An issue was discovered in Trusted Firmware-M through 2.1.0. User provided (and controlled) mailbox messages contain a pointer to a list of input arguments (in_vec) and output arguments (out_vec). These list pointers are never validated. Each argument list contains a buffer pointer and a buffer length field. After a PSA call, the length of the output arguments behind the unchecked pointer is updated in mailbox_direct_reply, regardless of the call result. This allows an attacker to write anywhere in the secure firmware, which can be used to take over the control flow, leading to remote code execution (RCE).