# CVE-2025-66390: Auth bypass in n/a

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

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 21 Jul 2026
- **Updated:** 22 Jul 2026
- **Also known as:** GHSA-984h-wx22-r594

## 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 (Improper Access Control)

## 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.

## 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:

- 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. (github_poc): https://github.com/dz-y/Azure-APIM-Dev-Portal-Signup-Bypass

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-66390 and fix it if it is affected.

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

Affected product: n/a. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-66390/ (Markdown: https://insights.sbomify.com/CVE-2025-66390/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-66390
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-66390
- OSV: https://osv.dev/vulnerability/CVE-2025-66390

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-66390/
Summaries are generated and may contain mistakes; the official description above is the record.