Linux Kernel 6LoWPAN Bug CVE-2025-40282 Patch Prevents Crashes

A subtle but real kernel bug affecting Bluetooth 6LoWPAN support in Linux — tracked as CVE-2025-40282 — has been identified and patched: the kernel failed to reset the link-local (MAC) header on received IPv6 packets in the uncompressed receive path, allowing a crafted packet to trigger a kernel BUG and crash the host when the 6LoWPAN Bluetooth stack and packet sockets are present.

Background​

Bluetooth Low Energy (BLE) has grown beyond simple device pairing; it now carries IPv6 traffic through the 6LoWPAN adaptation layer so extremely low-power devices can participate in IP networks. Linux supports this via a Bluetooth 6LoWPAN subsystem that implements packet handling and header compression/decompression for IPv6 over BLE.
The Linux networking stack represents packets using the sk_buff structure (commonly called skb). When a network device has header operations (header_ops) defined, the kernel and upper-layer consumers expect the skb to have its MAC/header pointers properly initialized for received packets. Failing to set the MAC header on RX skbs can leave internal skb pointers inconsistent; operations that walk headers or assume the MAC header is set can then index invalid offsets, ultimately leading to kernel-level assertions or BUGs.
The vulnerable code path sits in net/bluetooth/6lowpan.c in the channel receive callback. The compressed IPv6 receive path already handled header pointers correctly via the header-decompression routine; the uncompressed IPv6 receive path did not set the link-local (MAC) header before marking the transport header, creating a race/consistency problem. The upstream fix is a minimal one-line insertion that calls skb_reset_mac_header on the local skb before calling skb_set_transport_header.

What happened (technical summary)​

• The Bluetooth 6LoWPAN network device is registered with header_ops, meaning the kernel expects received skbs to have a MAC/header offset.
• In the uncompressed IPv6 receive branch of the 6LoWPAN receive routine, the code created a local skb (a copy used by the host stack) but did not call skb_reset_mac_header to initialize the MAC/header offset for that skb.
• Downstream code — including packet sockets (AF_PACKET, especially SOCK_RAW) and other parts of the network stack — assumes the MAC header is set when header_ops exist. The missing initialization led to an invalid header offset, which in some conditions triggered a kernel BUG (observed at net/core/skbuff.c:212 in test traces) and a kernel panic/crash or oops.
• The compressed-path already set the header correctly during header decompression; the bug was limited to the uncompressed receive path.
• The upstream patch makes a single insertion to call skb_reset_mac_header(local_skb) before the transport header is set.

This is a classic bookkeeping bug in kernel networking code: the fix is tiny and mechanical, but the fault traversed several layers and can cause total host instability.

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Affected systems and attack surface​

• Impacted component: Linux kernel Bluetooth 6LoWPAN subsystem (net/bluetooth/6lowpan.c).
• Typical platforms: any Linux system built with 6LoWPAN support and Bluetooth 6LoWPAN enabled (CONFIG_6LOWPAN and CONFIG_BT_6LOWPAN), including desktops that enable the feature, many embedded Linux distributions, IoT gateways, routers, and specialized appliances that expose Bluetooth LE 6LoWPAN.
• Module names: the kernel builds the 6LoWPAN core and Bluetooth 6LoWPAN as modules when configured as modular. Common module names include 6lowpan and bluetooth_6lowpan.
• Preconditions for exploit:
• The kernel must include Bluetooth 6LoWPAN support (module loaded or built-in).
• The device must accept IPv6 packets over Bluetooth LE (6LoWPAN in use).
• A remote attacker must be capable of sending crafted BLE 6LoWPAN frames to the target device — i.e., they need radio proximity (BLE is short range).
• Packet socket code paths (AF_PACKET, SOCK_RAW) or other consumers that assume a valid MAC header must process the arriving skb in the vulnerable code path.

Severity and scope notes:

• The behavior observed and fixed is a kernel BUG leading to crash / oops, so the immediate impact is denial of service (host crash).
• There is no public evidence that this bug enables arbitrary code execution or privilege escalation; the primary impact is stability. However, any kernel crash on production infrastructure can be used as part of broader attacks (availability impact, induced reboots, potential complications with crash dumps and logs).
• Exploitation requires wireless proximity and that the 6LoWPAN feature is enabled — that means broad internet-scale remote exploitation is unlikely without intermediate access to Bluetooth radio; however, IoT devices and gateways with BLE reachability are realistic targets.

