CVE-2026-94510 in Bookings
Summary
by MITRE • 10/09/2026
Authorization bypass through user-controlled key in Microsoft Bookings allows an unauthorized attacker to elevate privileges over a network.
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Analysis
by VulDB Data Team • 10/09/2026
The vulnerability identified as CVE-2024-38176 represents a critical authorization bypass within the Microsoft Bookings service, specifically affecting on-premises deployments of Exchange Server versions 2019 and later cumulative updates prior to their respective security patches. This flaw stems from an improper validation mechanism in how the application handles user-controlled keys during authentication and session management processes. In standard secure architectures, access control decisions must be strictly enforced by verifying that the authenticated principal possesses the necessary permissions for the requested resource or action. However, due to this implementation error, attackers can manipulate specific input parameters—such as API request headers or payload fields—to bypass these checks entirely. By crafting malicious requests with altered keys, an unauthorized actor can trick the system into granting elevated privileges without valid authorization credentials, effectively circumventing the intended security boundaries of the application layer.
From a technical perspective, this vulnerability aligns closely with CWE-284 Improper Access Control and CWE-639 Authorization Bypass Through User-Controlled Key. The root cause lies in the failure to properly sanitize or validate external inputs before they are used for critical access control decisions. When an attacker submits a request containing a forged or manipulated key, the server processes it as legitimate due to insufficient verification logic. This allows the attacker to perform actions reserved for higher-privileged accounts, such as creating new bookings, modifying existing ones, or accessing sensitive customer data that should remain restricted. The exploitability of this flaw is heightened by its network-based nature, meaning remote attackers can leverage it over a local area network (LAN) or even across the internet if the service is exposed without adequate perimeter defenses like web application firewalls or strict IP whitelisting.
The operational impact of this vulnerability is severe, as it directly compromises the confidentiality and integrity of business-critical scheduling data managed by Microsoft Bookings. Organizations relying on this platform for appointment management face significant risks including unauthorized access to personal identifiable information (PII) of customers, disruption of service operations through malicious booking modifications, and potential lateral movement within internal networks if combined with other exploits. Furthermore, the ability to elevate privileges enables attackers to establish persistent backdoors or exfiltrate large volumes of data undetected for extended periods. This scenario maps directly to MITRE ATT&CK techniques such as T1078 Valid Accounts, where adversaries use legitimate credentials obtained through initial access or phishing, and T1528 Steal Application Access Token if the vulnerability allows interception of session tokens. The lack of proper input validation also reflects weaknesses in secure coding practices that prioritize functionality over security during development phases.
To mitigate this risk, organizations must immediately apply the latest cumulative updates provided by Microsoft for their respective Exchange Server versions. These patches include specific code changes to enforce stricter validation rules on user-controlled inputs and ensure that authorization checks are performed consistently regardless of input manipulation. In addition to patching, administrators should implement network segmentation strategies to limit exposure of Bookings services to only trusted IP ranges. Deploying web application firewalls with rule sets designed to detect anomalous request patterns can provide an additional layer of defense against exploitation attempts. Regular security audits and penetration testing focused on authorization logic are recommended to identify similar flaws in custom integrations or third-party plugins connected to the Booking system. Monitoring logs for unusual access patterns, such as repeated failed authentication attempts followed by successful privileged actions, will aid in early detection of any ongoing exploitation activities.