When running on Microsoft Azure mishmash io uses Azure Active Directory to authenticate and authorize incoming requests from users and apps.
Access control is entirely done in Azure AD - you can set who should have access (and what kind of access) in Azure AD and mishmash io only allow access to those users and apps that have been specifically allowed by the AD.
A client (like an app for example - My app) gets a signed token from Azure AD, which is then used by a mishmash io cluster as a 'proof' that Azure AD allows access.