Keycloak vulnerable to session takeover with OIDC offline refreshtokens

An issue was discovered in Keycloak when using a client with the offline_access scope. Reuse of session ids across root and user authentication sessions and a lack of root session validation enabled attackers to resolve a user session attached to a different previously authenticated user.

This issue most affects users of shared computers. Suppose a user logs out of their account (without clearing their cookies) in a mobile app or similar client that includes the offline_access scope, and another user authenticates to the application. In that case, it will share the same root session id, and when utilizing the refresh token, they will be issued a token for the original user.