CVE-2026-54413

single-byte 0x27 SecurityAccess request that follows any earlier well-formed 0x27 message. The handler reads the SecurityAccess subFunction from recv_buf[1] without first checking that recv_len is at least 2, then computes the key-data length as the unsigned subtraction (uint16_t)(recv_len - UDS_0X27_REQ_BASE_LEN); when recv_len equals 1 the result underflows to 65535 and is passed as args.len to the application's SecAccessValidateKey or SecAccessRequestSeed callback, which typically iterates or copies that many bytes from the 4-KB receive buffer. Every other UDS sub-function handler in the library (0x10, 0x11, 0x14, 0x19, 0x22, 0x23, 0x28, and others) performs an explicit recv_len lower-bound check before indexing; Handle_0x27_SecurityAccess is the sole outlier. The vulnerable handler reaches over CAN bus, OBD-II, ISO-TP, and DoIP transports and is exposed in the default diagnostic session without prior authentication; deployments on automotive ECUs, industrial controllers, and IoT devices that ship iso14229 as their UDS server are affected.