CVE-2025-66390: Access Control Flaw (CVSS 9.8)

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).

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CVSS v3.1 Score

9.8

CRITICAL

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

EPSS — Exploit Prediction

0.0035

Probability of exploitation

0.27%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-284 CWE-284

References

Other References

https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass/security/advisories/GHSA-vcwf-73jp-r7mv https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass

Frequently Asked Questions

What is 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 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). It has a CVSS v3.1 base score of 9.8 (CRITICAL).

How severe is CVE-2025-66390? +

CVE-2025-66390 has a CVSS v3.1 score of 9.8 out of 10, rated CRITICAL. This is a critical vulnerability that should be patched immediately. The EPSS score is 0.0035, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2025-66390? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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