---
id: "CVE-2026-81039"
kind: "vulnerability"
aliases: ["CVE-2026-81039"]
title: "Zephyr Bluetooth host: remote peer can exhaust connection slots, blocking new connections"
severity: "medium"
cvss: 4.3
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["bluetooth", "embedded", "firmware", "runtime", "zephyr"]
not_affected_if: ["default-config", "single-advertising-set"]
published_at: "2026-10-26T17:17:30+00:00"
first_seen_at: "2026-10-26T17:31:22+00:00"
updated_at: "2026-10-26T17:33:30+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-81039", "first_seen_at": "2026-10-26T17:31:22+00:00"}]
---

# Zephyr Bluetooth host: remote peer can exhaust connection slots, blocking new connections

The Zephyr Bluetooth host mis-attributes an incoming connection to the wrong pre-allocated advertising reservation when two extended advertising sets share an identity. A nearby attacker can repeat this until the connection pool is exhausted, so the device stops accepting Bluetooth connections.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check whether `CONFIG_BT_EXT_ADV_MAX_ADV_SET` is greater than 1 and whether the application runs an undirected and a directed connectable extended advertising set on the same identity.
- Check the Zephyr version in use against the fixed release listed in the NVD entry; unknown if the sources do not give it.

## Fix

- Upgrade Zephyr to a release containing the fix for this issue; the fixed version is unknown from the sources.
- Until patched, run a single connectable advertising set per identity (`CONFIG_BT_EXT_ADV_MAX_ADV_SET=1`) or avoid running a directed set alongside an undirected one.

## Timeline

- 2026-10-26T17:31:22+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-81039

## From the sources

The Bluetooth host pre-allocates a bt_conn object (a "reservation") from the fixed acl_conns[] pool for every connectable advertising set it starts, in le_adv_start_add_conn() in subsys/bluetooth/host/adv.c. When an incoming peripheral connection completes, find_pending_connect() in subsys/bluetooth/host/hci_core.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 CONFIG_BT_EXT_ADV_MAX_ADV_SET > 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 BT_ADV_ENABLED flag has already been cleared, and le_adv_stop_free_conn() — the only path that returns a reservation to the pool — is reached only for sets that are still enabled, so bt_le_ext_adv_stop() returns early and the object is never released. Each occurrence permanently consumes one CONFIG_BT_MAX_CONN slot until bt_disable() 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 bt_hci_disconnect(). The impact is availability only; no memory corruption or information disclosure results, and configurations with a single advertising set are unaffected.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_