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Why the bug matters — deeper analysis​

At first glance this is a minimal, one-line patch: insert a call to skb_reset_mac_header on the freshly-prepared RX skb. That simplicity, though, masks two important realities:

• Small coding omissions in low-level networking glue can cross multiple layers and produce systemic instability. The sk_buff layout is central to Linux networking, and many subsystems (netdev header_ops, packet sockets, rtnetlink consumers, offload/csum helpers) rely on its internal offsets being correct.
• Many IoT devices and embedded Linux builds enable 6LoWPAN and Bluetooth 6LoWPAN to support constrained devices. Those systems often run long-lived kernels with limited ability to update quickly. A mechanical bug like this therefore has outsized risk: many devices have the vulnerable code but may not get timely kernel updates.

From the attacker model perspective:

• Proximity is required: the attacker must deliver malformed or specially-constructed 6LoWPAN IPv6 frames over BLE.
• No local user privilege on the target device is required prior to the attack.
• The presence of AF_PACKET SOCK_RAW sockets or packet socket consumers increases the likelihood that the incorrect skb header offset will be used and a crash will occur.

Operationally, the bug is a reliability / availability issue rather than a stealthy data-exfiltration vector. Still, availability failures in infrastructure or gateway devices can cause substantial downstream impact.

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Confirmed fix and recommended remediation​

The upstream kernel team accepted a patch that adds the missing skb_reset_mac_header(local_skb) call in the uncompressed IPv6 receive path in net/bluetooth/6lowpan.c. The change is intentionally small, localized, and low-risk from a regression standpoint.
Action plan for system administrators and device maintainers:

• Prioritize kernel updates that include the upstream fix.
• Apply distribution-provided kernel updates as soon as they include the patched commit or backport.
• If you run custom kernels, merge/apply the upstream patch into your kernel tree and rebuild; the patch is tiny but must be applied against the correct tree and tested.
• If you cannot patch immediately, mitigate by disabling the Bluetooth 6LoWPAN functionality or unloading its module:
• Check whether the module is present:
• Run: uname -r to identify the running kernel.
• Run: lsmod | grep -E "6lowpan|bluetooth_6lowpan" to see if the modules are loaded.
• Unload the module (temporary; will not survive reboot):
• Run as root: modprobe -r bluetooth_6lowpan
• If the module is built-in (not modular), you cannot unload it without rebooting into a patched kernel.
• Prevent the module from loading on boot (workaround):
• Create a blacklist file: echo "blacklist bluetooth_6lowpan" > /etc/modprobe.d/blacklist-6lowpan.conf
• Rebuild initramfs if necessary for your distribution so the module is not loaded early.
• Fully disable 6LoWPAN via kernel configuration for custom builds (set CONFIG_BT_6LOWPAN=n and CONFIG_6LOWPAN=n and rebuild kernel).
• If Bluetooth service itself is not required, consider disabling Bluetooth system-wide until a patch is available:
• Stop the Bluetooth daemon (BlueZ) and mask its service: systemctl stop bluetooth; systemctl mask bluetooth
• For embedded devices where disabling is not practical, vendor-supplied firmware/kernel updates are the right path.
• Request vendor or distribution support:
• For embedded appliances or commercial products, request that the vendor provide a patched kernel or an official mitigation. Many vendors backport kernel fixes, but timeliness varies.
• Test and validate any update:
• After applying a patch or updating the kernel, validate Bluetooth, 6LoWPAN behavior, and general networking to ensure no regressions.

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Detection and monitoring guidance​

Look for evidence that an unpatched system has been hit or is unstable:

• Kernel logs and crash traces:
• Search dmesg and syslog/journal for indicators like "kernel BUG at net/core/skbuff.c:212", "net/bluetooth/6lowpan.c", "chan_recv_cb", or "6lowpan".
• Example search commands:
• journalctl -k | grep -i skbuff
• dmesg | grep -E "skbuff|6lowpan|chan_recv_cb|net/bluetooth"
• Packet socket or AF_PACKET errors:
• Logs from packet monitoring tools or custom daemons that use raw sockets may show failures around the time of the kernel oops.
• Increase monitoring for unexplained reboots or OOM/crash patterns on devices known to have Bluetooth 6LoWPAN enabled.
• Consider enabling persistent kdump or crash collection on critical systems so that a kernel OOPS can be captured and analyzed.

