CVE-2026-19938CVE-2026-19938

The Bluetooth host's ATT transmit path releases a buffer it does not own when signing fails. chan_send() in subsys/bluetooth/host/att.c documents (and every other error exit implements) the contract that on failure the caller retains ownership of the net_buf; the BT_ATT_OP_SIGNED_WRITE_CMD branch instead called net_buf_unref(buf) when bt_smp_sign() returned an error and then returned that error to the caller. The callers act on the contract: process_queue() re-queues the buffer with k_queue_prepend(), bt_att_chan_send_rsp() puts it back on chan->tx_queue, bt_att_chan_req_send() restores it into the request, and att_req_retry() unreferences it — so the already-freed buffer was re-queued, later dereferenced, and eventually freed again. An ATT Signed Write Command is produced when an application calls bt_gatt_write_without_response_cb() with sign set on an unencrypted connection. bt_smp_sign() fails whenever no local CSRK exists for the peer (-ENOENT), and whether one exists is negotiate