Microsoft’s Azure API Management (APIM) Developer Portal, a platform commonly used for managing and securing APIs, is vulnerable to a significant security flaw that enables attackers to bypass access controls and create unauthorized accounts across different tenants. This issue stems from a fundamental design flaw where disabling signup within the Azure Portal UI only visually hides the registration form while leaving the underlying API endpoint active.
The vulnerability allows attackers to exploit basic authentication settings and manipulate the Host header in signup requests. By intercepting legitimate requests, they can redirect them to another organization’s APIM instance, effectively creating an account despite “disabled” signup on the victim’s portal. This poses a serious threat to organizations with APIM instances vulnerable to this flaw, as it compromises control over user access and exposes sensitive data and APIs.
Understanding the Flaws
At its core, this vulnerability relies on Microsoft’s current design of the APIM Developer Portal. It allows for Basic Authentication to be enabled on the backend API even when the UI setting suggests disabled signup. The result is that the underlying functionality remains active, accepting registration requests and bypassing any boundary restrictions or verification mechanisms. This means that attacks can occur even after seemingly disabling account creation through the portal interface.
The Scope of Impact
This flaw has a wide-ranging impact on businesses that utilize APIM, potentially exposing them to numerous security risks:
- Cross-tenant Account Creation: Attackers can create accounts in different tenant instances using this vulnerability. This directly threatens data privacy and exposes organizations to potential breaches or unauthorized access.
- Bypass of Administrative Controls: The flaw bypasses the normal administrative controls and restrictions, allowing attackers to gain unauthorized access and operate within a targeted organization’s API ecosystem.
- Data Exposure: Sensitive information like API documentation and subscription keys can be accessed and used by malicious actors if the vulnerability remains exploited.
Addressing the Vulnerability
Microsoft has acknowledged this design flaw as “by design,” yet it remains unpatched, leaving organizations vulnerable. It is crucial to understand that simply disabling signup on the UI does not solve the problem. To mitigate the risk, organizations must take the following steps:
- Completely Remove Basic Authentication: The most critical step involves removing basic authentication entirely from the Azure Portal and deleting all associated identity providers. This removes any reliance on the vulnerable API endpoint for user registration.
- Leverage Azure Active Directory: Switching to Azure Active Directory (AAD) authentication, which enforces proper tenant boundaries and user access controls, is crucial in securing APIM instances.
- Audit Existing Accounts: Auditing existing Developer Portal user accounts is essential to identify any potential unauthorized registrations created after the supposed disabling of signup.
- Continuous Monitoring: Implementing continuous monitoring of signup activity and API calls is crucial for detecting suspicious activity or attempts to exploit the vulnerability.
Identifying Vulnerable Instances
Researchers have developed publicly available tools that can help organizations quickly identify instances of this vulnerability in their systems: A Python verification script and Nuclei template, released by researcher Mihalis Haatainen, can be utilized to identify vulnerable APIM instances within an organization.