CVE-2026-43021 Overview
CVE-2026-43021 is a memory leak vulnerability in the Linux kernel's Bluetooth subsystem. The flaw resides in the hci_sync component, specifically in the hci_cmd_sync_queue_once() function. When this function returns an error, the destroy callback is not invoked, leaving allocated references and memory unfreed.
The vulnerability has been resolved upstream through kernel commits 7fd74178d4b1 and aca377208e7f. While not classified as a code execution flaw, repeated triggering of the error path can exhaust kernel memory resources over time, leading to system instability.
Critical Impact
Repeated failures of hci_cmd_sync_queue_once() leak kernel memory and reference counts, which can degrade system stability and contribute to denial-of-service conditions on affected Linux hosts using Bluetooth.
Affected Products
- Linux kernel versions containing the vulnerable hci_sync Bluetooth implementation
- Distributions shipping the affected kernel prior to upstream patch integration
- Systems with Bluetooth (BlueZ/HCI) functionality enabled
Discovery Timeline
- 2026-05-01 - CVE-2026-43021 published to NVD
- 2026-05-01 - Last updated in NVD database
Technical Details for CVE-2026-43021
Vulnerability Analysis
The vulnerability exists in the Linux kernel Bluetooth Host Controller Interface (HCI) synchronization code. The function hci_cmd_sync_queue_once() queues commands for synchronous execution against the Bluetooth controller. When the queue operation succeeds, a destroy callback later releases resources tied to the queued command.
When hci_cmd_sync_queue_once() returns an error before the command is successfully queued, the destroy callback is never invoked. This leaves heap-allocated memory and held references stranded with no cleanup path. Each failed call increases the leaked footprint.
The EPSS score for this issue is 0.024% with a percentile of 6.841, reflecting low likelihood of opportunistic exploitation in the wild.
Root Cause
The root cause is missing error-path cleanup. The original implementation assumed the destroy callback would always run after hci_cmd_sync_queue_once(), regardless of return status. The patch corrects this by ensuring resources are released directly on the error path, removing the leak.
Attack Vector
The attack vector is local. A user or process able to interact with the Bluetooth stack can trigger conditions causing repeated failures of hci_cmd_sync_queue_once(). Sustained failures gradually consume kernel memory. The issue is a reliability and resource-exhaustion concern rather than a path to code execution or privilege escalation.
No public proof-of-concept code is available. Refer to the upstream commits 7fd74178d4b16dcf47179da634ea9d7c02e3608b and aca377208e7f7322bf4e107cdec6e7d7e8aa7a88 for the exact code change. See Kernel Git Commit (7fd74178) and Kernel Git Commit (aca37720) for technical details.
Detection Methods for CVE-2026-43021
Indicators of Compromise
- Gradual increase in kernel slab allocator usage tied to Bluetooth-related caches over time.
- Repeated kernel log entries from the hci_sync subsystem indicating command queue failures.
- Unexplained memory pressure on Linux hosts that actively use Bluetooth.
Detection Strategies
- Track kernel memory consumption via /proc/meminfo and /proc/slabinfo for sustained growth without workload increase.
- Monitor dmesg and journald for Bluetooth HCI error patterns produced by the affected code path.
- Compare installed kernel package versions against distribution advisories listing the fix.
Monitoring Recommendations
- Collect kernel version inventory across Linux endpoints and servers and flag hosts running unpatched kernels.
- Alert on abnormal kernel memory growth on hosts where Bluetooth is in active use, including embedded and IoT systems.
- Audit which systems require Bluetooth and disable the bluetooth kernel module where it is not needed.
How to Mitigate CVE-2026-43021
Immediate Actions Required
- Apply vendor kernel updates that include commits 7fd74178d4b1 and aca377208e7f once available from your Linux distribution.
- Identify systems running affected kernel versions and prioritize patching for hosts using Bluetooth.
- Disable Bluetooth on systems where it is not required to remove exposure to the vulnerable code path.
Patch Information
The upstream fix is available in the stable kernel tree. The change ensures that when hci_cmd_sync_queue_once() returns an error, allocated memory and references are released directly rather than relying on the destroy callback. Apply the corresponding distribution kernel update from Debian, Ubuntu, Red Hat, SUSE, or other vendor security feeds. Reference: Kernel Git Commit (7fd74178) and Kernel Git Commit (aca37720).
Workarounds
- Unload the Bluetooth kernel module on systems that do not need it using rmmod bluetooth and blacklist it persistently.
- Restrict access to Bluetooth management interfaces to privileged users only.
- Reboot affected systems periodically until patching is complete to reclaim leaked memory.
# Configuration example: disable Bluetooth where not required
sudo systemctl stop bluetooth.service
sudo systemctl disable bluetooth.service
echo "blacklist bluetooth" | sudo tee /etc/modprobe.d/blacklist-bluetooth.conf
echo "blacklist btusb" | sudo tee -a /etc/modprobe.d/blacklist-bluetooth.conf
sudo update-initramfs -u
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
