UBUNTU-CVE-2026-81039: Medium severity ubuntu/zephyr vulnerability

The Bluetooth host pre-allocates a btconn object (a "reservation") from the fixed aclconns[] pool for every connectable advertising set it starts, in leadvstartaddconn() in subsys/bluetooth/host/adv.c. When an incoming peripheral connection completes, findpendingconnect() in subsys/bluetooth/host/hcicore.c looked that reservation up using only the identity and the peer address, with a directed-then-undirected fallback, and had no knowledge of which advertising set actually terminated. With CONFIGBTEXTADVMAXADVSET > 1, where the LE Enhanced Connection Complete event is cached until the LE Advertising Set Terminated event identifies the set, this lookup can return the reservation belonging to a different, still-running advertising set. The defect is reachable over the air by any adjacent device. If an application runs two connectable extended advertising sets on the same identity — one undirected and one directed at a peer P (the common "directed reconnect to a bonded central while still accepting new centrals" pattern) — a remote initiator whose address maps to P connecting on the undirected set causes the directed set's reservation to be consumed instead. The initiator address is chosen freely by the remote device and is not authenticated at link establishment, so no bonding, pairing or prior relationship is required to trigger the mis-attribution. The undirected set's own reservation is then stranded: its BTADVENABLED flag has already been cleared, and leadvstopfreeconn() — the only path that returns a reservation to the pool — is reached only for sets that are still enabled, so btleextadvstop() returns early and the object is never released. Each occurrence permanently consumes one CONFIGBTMAXCONN slot until btdisable() or reboot. In application patterns that restart the directed advertising set after each disconnect, an attacker can repeat the sequence until the pool is exhausted, after which connectable advertising fails with -ENOMEM and the device can no longer accept connections. As a side effect the directed set keeps advertising without a reservation, so a genuine connection on it is dropped by bthcidisconnect(). The impact is availability only; no memory corruption or information disclosure results, and configurations with a single advertising set are unaffected.