CVE-2026-77684 in Zephyr
Summary
by MITRE • 10/20/2026
The Bluetooth Classic Hands-Free Profile HF role in subsys/bluetooth/host/classic/hfp_hf.c does not release the AT command buffers that are still queued on hf->tx_pending when the RFCOMM DLC goes down. hfp_hf_disconnected() only cancelled the work items and cleared hf->acl, and no other code path ever dequeues those buffers, so each one is permanently lost from the fixed hf_pool (CONFIG_BT_MAX_CONN + 1 buffers, i.e. two in the default configuration). The same function also left BT_HFP_HF_FLAG_CONNECTED set, and hfp_hf_create() wiped the object with memset() as soon as hf->acl was NULL, without checking whether hf->work, hf->slc_work or hf->deferred_work was still pending or running.
The remote Audio Gateway peer controls the trigger end to end and needs no application involvement: an unsolicited +BCS: 1 response makes bcs_handle() call bt_hfp_hf_select_codec() and queue an AT+BCS buffer, which cannot be transmitted while BT_HFP_HF_FLAG_RX_ONGOING is set, and an immediately following RFCOMM DISC strands that buffer. Repeating the connect / +BCS / disconnect cycle a couple of times empties hf_pool for good.
With the pool exhausted, HFP command sending fails permanently, and because hfp_hf_send_cmd() allocates with K_FOREVER (bt_rfcomm_create_pdu() → bt_conn_create_pdu()) from the Bluetooth RX context — where that timeout is not downgraded to K_NO_WAIT — one further +BCS from the peer parks the Bluetooth RX thread inside net_buf_alloc() with no possible waker, halting all HCI event processing until the device is rebooted. Separately, reusing the HF slot while a work handler was still queued or running allowed memset() to wipe live k_work items out from under the system workqueue, corrupting shared kernel workqueue state. Exploitation requires an established HFP session (for SSP devices L2CAP enforces authentication before the RFCOMM channel is reachable) and a build with CONFIG_BT_HFP_HF enabled.
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Analysis
by VulDB Data Team • 10/20/2026
The vulnerability described constitutes a critical resource exhaustion and memory corruption flaw within the Bluetooth Classic Hands-Free Profile Host Function implementation, specifically affecting the HF role in Linux kernel subsystems or similar embedded operating systems utilizing Zephyr RTOS components. The core technical defect lies in improper state management during connection teardown sequences. When an RFCOMM Data Link Connection Identifier (DLC) is terminated, typically triggered by a remote Audio Gateway peer sending a DISC frame, the hfp_hf_disconnected function fails to properly clean up pending transmission buffers queued on hf->tx_pending. Instead of dequeuing and releasing these memory allocations back into the fixed-size pool defined by CONFIG_BT_MAX_CONN plus one buffer slot, the code merely cancels work items and clears the ACL connection pointer. This oversight results in a permanent leak of each buffered AT command that was awaiting transmission but could not be sent due to ongoing receive operations or other blocking conditions. In default configurations where only two buffers are available for this pool, repeated exploitation can exhaust all resources rapidly.
The operational impact is severe and manifests as both a denial-of-service condition through resource exhaustion and potential kernel instability via memory corruption. The vulnerability allows an attacker with the ability to establish a Bluetooth Classic connection and initiate HFP sessions to systematically drain the buffer pool. By exploiting the logic where unsolicited +BCS responses trigger codec selection and subsequent AT+BCS command queuing, followed immediately by RFCOMM disconnection frames, an adversary can strand buffers in memory without releasing them. Once the hf_pool is exhausted, any attempt to send new HFP commands fails permanently because no buffer space remains for allocation via bt_rfcomm_create_pdu. Crucially, this failure triggers a secondary catastrophic effect: hfp_hf_send_cmd utilizes K_FOREVER as its timeout parameter when allocating from the Bluetooth RX context. Since timeouts are not downgraded to non-blocking modes in this specific execution path, subsequent +BCS events cause the Bluetooth RX thread to park indefinitely inside net_buf_alloc waiting for memory that will never be freed. This results in a complete halt of HCI event processing and effectively bricks the device until a manual reboot is performed.
Beyond simple denial-of-service, the vulnerability introduces significant integrity risks through improper handling of kernel work structures during connection teardown. The hfp_hf_disconnected function leaves the BT_HFP_HF_FLAG_CONNECTED flag set while simultaneously allowing subsequent code paths to reuse or wipe the hf object structure using memset when hf->acl becomes NULL. This sequence fails to check whether active k_work items, such as hf->work, hf->slc_work, or deferred work handlers, are still pending or executing in the system workqueue. Consequently, wiping the memory associated with these structures while they remain queued for execution corrupts shared kernel workqueue state. This corruption can lead to unpredictable behavior, including crashes, privilege escalation vectors if specific control flow is manipulated, or general system instability depending on how the corrupted data interacts with subsequent scheduler operations.
Exploitation of this vulnerability requires an established HFP session and a device configuration where CONFIG_BT_HFP_HF is enabled. For devices implementing Secure Simple Pairing (SSP), L2CAP enforces authentication before the RFCOMM channel becomes reachable, meaning that physical proximity or prior pairing credentials are necessary prerequisites for triggering the flaw remotely. The attack vector relies on precise timing of AT command generation and disconnection frames to maximize buffer stranding. Mitigation strategies must focus on rigorous resource lifecycle management within the HFP state machine. Developers should ensure that hfp_hf_disconnected explicitly dequeues and frees all pending buffers in hf->tx_pending before clearing connection states. Additionally, work item cancellation and memory zeroing operations must be synchronized with the kernel workqueue to prevent use-after-free scenarios or corruption of active handlers. Implementing strict checks for ongoing work execution prior to structure reuse is essential to maintain system integrity. From a standards perspective, this flaw aligns with CWE-401 Missing Release of Resource after Effective Lifetime and CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, while the exploitation technique maps to ATT&CK Tactic TA0005 Defense Evasion through resource manipulation or potentially TA0008 Lateral Movement if used in conjunction with other vulnerabilities.