Saturday, 23 January 2021 17:56

FireEye: Remediation and Hardening Strategies for Microsoft 365 to Defend Against UNC2452

By Mike Burns, Matthew McWhirt, Douglas Bienstock, Nick Bennett

GUEST RESEARCH: In December 2020, FireEye uncovered and publicly disclosed a widespread attacker campaign that is being tracked as UNC2452. In some, but not all, of the intrusions associated with this campaign where Mandiant has visibility, the attacker used their access to on-premises networks to gain unauthorized access to the victim’s Microsoft 365 environment.

Goals and Objectives

Methodologies that UNC2452 and other threat actors have used to move laterally from on-premises networks to the Microsoft 365 cloud have been detailed in our white paper, Remediation and Hardening Strategies for Microsoft 365 to Defend Against UNC2452. The paper also discusses how organisations can proactively harden their environments and remediate environments where similar techniques have been observed.

Mandiant is releasing an auditing script, Azure AD Investigator, through its GitHub repository that organizations can use to check their Microsoft 365 tenants for indicators of some of the techniques used by UNC2452. The script will alert administrators and security practitioners to artifacts that may require further review to determine if they are truly malicious or part of legitimate activity. Many of the attacker techniques detailed in the white paper are dual-use in nature—they can be used by threat actors but also by legitimate tools. Therefore, a detailed review for specific configuration parameters may be warranted, including correlating and verifying that configurations are aligned with authorized and expected activities.

Attacker Tactics, Techniques and Procedures (TTPs)

Mandiant has observed UNC2452 and other threat actors moving laterally to the Microsoft 365 cloud using a combination of four primary techniques:

1. Steal the Active Directory Federation Services (AD FS) token-signing certificate and use it to forge tokens for arbitrary users (sometimes described as Azure AD). This would allow the attacker to authenticate into a federated resource provider (such as Microsoft 365) as any user, without the need for that user’s password or their corresponding multi-factor authentication (MFA) mechanism.

2. Modify or add trusted domains in Azure AD to add a new federated Identity Provider (IdP) that the attacker controls. This would allow the attacker to forge tokens for arbitrary users and has been described as an Azure AD backdoor.

3. Compromise the credentials of on-premises user accounts that are synchronized to Microsoft 365 that have high privileged directory roles, such as Global Administrator or Application Administrator.

4. Backdoor an existing Microsoft 365 application by adding a new application or service principal credential in order to use the legitimate permissions assigned to the application, such as the ability to read email, send email as an arbitrary user, access user calendars, etc.

Read the white paper for a detailed overview of each technique, including practical remediation and hardening strategies, and check out our auditing script, Azure AD Investigator

More information on six FireEye Helix Detections, the MITRE Technique and the Detection Logic here.


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