A recent analysis has uncovered a significant design flaw within Microsoft Entra ID, formerly known as Azure Active Directory, that could potentially allow unauthorized users to gain elevated privileges within an organization's Azure environment. This vulnerability centers around the default permissions granted to guest users, which, if exploited, could lead to unauthorized control over Azure subscriptions.
Understanding the Design Flaw
The core of this issue lies in the default billing permissions assigned to guest users within an Azure tenant. When an external user is invited as a guest into an organization's Azure environment, they inherit certain permissions that, under specific conditions, enable them to create and manage subscriptions without explicit administrative approval. This behavior is not a result of a coding error but is an intentional design choice by Microsoft.
Security researchers at BeyondTrust have demonstrated how this design can be exploited:
- Guest Invitation: An attacker creates their own Azure tenant and sets up a subscription under a trial or paid plan.
- Invitation Acceptance: The attacker is then invited as a guest user into the target organization's Azure tenant.
- Subscription Transfer: Leveraging the default billing permissions, the attacker initiates a transfer of their existing subscription to the target tenant.
- Elevation of Privileges: Upon successful transfer, the attacker gains "Owner" rights over the subscription within the target tenant, granting them extensive control over resources and services.
Microsoft's Stance and Industry Response
Microsoft has acknowledged this behavior but maintains that it is by design. The company asserts that the ability for guest users to transfer subscriptions is intended to facilitate collaboration and resource sharing between organizations. However, this stance has raised concerns within the cybersecurity community.
Simon Maxwell-Stewart, Senior Data Engineer at BeyondTrust, highlighted the inherent risks:
This perspective underscores the need for organizations to proactively manage and restrict guest user permissions to mitigate potential security threats."The problem lies in the default behavior: if this capability were opt-in, meaning guests were blocked from creating subscriptions by default, the risk would be significantly reduced, and this wouldn't pose a security concern."
Potential Risks and Implications
The implications of this design flaw are far-reaching:
- Unauthorized Resource Access: Attackers could deploy, modify, or delete resources within the compromised subscription, leading to data breaches, service disruptions, or financial losses.
- Lateral Movement: Gaining a foothold within the Azure environment could allow attackers to explore and exploit other vulnerabilities, potentially compromising additional systems and data.
- Compliance Violations: Unauthorized access and control over resources may lead to non-compliance with industry regulations and standards, resulting in legal and financial repercussions.
To protect against potential exploitation of this design flaw, organizations should consider implementing the following measures:
- Review and Restrict Guest Permissions: Assess the default permissions assigned to guest users and modify them to align with the principle of least privilege.
- Implement Subscription Policies: Enforce policies that prevent guest users from transferring or creating subscriptions without explicit administrative approval.
- Monitor and Audit Activities: Regularly monitor and audit activities related to subscription management to detect and respond to unauthorized actions promptly.
- Educate and Train Administrators: Ensure that administrators are aware of the potential risks associated with guest user permissions and are trained to implement and enforce appropriate security measures.
While Microsoft's design choice aims to facilitate collaboration, it inadvertently introduces a significant security risk. Organizations must take proactive steps to review and adjust their Azure configurations to prevent unauthorized privilege escalation. By implementing stringent access controls and monitoring mechanisms, organizations can safeguard their Azure environments against potential compromises stemming from this design flaw.
Source: SC Media Azure compromise likely with Microsoft Entra ID design issue