CVE-2025-66390 Auth bypass in n/a

Critical 9.8 Published 21 Jul 2026 Updated 22 Jul 2026

Attackers can bypass disabled signup UI restrictions by manipulating host headers to register accounts in other tenants via the underlying API.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: None listed

What is CVE-2025-66390?

CVE-2025-66390 is a critical authentication bypass vulnerability in n/a, published on 21 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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).

Weakness: CWE-284

Which versions are affected by CVE-2025-66390?

How do I fix CVE-2025-66390?

No fix has been published for CVE-2025-66390 yet. Limit exposure of the affected software until one is available.

Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.

Is CVE-2025-66390 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2025-66390 exists.

Public exploit code: 1 proofs of concept on GitHub.

Sources

Also known as GHSA-984h-wx22-r594.