The Bluetooth host's GATT client passed the value of a received Handle Value Notification, Handle Value Indication or Multiple Handle Value Notification to bt_gatt_subscribe_params::notify callbacks using only the length derived from the received ATT PDU. bt_att_recv() in subsys/bluetooth/host/att.c validates only a minimum PDU length and does not enforce the negotiated ATT MTU, and the fixed ATT L2CAP channel in subsys/bluetooth/host/l2cap.c applies no rx.mtu check either, so the only ceiling was the ACL reassembly buffer CONFIG_BT_BUF_ACL_RX_SIZE. Values longer than the 512-octet maximum attribute length defined by the Bluetooth Core Specification (Vol 3, Part F, Section 3.2.9) could therefore be handed to subscribers that size their buffers to that documented limit.

A remote peer that a device has connected and subscribed to — a malicious, spoofed or compromised GATT server within radio range — triggers the flaw by sending a notification or indication whose value exceeds 512 octets on a subscribed handle. In the LE Audio unicast client this reaches a concrete out-of-bounds write: unicast_client_ep_notify() in subsys/bluetooth/audio/bap_unicast_client.c copies the notification into the 512-byte att_buf of struct unicast_client with net_buf_simple_add_mem() and no tailroom check whenever length equals bt_audio_get_max_ntf_size(), i.e. the negotiated ATT MTU minus three. notify_handler() in subsys/bluetooth/audio/bap_broadcast_assistant.c has the same shape. Reaching the ov…