Vulnerability Description
In Microsoft Azure API Management through 2025-10-17, when self-service signup (username/password Basic Authentication) is enabled in Tenant A, an attacker can reuse the registration flow by changing the hostname or tenant identifier to Tenant B, even when Tenant B has signup disabled at the UI level. In other words, disabling signup in the UI does not disable the underlying API endpoint (which still accepts cross-tenant requests based on the Host header). NOTE: The supplier states that they evaluated the report and determined it did not cross a security boundary (i.e., the observed behavior was a configuration/state issue rather than an exploitable product vulnerability affecting tenant isolation). NOTE: The supplier evaluated this report and determined that it did not cross a security boundary (i.e., the observed behavior was a configuration/state issue rather than an exploitable product vulnerability affecting tenant isolation).
CVSS Score
CRITICAL
Related Weaknesses (CWE)
References
- https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass
- https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass/security/advi
- https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass
FAQ
What is CVE-2025-66390?
CVE-2025-66390 is a vulnerability with a CVSS score of 9.8 (CRITICAL). In Microsoft Azure API Management through 2025-10-17, when self-service signup (username/password Basic Authentication) is enabled in Tenant A, an attacker can reuse the registration flow by changing ...
How severe is CVE-2025-66390?
CVE-2025-66390 has been rated CRITICAL with a CVSS base score of 9.8/10. This is considered a critical vulnerability requiring immediate attention.
Is there a patch for CVE-2025-66390?
Check the references section above for vendor advisories and patch information. Review vendor security bulletins for remediation guidance.