CVE-2026-106078 in Backup for Microsoft 365
Summary
by MITRE • 10/27/2026
A vulnerability allowing an unauthenticated attacker to bypass REST API authentication with a forged federation token and receive responses from REST API endpoints that do not call the Veeam Backup for Microsoft 365 backup service.
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Analysis
by VulDB Data Team • 10/27/2026
The identified vulnerability represents a critical security flaw within the Veeam Backup for Microsoft 365 application, specifically targeting its REST API authentication mechanism. This issue stems from an insufficient validation of federation tokens used to authenticate requests against specific API endpoints. In modern enterprise environments, applications often rely on federated identity providers such as Azure Active Directory or other SAML-based systems to manage user access and session management. The vulnerability arises because the application fails to strictly enforce that incoming authentication tokens are bound exclusively to services that require active backup operations. Consequently, an attacker who possesses a valid but improperly scoped federation token can exploit this lack of binding validation to gain unauthorized access.
From a technical perspective, the core flaw lies in the logic governing how REST API endpoints verify the authenticity and scope of presented credentials. When a request is made, the application accepts the provided federation token without adequately checking whether the target endpoint necessitates interaction with the Veeam Backup for Microsoft 365 backup service itself. This oversight allows an unauthenticated or improperly authenticated attacker to bypass standard access controls by presenting a forged or misused token that would normally be rejected if proper scope validation were in place. The absence of strict context-aware authentication checks means that tokens intended for general identity verification can be repurposed to interact with administrative API functions, effectively circumventing the intended security boundaries.
The operational impact of this vulnerability is severe due to its potential for unauthenticated access and data exfiltration or manipulation. An attacker who successfully exploits this flaw could retrieve sensitive backup metadata, configuration details, or potentially interfere with backup jobs depending on the specific endpoints accessible through the bypassed authentication. Since REST APIs often serve as the primary interface for automation and management in cloud infrastructure, compromising these endpoints can lead to a broader compromise of the organization's data protection strategy. The ability to receive responses from API endpoints that do not call the core backup service suggests that even non-critical administrative functions may be exposed, providing attackers with valuable reconnaissance information about the environment's structure and security posture.
This vulnerability aligns closely with CWE-287, which describes Improper Authentication, as well as CWE-345, Insufficient Verification of Data Authenticity. In terms of offensive cybersecurity frameworks, this behavior is consistent with MITRE ATT&CK technique T1078, Valid Accounts, where attackers leverage legitimate credentials or tokens in unauthorized ways to maintain access and move laterally within a network. The exploitation path typically involves obtaining a valid federation token through phishing, credential theft, or other initial access vectors, followed by crafting requests that exploit the validation gap in the REST API layer.
Mitigation strategies must focus on strengthening authentication logic and implementing stricter scope validations for all API endpoints. Veeam has addressed this issue in subsequent software updates, making it imperative for administrators to apply the latest patches immediately. Beyond patching, organizations should implement network-level controls such as firewalls or Web Application Fireplaces that restrict access to management APIs based on IP reputation and geographic location where possible. Additionally, enforcing multi-factor authentication for all administrative accounts and monitoring API logs for anomalous request patterns can provide early detection of exploitation attempts. Regular audits of federated identity configurations ensure that tokens are issued with the minimum necessary privileges, reducing the blast radius if a token is compromised.