Family Matters : Abusing Family Refresh Tokens to Gain Unauthorised Access to Microsoft Cloud Services Exploratory Study of Azure Active Directory Family of Client IDs

Azure Active Directory (Azure AD) is an identity and access management service used by Microsoft 365 and Azure services and thousands of third-party service providers. Azure AD uses OIDC and OAuth protocols for authentication and authorisation, respectively. OAuth authorisation involves four parties: client, resource owner, resource server, and authorisation server. The resource owner can access the resource server using the specific client after the authorisation server has authorised the access. The authorisation is presented using a cryptographically signed Access Token, which includes the identity of the resource owner, client, and resource. During the authorisation, Azure AD assigns Access and Id Tokens that are valid for one hour and a Refresh Token that is valid for 90 days. Refresh Tokens are used for requesting new Access and Id token after their expiration. By OAuth 2.0 standard, Refresh Tokens should only be able to be used to request Access Tokens for the same resource ow ner, client, and resource. In this paper, we will present findings of a study related to undocumented feature used by Azure AD, the Family of Client ID (FOCI). After studying 600 first-party clients, we found 16 FOCI clients which supports a special type of Refresh Tokens, called Family Refresh Tokens (FRTs). These FRTs can be used to obtain Access Tokens for any FOCI client. This non-standard behaviour makes FRTs primary targets for a token theft and privilege escalation attacks.