A critical security flaw has been identified in Microsoft Bookings, a scheduling tool integrated within Microsoft 365, which could allow attackers to create unauthorized Entra (formerly Azure Active Directory) accounts and impersonate legitimate users. This vulnerability poses significant risks, including sophisticated phishing attacks and unauthorized access to sensitive information.
Understanding the Vulnerability
The issue originates from the Shared Booking Pages feature in Microsoft Bookings, which is enabled by default for users with appropriate Microsoft 365 licenses. When a user creates a shared booking page, the system automatically generates a corresponding account in Entra without requiring administrative permissions. This process includes:
– Display Name: Matches the name of the booking page.
– Email Address: Formed by removing spaces from the booking page name (e.g., a page named John Doe results in the email address [email protected]).
– Mailbox Functionality: The generated account can send and receive emails both internally and externally, regardless of sharing settings.
This automatic account creation can be exploited by attackers who have compromised a Microsoft 365 user account. By creating a shared booking page, they can:
1. Impersonate Legitimate Users: Craft email addresses that closely resemble those of existing employees, enabling convincing phishing attacks.
2. Hijack Former Employee Accounts: Create booking pages with email addresses matching those of former employees, allowing them to intercept communications and reset passwords for external services.
3. Establish Hidden Mailboxes: Set up fully functional mailboxes that do not consume Microsoft 365 licenses, making detection and monitoring challenging.
Potential Impacts
The exploitation of this vulnerability can lead to several severe consequences:
– Sophisticated Phishing Attacks: Attackers can impersonate high-profile individuals within an organization, such as executives or IT administrators, to deceive employees into divulging sensitive information or authorizing financial transactions.
– Unauthorized Access to External Services: By hijacking email addresses of former employees, attackers can reset passwords and gain access to external services linked to those emails, including cloud storage, financial platforms, and more.
– Compromise of Sensitive Communications: The ability to send and receive emails from these unauthorized accounts allows attackers to intercept confidential communications, potentially leading to data breaches.
Mitigation Strategies
To protect against the risks associated with this vulnerability, organizations should implement the following measures:
1. Audit Existing Shared Booking Pages: Use ExchangeOnline PowerShell to identify and review all shared booking pages within the organization.
2. Restrict Creation of Shared Booking Pages: Disable the ability for end users to create shared booking pages unless absolutely necessary. This can be achieved by setting the `BookingsEnabled` parameter to `false` using PowerShell.
3. Monitor Entra Account Activity: Regularly review Entra accounts for unusual creation activity, focusing on accounts that do not correspond to legitimate users.
4. Review Mailbox Permissions: Regularly assess and revoke unnecessary mailbox permissions to minimize the risk of unauthorized access.
5. Implement Strong Input Validation: Ensure that all web applications, including Microsoft Bookings, have robust input validation to prevent HTML injection and other forms of input manipulation.
6. Educate Employees: Conduct regular training sessions to raise awareness about phishing attacks and the importance of verifying the authenticity of emails, especially those requesting sensitive information or financial transactions.
Conclusion
The discovery of this vulnerability in Microsoft Bookings underscores the importance of vigilant security practices within organizations. By understanding the potential risks and implementing proactive measures, businesses can safeguard their systems against unauthorized access and protect sensitive information from malicious actors.
