CVE-2026-19739 in Zephyr
Summary
by MITRE • 10/11/2026
The Bluetooth Link Layer control procedure code in subsys/bluetooth/controller/ll_sw/ull_llcp_conn_upd.c retains the received RX node while a Connection Update / Connection Parameter procedure waits for its instant, so the node can later carry the host notification (llcp_rx_node_retain(), which marks it NODE_RX_TYPE_RETAIN and thereby suppresses the normal recycling in ull.c). The default: arm of llcp_rp_cu_rx() and llcp_lp_cu_rx() — the "invalid PDU, terminate the connection" path — completed the procedure without releasing that retained node. llcp_rr_check_done() then dequeued and freed the procedure context with ctx->node_ref.rx still pointing at the retained node, dropping the last reference to it.
A peer device in radio range can reach this without pairing, bonding, or encryption. Against a peripheral, the peer sends a well-formed LL_CONNECTION_UPDATE_IND with an instant a few connection events in the future (the node is retained and the procedure enters RP_CU_STATE_WAIT_INSTANT), then, before the instant is reached, sends any other LL Control PDU such as LL_LENGTH_REQ. ull_cp_rx() routes that PDU into the active remote Connection Update procedure, which takes the invalid-PDU path. The central role is reachable symmetrically after accepting an LL_CONNECTION_PARAM_REQ, and the local-procedure variant is reachable with LL_REJECT_IND.
With CONFIG_BT_CTLR_ASSERT_DEBUG enabled (its default), the resulting state violates the invariant asserted in llcp_rr_check_done(), so the two-PDU sequence produces an immediate fatal error in the controller. With those asserts disabled, each occurrence permanently loses one node from the controller's small fixed RX pool (sized from CONFIG_BT_CTLR_RX_BUFFERS, which defaults to 1); repeating the sequence across reconnections exhausts the pool, after which the link-layer receive path operates on a NULL node. The impact is an unauthenticated, remotely triggerable denial of service persisting until reboot; there is no memory-disclosure or memory-corruption consequence, since the leaked node simply becomes unreachable.
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Analysis
by VulDB Data Team • 10/11/2026
The vulnerability resides within the Bluetooth Link Layer control procedure implementation in the subsys/bluetooth/controller/ll_sw/ull_llcp_conn_upd.c module of a BLE stack. The core technical flaw involves improper resource management during Connection Update and Connection Parameter procedures. Specifically, when a device receives an LL_CONNECTION_UPDATE_IND or similar PDU that initiates such a procedure, the system retains the received RX node to handle future notifications associated with the connection update instant. This retention is marked by setting the NODE_RX_TYPE_RETAIN flag, which suppresses the normal recycling of the buffer in the upper link layer processing routine ull.c. However, if an invalid PDU is subsequently received while this procedure is still waiting for its scheduled instant, the default error handling path terminates the connection without releasing the retained node reference. Subsequently, when llcp_rr_check_done() dequeues and frees the procedure context, it leaves ctx->node_ref.rx pointing to a memory location that has effectively been orphaned because the final reference count was dropped during cleanup but not properly nullified or returned to the pool before deallocation logic completed its state checks.
This flaw creates a significant denial of service vector against Bluetooth Low Energy devices operating in peripheral, central, or bridge roles. An attacker within radio range can exploit this without requiring any form of authentication, pairing, bonding, or encryption. The attack sequence involves sending a well-formed LL_CONNECTION_UPDATE_IND with an instant set for several connection events into the future to trigger the retention state. Before that instant is reached, the attacker sends another LL Control PDU, such as LL_LENGTH_REQ, which routes through the active remote Connection Update procedure and triggers the invalid-PDU termination path. For central devices, a symmetric attack vector exists after accepting an LL_CONNECTION_PARAM_REQ, while local-procedure variants can be triggered using LL_REJECT_IND. This allows for unauthenticated, remotely triggerable denial of service attacks that persist until the device is rebooted or powered off.
The operational impact varies depending on whether debug assertions are enabled in the build configuration. With CONFIG_BT_CTLR_ASSERT_DEBUG enabled, which is often the default setting during development and testing, the violation of internal state invariants triggers an immediate fatal error via llcp_rr_check_done(), causing a hard crash of the controller software. In production builds where these asserts are disabled, each successful exploitation permanently leaks one node from the controller's fixed RX buffer pool. The size of this pool is determined by CONFIG_BT_CTLR_RX_BUFFERS, which defaults to just one buffer in many configurations. Repeating the attack sequence across multiple reconnections will exhaust all available buffers. Once the pool is depleted, the link-layer receive path attempts to operate on a NULL node, leading to system instability or complete loss of Bluetooth connectivity. Although this vulnerability does not result in memory disclosure or arbitrary code execution due to the nature of the leak being an unreachable reference rather than corruption, it effectively disables critical wireless communication capabilities for extended periods until manual intervention occurs.
From a security standards perspective, this issue aligns with CWE-401 Missing Release of Memory after Effective Lifetime and CWE-755 Improper Handling of Unexpected Unusual Input, as the system fails to properly clean up resources when encountering unexpected control flow during protocol state transitions. In terms of MITRE ATT&CK for IoT or network protocols, this represents a Denial of Service (T1499) specifically targeting resource exhaustion through protocol manipulation rather than volumetric flooding. Mitigation strategies should focus on ensuring that all code paths terminating a procedure correctly release any retained resources before freeing the context structure. Developers must implement rigorous state machine validation to ensure that reference counts are decremented and buffers returned to the pool regardless of whether the procedure completes normally or is aborted due to an invalid PDU. Additionally, enabling robust assertion checks in production builds where feasible can help detect such logic errors early, although proper resource management remains the primary defense against this class of vulnerability.