---
id: CVE-2025-66390
title: >-
  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 …
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 …
severity: critical
cvss: 9.8
cvssVector: 'CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H'
cwe:
  - CWE-284
published: '2026-07-21'
updated: '2026-10-05'
sourceUpdated: '2026-10-05T18:10:00.200'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2025-66390'
references:
  - url: 'https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass'
    label: cve@mitre.org
  - url: >-
      https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass/security/advisories/GHSA-vcwf-73jp-r7mv
    label: cve@mitre.org
  - url: 'https://github.com/bountyyfi/Azure-APIM-Cross-Tenant-Signup-Bypass'
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
tags:
  - nvd
  - exploit-available
epss: 0.00642
epssPercentile: 0.48996
exploits:
  github: 1
  githubRepos:
    - 'https://github.com/dz-y/Azure-APIM-Dev-Portal-Signup-Bypass'
  checkedAt: '2026-10-05T18:29:46.182Z'
exploitAvailable: true
ingestedAt: '2026-10-05T18:29:11.174Z'
---

## Overview

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).

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.