A method, system and computer program product are provided for implementing and processing a block extent granularity authorization mechanism for a Coherent Accelerator Processor Interface (CAPI) adapter. The CAPI adapter generates an authorization table with multiple authorization entries, each authorization entry including an Authorization Handle with CAPI server registers identification (ID) including a start Logical Block Address of the extent and range of Logical Block Addresses for each extent. When a command is received an authentication process uses the Authorization Handle contained in the received command and an Authorization Entry in the Authorization Table indexed by the Authorization Handle to authenticate the received command to prevent unauthorized data access.