CVE-2026-11368 in Zephyr
Summary
by MITRE • 08/04/2026
The Bluetooth host ATT layer (subsys/bluetooth/host/att.c) associates each in-flight ATT TX buffer with its owning channel via the static tx_meta_data_storage[] array (data->att_chan = chan). When a buffer's last reference is dropped, its net-buf destroy callback defers the completion handling to the system workqueue (att_tx_destroy -> att_tx_destroy_work_handler -> att_on_sent_cb -> bt_att_sent), where bt_att_sent dereferences the channel and its ATT context (sys_slist_get(&att->reqs)).
When a peer disconnects while an ATT PDU (a server notification/indication or any response) is still in flight in the controller TX path, L2CAP tears the channel down in l2cap_chan_del(): it runs the disconnected callback and then the released callback (bt_att_released), which frees the channel slab slot. Because the in-flight buffer is held by the connection TX path rather than the channel's own queue, its deferred destroy work can run after the channel has been freed. The att_on_sent_cb guard intended to drop the stale callback itself dereferences meta->att_chan, which is now a dangling pointer into a freed (and possibly reused) slab slot.
A remote peer with an ATT connection can drive this by disconnecting during routine ATT traffic; no pairing or user interaction is required to reach the ATT bearer. The result is a use-after-free read/write of freed channel memory, reliably crashing the Bluetooth host (denial of service) and, because the channel slab slot may be reused, potentially corrupting live memory.
The fix makes bt_att_released() NULL the att_chan field of every tx_meta_data_storage[] entry still referencing the channel before freeing it, so the deferred guard observes a NULL pointer and drops the callback. Teardown and the destroy work both run on the cooperative system workqueue, so the array update is serialized and needs no lock.
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Analysis
by VulDB Data Team • 08/04/2026
The vulnerability described represents a critical use-after-free condition in the Bluetooth host implementation within the ATT layer, specifically affecting how in-flight transmission buffers are managed during channel teardown. This flaw exists in the subsystem responsible for handling attribute protocol exchanges between Bluetooth devices, where each active ATT transaction maintains a reference to its associated L2CAP channel through a static array tx_meta_data_storage[]. The system employs a callback mechanism to handle buffer destruction, deferring completion processing to a dedicated workqueue to avoid blocking the main execution path. When an ATT PDU is being transmitted and the peer disconnects during this process, the L2CAP layer initiates channel cleanup by calling l2cap_chan_del(), which executes both disconnected and released callbacks including bt_att_released(). This function frees the channel memory slab, but leaves behind references in the tx_meta_data_storage[] array that point to the now-invalidated channel structure. The deferred destruction work continues to execute after channel cleanup, causing a dereference of meta->att_chan that points to freed memory, resulting in either immediate crash or memory corruption when the freed slot gets reallocated.
The technical implementation of this vulnerability stems from improper synchronization between channel lifecycle management and asynchronous buffer cleanup operations. During normal operation, ATT transactions maintain references to their parent channels through the tx_meta_data_storage[] array structure. When a disconnection occurs, the channel's memory is immediately released, but any pending transmission buffers that were queued for transmission may still be in the controller's TX path. The destruction callbacks execute on a separate workqueue, creating a window where the channel reference becomes stale while the cleanup process continues to execute. This race condition allows for a scenario where bt_att_sent() function attempts to access metadata associated with a channel that has already been freed and potentially reallocated, leading to unpredictable behavior including system crashes or data corruption.
The operational impact of this vulnerability extends beyond simple denial of service, as it can result in memory corruption that affects the stability and security of the entire Bluetooth subsystem. An attacker positioned as a remote peer on an active ATT connection can trigger this condition by simply disconnecting during routine attribute protocol traffic, requiring no authentication or pairing prerequisites to exploit. The vulnerability affects the core Bluetooth host functionality and demonstrates a failure in proper resource management during asynchronous cleanup operations. This represents a fundamental flaw in the design pattern where references between subsystems are not properly synchronized, allowing for dangling pointer dereferences that can be reliably reproduced under specific timing conditions.
The mitigation implemented addresses this issue by ensuring proper nullification of channel references before memory deallocation occurs. The fix modifies bt_att_released() to iterate through all tx_meta_data_storage[] entries that reference the channel being torn down, setting their att_chan fields to NULL before freeing the channel structure. This approach ensures that when the deferred destroy work executes, it encounters a NULL pointer rather than a dangling reference, preventing any further access to freed memory. The solution leverages the cooperative nature of the system workqueue where both teardown operations and destruction callbacks execute serially, eliminating the need for additional locking mechanisms while maintaining thread safety through natural serialization. This approach aligns with security best practices by preventing information disclosure through memory corruption and ensuring proper resource cleanup in asynchronous systems.
This vulnerability classifies under CWE-416 Use After Free, which specifically addresses conditions where software continues to reference memory after it has been freed, leading to undefined behavior. The flaw also demonstrates characteristics of CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, highlighting the improper handling of shared resources between different execution contexts. From an ATT&CK framework perspective, this represents a privilege escalation vector through system instability and potential memory corruption, falling under techniques that leverage software vulnerabilities for denial of service or arbitrary code execution. The vulnerability underscores the critical importance of proper resource management in asynchronous systems where cleanup operations may occur after the referenced objects have been freed, particularly in embedded systems where such flaws can lead to complete system compromise rather than simple service disruption.