CVE-2025-66390: In Microsoft Azure API Management through 2025-10-17, when self-service signup (username/password Basic Authentication) is enabled in Tenant A, an attacker... | Kitploit

CVE-2025-66390

CriticalPublished

In Microsoft Azure API Management through 2025-10-17, when self-service signup (username/password Basic Authentication) is enabled in Tenant A, an attacker...

Published: Jul 21, 2026
Updated: Jul 22, 2026
Assigning CNA: mitre
Evidence observed: Aug 5, 2026

Primary CVSS

9.8/ 10Critical

nvd · CVSS 3.1

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

EPSS probability estimate

0.6%

Low · next 30 days

Percentile: 49.0%
Model date: Sep 21, 2026

EPSS is a statistical estimate, not a certainty or a measure of impact. Combine it with CVSS, KEV status, exposure and your environment.

Summary

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).

Sources

Azure-APIM-Cross-Tenant-Signup-BypassResearch
Security advisory: Azure APIM Developer Portal allows cross-tenant account registration by bypassing UI signup restrictions. Reported to MSRC twice - closed as "by design".
Azure-APIM-Dev-Portal-Signup-BypassExploit
Python script to bypass Azure APIM signup when UI is disabled, this is different from the CVE-2025-66390 as it does not require you to setup anything cross tenant.

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