, or several of these together? I am especially interested in protecting personal devices without full Intune enrollment and limiting the impact after someone submits credentials to a phishing site.
5 Answers
For BYOD phones, use App Protection Policies with Conditional Access requiring an approved, protected application rather than requiring full device enrollment. That helps protect college data without managing the entire personal phone. For school-owned devices, enforce compliance through Intune. Also make sure password resets revoke refresh tokens and active sessions; otherwise a stolen session can remain useful after the password has changed. Monitor and respond to inbox rules, forwarding, and malicious OAuth consent, since attackers often establish persistence there.
Before selecting more products, separate the different attack paths. Failed sign-ins followed by success may indicate password spraying or credential stuffing, while stolen-session attacks may not produce a useful risky-sign-in signal at all. Use phishing-resistant MFA to address credential theft, block legacy protocols, apply app protection to BYOD, and build automated containment around session revocation, forwarding-rule creation, OAuth consent, and unusual outbound mail. An MDR provider is most valuable when it can perform those containment steps rather than simply report them.
Make compromised accounts self-limiting, especially after hours. Set a reasonable outbound-mail threshold for student accounts and automatically suspend or restrict accounts that suddenly send dozens of messages. Add detection for suspicious forwarding and inbox rules, and review unusual links or external services that appear repeatedly in phishing campaigns. This will not prevent the initial compromise, but it can stop one account from becoming a campus-wide mail relay while nobody is watching.
That makes sense. I am particularly interested in controls that can automatically contain the account, revoke sessions, and remove persistence without requiring someone to be online immediately.
The most important change is phishing-resistant authentication. This pattern is consistent with password harvesting or adversary-in-the-middle attacks, where the sign-in can look legitimate to Entra even though the credentials were stolen. Passkeys or FIDO2 security keys are much stronger than ordinary MFA because a harvested password and one-time code cannot simply be replayed. Block legacy authentication and require strong authentication across the student population. Safe Links and automatic remediation are still useful for catching messages and reducing outbound spam, but they should be treated as layers rather than the main fix.
Safe Links, external-mail warnings, and targeted user training are worthwhile defense-in-depth measures. Training and simulated phishing can reduce susceptibility, but they should not be the primary control because users will eventually make mistakes at scale. A third-party MDR or ITDR service can help if it is authorized to take action, such as disabling the account, revoking sessions, removing malicious rules or OAuth grants, and stopping outbound mail. A service that only sends alerts will not solve the after-hours problem.
Outbound limits are a useful overnight control, but they need to be tuned by account type. Student limits can be much lower than faculty or administrative limits, and exceptions should be handled carefully so a legitimate mailing or reply-all incident does not create constant helpdesk work.