Cross device flows lets a user initiate an action on one device (e.g. a SmartTV) and authenticate or authorize that action on a trusted personal device (e.g. a mobile phone). Examples includes authorizing a smart TV to access streaming content, or authenticating to a service by scanning a QR code with a mobile phone and completing the authentication on the mobile phone. This process of authorizing an action on a separate (but trusted) device from the one on which an action is initiated is an increasingly common flow, whether used for devices with limited input capabilities, multi-factor authentication or credential presentation. A number of standards have adopted this pattern including Device Authorization Grant (formerly Device Code Flow), Client Initiated Backchannel Authentication (CIBA) and Self Issued OpenID Provider (SIOP). These flows solve important business problems, but is vulnerable to attacks where the user is tricked into granting consent to an attacker. The IETF OAuth working group has recognised this challenge and is creating new guidance that leverages zero-trust principles to defend against these "illicit consent grant" attacks. This session will discuss the attacks and how the new guidance can mitigate these threats against cross device flows.