CVE-2026-85033 in Zephyr
Summary
by MITRE • 11/03/2026
The AVRCP Target role in subsys/bluetooth/host/classic/avrcp.c keeps its vendor-dependent response transmit state in a per-connection object selected by connection index (get_avrcp_tg()), with the per-transfer context (tid, sent_len, and the BT_AVRCP_TG_FLAG_TX_ONGOING flag) stored in the user_data of the queued response net_buf. The AVCTP teardown handler avrcp_disconnected() did not cancel tg->vd_rsp_tx_work and did not release the buffers still queued on tg->vd_rsp_tx_pending, so a link drop in the middle of a fragmented response left the buffer on the list with TX_ONGOING still set.
A remote AVRCP Controller can trigger this deterministically: it issues a vendor-dependent command whose response exceeds one AVRCP fragment (for example GetElementAttributes or GetCapabilities), lets bt_avrcp_tg_vendor_tx_work() emit the START fragment, and then drops the ACL link instead of sending REQUEST_CONTINUING_RESPONSE. The AVCTP control channel requires an encrypted BR/EDR link (BT_SECURITY_L2), so the attacker must be a paired peer within radio range; no user interaction is needed.
Because the Target objects are static and reused, the next connection landing on the same connection index finds the stale buffer at the head of vd_rsp_tx_pending. Every run of the TX work sees TX_ONGOING and returns immediately, so all further vendor-dependent responses queue behind the stale head and are never sent — a persistent loss of AVRCP Target function that no timeout clears and only a reboot recovers. Each cycle also leaks one response buffer from a shallow application pool, and a new peer that sends REQUEST_CONTINUING_RESPONSE clears the stale TX_ONGOING via bt_avrcp_tg_process_continuing_cmd(), causing the un-sent remainder of the previous peer's response to be transmitted to it.
The fix cancels tg->vd_rsp_tx_work and unrefs every buffer on tg->vd_rsp_tx_pending in avrcp_disconnected(); since the TX context lives in the buffer user_data, releasing the buffers also resets the TX state machine. There is no memory corruption: the pre-fix buffer is leaked with a live reference, so no use-after-free arises.
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Analysis
by VulDB Data Team • 11/03/2026
The vulnerability resides within the AVRCP Target role implementation of the Bluetooth Classic stack, specifically in how fragmented vendor-dependent responses are managed during connection teardown. The system maintains transmit state for these responses using per-connection objects selected by index, with critical transfer context such as transaction ID, sent length, and a flag indicating ongoing transmission stored directly in the user data of queued network buffers. When an AVCTP control channel is disconnected, the handler fails to cancel the pending vendor-dependent response work item or release the associated buffer queue. This oversight leaves stale state flags set on buffers that remain linked in the transmit list even after the logical connection has been terminated by a remote peer.
A remote attacker acting as an AVRCP Controller can exploit this flaw deterministically without requiring user interaction. By issuing a vendor-dependent command, such as GetElementAttributes or GetCapabilities, which generates a response larger than a single fragment, and then abruptly dropping the ACL link after the initial START fragment is emitted but before sending REQUEST_CONTINUING_RESPONSE, the attacker forces the system into an inconsistent state. Because AVCTP control channels require encrypted BR/EDR links, the attacker must be a paired peer within radio range, limiting the attack surface to authenticated connections but removing any need for social engineering or user consent.
The operational impact is severe and persistent, resulting in both denial of service and potential information leakage due to improper state management across connection boundaries. Since Target objects are static and reused based on connection index, a new peer connecting with the same index inherits the stale buffer at the head of the transmit queue. The transmission work routine detects the lingering TX_ONGOING flag from the previous session and returns immediately without processing any subsequent commands. This causes all further vendor-dependent responses to queue behind the stuck entry and remain unsent indefinitely, effectively disabling AVRCP Target functionality for that connection slot until a system reboot clears the state.
Furthermore, each exploitation cycle leaks one response buffer from the application pool, leading to gradual resource exhaustion over time if repeated. More critically, when a new peer sends REQUEST_CONTINUING_RESPONSE, it triggers processing logic that clears the stale TX_ONGOING flag on the previous connection's buffer. This causes the un-sent remainder of the original attacker's fragmented response to be transmitted to the innocent third party, resulting in an information disclosure vulnerability where sensitive data from one session is leaked into another unrelated session.
The remediation involves modifying the disconnection handler to explicitly cancel the pending vendor-dependent transmit work and release all buffers queued on the transmission list. Since the transaction context resides within the buffer's user data structure, releasing these references effectively resets the state machine for that connection slot. This fix ensures proper cleanup of both software state and memory resources without introducing use-after-free conditions, as the pre-fix behavior resulted in a simple leak rather than dangling pointer dereference. The vulnerability aligns with CWE-401 Missing Release of Memory after Effective Lifetime and CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization due to race-like state persistence across connection boundaries. In terms of MITRE ATT&CK, this represents an impact on availability through resource exhaustion and a confidentiality breach via improper isolation of session data in shared memory structures.