CVE-2026-85034: Out-of-bounds write in the Bluetooth GATT client: oversized notification and indication values delivered to subscribers
The Bluetooth host's GATT client passed the value of a received Handle Value Notification, Handle Value Indication or Multiple Handle Value Notification to btgattsubscribeparams::notify callbacks using only the length derived from the received ATT PDU. btattrecv() 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 CONFIGBTBUFACLRXSIZE. 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: unicastclientepnotify() in subsys/bluetooth/audio/bapunicastclient.c copies the notification into the 512-byte attbuf of struct unicastclient with netbufsimpleaddmem() and no tailroom check whenever length equals btaudiogetmaxntfsize(), i.e. the negotiated ATT MTU minus three. notifyhandler() in subsys/bluetooth/audio/bapbroadcastassistant.c has the same shape. Reaching the overflow requires a build whose ATT MTU exceeds 515 octets (CONFIGBTL2CAPTXMTU and CONFIGBTBUFACLRXSIZE set for full-length attribute support); the shipped defaults are far smaller.
The overflow writes peer-controlled bytes past attbuf directly over the adjacent struct netbufsimple netbuf — 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. netbufsimpleadd() only asserts on insufficient tailroom unless CONFIGNETBUFHARDENING 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 BTATTMAXATTRIBUTELEN in callnotifycbandmaybeunsubscribe() 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.