US Government Security Agency Warns of Office 365 Security Misconfigurations

On Monday, 5/13/2019, the Cybersecurity and Infrastructure Security Agency (CISA) warned Office 365 users and their technology partners about common Office 365 misconfigurations.

Note: CISA is the new standalone agency within the Department of Homeland Security that functions as the lead national government unit on civilian cybersecurity.

A CISA analysis report titled “Microsoft Office 365 Security Observations,” described four common security misconfigurations found during a multi-month investigation that began last fall. The investigation focused on customers who have used third-party partners to migrate their e-mail services to Office 365.

CISA analysis report conclusion: “The organizations that used a third party have had a mix of configurations that lowered their overall security posture (e.g., mailbox auditing disabled, unified audit log disabled, multi-factor authentication disabled on admin accounts),” the report said. “In addition, the majority of these organizations did not have a dedicated IT security team to focus on their security in the cloud. These security oversights have led to user and mailbox compromises and vulnerabilities.”

CISA found five Office 365 configuration problems, with some of them exposing administrator username/password prompts to attack without multi-factor authentication (MFA) protections in place, others involving audit logs being left off, and another allowing attackers who had compromised on-premises accounts to move laterally into the cloud.

The main MFA problem involved organizations that didn’t set up MFA for the Azure Active Directory (AD) Global Administrators in an Office 365 environment. Microsoft does not require MFA by default in creating the accounts, and many organizations don’t change the setting. The report notes that the Azure AD Global Administrator accounts are the first ones created and are required to configure the tenant and migrate users. “These accounts are exposed to internet access because they are based in the cloud. If not immediately secured, these cloud-based accounts could allow an attacker to maintain persistence as a customer migrates to O365,” the report warned.

A related problem flagged by CISA involves legacy protocols that don’t support MFA. Those include POP3, IMAP and SMTP. While the report acknowledges that users with older e-mail clients may need these less secure protocols, efforts should be made to limit their use to specific users and to wean the organization off of those protocols as quickly as possible.

Of all the problems highlighted in the report, CISA stressed enabling MFA as a best practice: “This is the best mitigation technique to use to protect against credential theft for O365 users.”

Auditing is another commonly discussed problem in Office 365 security circles. Mailbox auditing was disabled by default prior to January 2019, meaning organizations trying to investigate potential breaches often discovered they had no logs to look at if they hadn’t enabled the feature. CISA urged organizations whose Office 365 configuration was set up prior to January of this year to ensure that mailbox auditing is enabled.

The analysis report also pointed to another logging feature which is still disabled by default — the unified audit log. That log records events from several Office 365 services, including Exchange Online, SharePoint Online, OneDrive, Azure AD, Microsoft Teams and Power BI. Administrators can enable the unified audit log in the Security and Compliance Center.

Another configuration choice that can lead to security problems involves password sync using Azure AD Connect, the report states. A useful migration tool designed to create Azure AD identities from on-premises identities or to match previously created Azure AD identities with on-premises AD identities, Azure AD Connect can cause security problems in certain cases.

CISA recommends that administrators implement the following mitigations and best practices:

  • Use multi-factor authentication. This is the best mitigation technique to use to protect against credential theft for O365 users.
  • Enable unified audit logging in the Security and Compliance Center.
  • Enable mailbox auditing for each user.
  • Ensure Azure AD password sync is planned for and configured correctly, prior to migrating users.
  • Disable legacy email protocols, if not required, or limit their use to specific users.

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