Synapse's invalid device keys degrade federation functionality

ImpactLack of validation for device keys in Synapse before 1.138.3 and in Synapse 1.139.0 allow an attacker registered on the victim homeserver to degrade federation functionality, unpredictably breaking outbound federation to other homeservers. PatchesPatched in Synapse 1.138.3, 1.138.4, 1.139.1...