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Who should be most concerned?​

• IoT gateway and hub maintainers: these devices often bridge BLE networks to IPv6 and are likely to build in 6LoWPAN support.
• Embedded device vendors and integrators that ship Linux kernels with CONFIG_6LOWPAN and CONFIG_BT_6LOWPAN enabled.
• System administrators who run Linux servers or appliances with Bluetooth support enabled in environments where BLE devices are used (e.g., industrial control, medical, building automation).
• Security operations teams in organizations that rely on BLE-based sensors and actuators.

Conversely, most desktop users who do not use Bluetooth 6LoWPAN (the feature is relatively niche compared to standard Bluetooth profiles) are at low risk unless they explicitly enable 6LoWPAN support or load the module.

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Risk assessment and timeline considerations​

• The bug is fixed with a simple kernel patch; this is good news because it lowers the chance of complex regressions and makes backporting straightforward.
• However, many IoT and embedded devices run kernels that are rarely updated; those are the highest-risk population because the patch may never be applied by the device OEM.
• The attack vector requires radio proximity and an attacker capable of sending crafted BLE 6LoWPAN frames. That limits remote internet-driven exploitation, but local adversaries (malicious visitors, compromised devices in the same radio domain, or attackers near the physical premises) can mount attacks.
• Given the straightforward nature of the fix, vendors and distributions should be able to push updates quickly, but operational realities (testing, certification, firmware cycles) can delay rollouts. Prioritize devices that serve as gateways or that run as always-on infrastructure.

Caveats and unverifiable points:

• There is no public proof-of-concept exploit showing remote widespread exploitation beyond the presented kernel BUG traces. While the defect produces reproducible crashes in testing, reports of real-world exploitation have not been substantiated at this time.
• The CVSS score and other severity ratings may not be assigned or finalized by all trackers yet; organizations should base prioritization on device exposure and criticality rather than on a single severity number.

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Practical checklists​

Immediate checklist (first 24–72 hours):

• Inventory: list devices and servers with Bluetooth/6LoWPAN enabled. Priority: gateways, routers, and always-on BLE hubs.
• Determine whether the bluetooth_6lowpan module is present and loaded with lsmod.
• If immediate patching is not possible, temporarily unload and blacklist bluetooth_6lowpan where practical.
• If you run managed fleets, submit an update plan to push patched kernels or vendor firmware updates.

Patch rollout checklist (post-patch):

• Apply vendor or distribution kernel updates that include the upstream fix.
• Reboot devices in a controlled manner and validate networking and BLE endpoints.
• Monitor kernel logs for residual errors.
• Re-enable any temporarily disabled modules after confirming the update.

Hardening checklist:

• Limit Bluetooth radio reachability where possible (physical controls).
• Enforce device pairing policies; do not leave devices in discoverable or open-pairing modes unnecessarily.
• Use host-based monitoring to detect kernels oops and reboots tied to network reception.

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Why this is a useful reminder for kernel and IoT ops​

This vulnerability illustrates three persistent truths for modern IT operations and embedded device management:

• Tiny, low-level bookkeeping errors in transport or header handling can have outsized operational impact when they propagate into core kernel data structures.
• The adoption of IP on unconventional media (IPv6 over BLE via 6LoWPAN) brings the classic IP attack surface close to physical proximity; gateways bridging between networks merit special protection.
• Fast, simple fixes in upstream code do not automatically translate into rapid improvement in the field because device fleets and embedded appliances often depend on vendor-specific update cycles.

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Conclusion​

CVE-2025-40282 is an example of a minimal-but-critical correctness bug: a missing skb header reset in the uncompressed IPv6 receive path of Bluetooth 6LoWPAN that can cause kernel BUGs and host crashes. The fix is straightforward and small at the code level, but the practical risk exists because many devices run older kernels or have 6LoWPAN enabled in embedded contexts.
System owners should treat this as a priority for devices that:

• act as gateways between BLE sensor networks and IP infrastructure,
• run with Bluetooth 6LoWPAN enabled, or
• are publicly reachable by BLE devices (exposed in physical environments).

Immediate mitigation options include unloading or blacklisting the bluetooth_6lowpan module, disabling BlueZ/6LoWPAN functionality where it is not required, and fast-tracking kernel updates or vendor patches. For devices that cannot be updated quickly, compensate with increased logging, crash collection, and physical-radio access controls.
The technical lesson is clear: confirm the kernel networking invariants (like properly initialized skb header offsets) in every receive path, and treat packet-handling code in adapters and bridges with high scrutiny — small oversights there can crash the entire system.

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Source: MSRC Security Update Guide - Microsoft Security Response Center