CVE-2026-85034

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 overflow requires a build whose ATT MTU exceeds 515 octets (CONFIG BT L2CAP TX MTU and CONFIG BT BUF ACL RX SIZE set for full-length attribute support); the shipped defaults are far smaller.

The overflow writes peer-controlled bytes past att buf directly over the adjacent struct net buf simple net buf — its data, buf, len and size fields — which the subsequent long-read accumulation then uses as its destination, turning a small linear overflow into a controlled write. net buf simple add() only asserts on insufficient tailroom unless CONFIG NET BUF HARDENING is enabled, and that option is disabled by default, so ordinary production builds perform the copy. Beyond the in-tree LE Audio clients, any application subscriber that sized its receive buffer to the specification's 512-octet limit was exposed to the same overflow.

The fix rejects notification, indication and multiple-notification values longer than BT ATT MAX ATTRIBUTE LEN in call notify cb and maybe unsubscribe() in subsys/bluetooth/host/gatt.c before any subscriber callback is invoked, so no subscriber can be handed more than the specified maximum. Remaining tuples of a multiple handle value notification are still delivered and oversized indications are still confirmed.