Description

A recently disclosed flaw in Microsoft Azure API Management (APIM) Developer Portal allows attackers to create user accounts across different tenant environments, even when organizations have intentionally disabled public signups. The issue occurs because the APIM portal’s “disable signup” option only hides the registration form in the interface, while the underlying /signup API endpoint continues to function. This design oversight leaves the service open to unauthorized access whenever Basic Authentication is enabled on the Developer Portal. The vulnerability can be exploited through the manipulation of the Host header in a signup request. An attacker only needs access to any APIM instance where registrations are allowed, this can be their own instance to capture a legitimate signup request. By altering the Host header to the target organization’s domain, the attacker can successfully register an account on the victim’s APIM instance. This bypasses admin controls, enabling access to internal API documentation, subscription keys, and potentially sensitive operational information. APIM instances using Basic Authentication across Developer, Basic, Standard, or Premium tiers are affected. Despite the security impact, Microsoft has classified the issue as “by design” and has not provided a patch. The flaw was reported by Finnish researcher Mihalis Haatainen, whose submissions to Microsoft Security Response Center were closed without remediation. The issue was later escalated to CERT-FI and publicly disclosed in November 2025. Since no official fix exists, organizations must remove the Basic Authentication provider entirely instead of relying on UI settings. Switching to Azure Active Directory, reviewing existing user accounts, and monitoring signup activity are strongly recommended. Verification tools released by the researcher can help security teams identify exposed APIM instances.