Description

The Hearing Access Service (HAS) GATT server in subsys/bluetooth/audio/has.c installs a connection-callback set unconditionally via BT_CONN_CB_DEFINE, so security_changed() runs for every connection that establishes security even before the application has called bt_has_register(). The service attribute pointers hearing_aid_features_attr, preset_control_point_attr, and active_preset_index_attr remain NULL until bt_has_register() resolves them and sets has.registered. With CONFIG_BT_SETTINGS, settings_set_cb() restores each bonded client's persisted context at boot and unconditionally sets context->flags to BONDED_CLIENT_INIT_FLAGS (non-zero). When a previously bonded peer reconnects and re-establishes security during the startup window before bt_has_register() has been called, security_changed() sees the non-zero flags and schedules notify_work_handler, which calls bt_gatt_is_subscribed() with a still-NULL attribute pointer. That triggers an assertion (__ASSERT(attr, ...) in bt_gatt_is_subscribed()), or a NULL dereference of attr->uuid when assertions are compiled out. The result is a remotely triggerable (Bluetooth, adjacent) crash of the HAS peripheral. Exploitation requires the peer to have previously bonded with the device and to reconnect within the boot-time race window before the application registers the service; a peer that reconnects persistently can prolong the outage. Impact is denial of service only, with no memory corruption or information disclosure. The fix adds an early if (!has.registered) { return; } guard in security_changed(), so no notification work is scheduled until the GATT service is registered and its attribute pointers are valid.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.3",
  "pubDate": "2026-08-24T18:16:59.450Z",
  "pubdate": "2026-08-24T18:16:59.450Z",
  "executiveSummary": "A denial of service vulnerability exists within the Hearing Access Service (HAS) GATT server implementation in subsys/bluetooth/audio/has.c.\nThe vulnerability is triggered by a race condition during the device boot window, where an uninitialized service state interacts with connection security callbacks.\nThe impact is a remote, adjacent-network crash of the HAS peripheral, leading to a Denial of Service (DoS) condition with no memory corruption or information disclosure capabilities.\nAffected systems include products utilizing the Zephyr Bluetooth audio subsystem with CONFIG_BT_SETTINGS enabled.\nAttacker capabilities require an adjacent Bluetooth connection from a peer that has previously bonded with the target device.\nExploitation requires the peer to reconnect during the specific boot-time window before the application calls bt_has_register(), and persistent reconnection attempts can continuously prolong the outage.",
  "technicalDetails": "The vulnerability resides in subsys/bluetooth/audio/has.c within the Hearing Access Service (HAS) GATT server component of Zephyr.\nThe root cause stems from the unconditional installation of a connection-callback via BT_CONN_CB_DEFINE, which causes the security_changed() function to execute for every connection establishing security, even prior to the application invoking bt_has_register().\nService attribute pointers such as hearing_aid_features_attr, preset_control_point_attr, and active_preset_index_attr remain initialized to NULL until bt_has_register() successfully resolves them and sets has.registered to true.\nWhen CONFIG_BT_SETTINGS is enabled, settings_set_cb() restores persisted bonded client contexts at boot and unconditionally sets context->flags to a non-zero value defined by BONDED_CLIENT_INIT_FLAGS.\nThe attack flow proceeds as follows: 1) The peripheral boots up. 2) A previously bonded peer immediately reconnects and re-establishes security during the startup race window before the application registers the HAS service. 3) The security_changed() callback evaluates the connection, observes the non-zero flags restored from the bonded context, and schedules notify_work_handler(). 4) The handler invokes bt_gatt_is_subscribed() passing a still-NULL attribute pointer.\nThis condition triggers a fatal assertion failure (__ASSERT(attr, ...)) in bt_gatt_is_subscribed(), or results in a direct NULL pointer dereference of attr->uuid when assertions are compiled out, directly crashing the peripheral.\nAuthentication and privilege requirements are minimal from an attacker perspective, requiring only a previously established Bluetooth bond with the peripheral.\nNetwork exposure is limited to the adjacent Bluetooth radio range.\nPost-exploitation impact is strictly restricted to denial of service, as persistent reconnection attempts by the bonded peer can maintain the peripheral in a continuous crash loop."
}