Zephyr Project up to 4.3.1/4.4.x Bluetooth Host bap_unicast_client.c unicast_client_ep_notify out-of-bounds write

Summaryinfo

A vulnerability, which was classified as very critical, has been found in Zephyr Project Zephyr up to 4.3.1/4.4.x. Affected by this issue is the function unicast_client_ep_notify of the file subsys/bluetooth/audio/bap_unicast_client.c of the component Bluetooth Host. The manipulation leads to out-of-bounds write. This vulnerability is uniquely identified as CVE-2026-85034. The attack is possible to be carried out remotely. No exploit exists. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in Zephyr Project Zephyr up to 4.3.1/4.4.x and classified as very critical. This issue affects the function unicast_client_ep_notify of the file subsys/bluetooth/audio/bap_unicast_client.c of the component Bluetooth Host. The manipulation with an unknown input leads to a out-of-bounds write vulnerability. Using CWE to declare the problem leads to CWE-787. The product writes data past the end, or before the beginning, of the intended buffer. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. The identification of this vulnerability is CVE-2026-85034 since 09/02/2026. The exploitation is known to be easy. The attack may be initiated remotely. Technical details are known, but no exploit is available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 11/03/2026).

Upgrading to version 4.3.2 or 4.5.0 eliminates this vulnerability. Applying the patch 64b1e7447856f635ee8bce9eab7c615da78133b8 is able to eliminate this problem. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Type

• Project Management Software

Vendor

• Zephyr Project

Name

• Zephyr

Version

• 4.0
• 4.1
• 4.2
• 4.3
• 4.3.0
• 4.3.1
• 4.4

CPE 2.3info

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CPE 2.2info

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