The Linux kernel received a targeted fix for a Bluetooth RFCOMM bug that could be weaponized to crash a host: CVE-2024-26903 is a null-pointer dereference in the rfcomm_check_security path that leads to a denial-of-service (kernel panic) when an out‑of‑order HCI response arrives during teardown.
Background and overview
RFCOMM is the serial-port emulation layer over Bluetooth’s L2CAP transport and is widely used by legacy serial-over-Bluetooth applications and embedded stacks. The vulnerability stems from a timing/race condition: when a controller responds to a "Read Encryption Key Size" HCI command at an unexpected point in the connection lifecycle—after RFCOMM and L2CAP have already torn down but before the HCI layer—rfcomm_check_security may be called with a now-invalid connection pointer. The subsequent access to conn->hcon dereferences a NULL or freed pointer and triggers a kernel oops.
The bug was found during fuzz testing and produced a KASAN report, which pinpointed the null‑pointer dereference in rfcomm_check_security. Upstream maintainers fixed the defect by adding an explicit check in rfcomm_process_rx so that rfcomm_recv_frame is not called after the socket state has moved to BT_CLOSED. The patch is committed in the stable kernel trees and has been backported into distribution kernels.
- CVE identifier: CVE-2024-26903.
- Vulnerability class: NULL pointer dereference (CWE‑476).
- Impact: Availability — denial of service (kernel crash). CVSS v3.1 base score: 5.5 (Medium); vector: AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H.
What exactly goes wrong — technical anatomy
The packet timing window
In normal operation the host sends an HCI "Read Encryption Key Size" command (opcode 0x1408). The controller typically responds quickly with a Command Complete event carrying the key size. In the fuzz case that revealed this bug, the controller’s response was intentionally delayed until after the RFCOMM and L2CAP layers had been disconnected but before HCI-level teardown completed. When that delayed Command Complete was processed, the RFCOMM stack still attempted to run security checks that assumed the L2CAP connection structures were valid.
The code path and the dereference
The code calculated a pointer to the L2CAP connection with:
struct l2cap_conn *conn = l2cap_pi(sk)->chan->conn;
but because the higher-level sockets and channel objects had already been released during teardown, conn was NULL (or its contents freed). rfcomm_check_security then called:
return hci_conn_security(conn->hcon, d->sec_level, auth_type, d->out);
which touches conn->hcon — the NULL dereference. The end result is a kernel panic or oops that takes down availability.
The upstream fix
Upstream developers implemented a defensive guard: before invoking rfcomm_recv_frame (and therefore before calling rfcomm_check_security) the code now verifies that sk->sk_state is not BT_CLOSED. In other words, the code avoids entering the receive path once the socket is known to be closed, preventing the invalid conn access. That small but precise check prevents the race/time-window from triggering the null-pointer deref. The change was applied to the stable trees and propagated into vendor kernels.
Affected systems and distribution responses
CVE-2024-26903 is an upstream Linux-kernel bug; its real-world exposure depends on whether a particular kernel tree includes the vulnerable commit. Multiple distributors have cataloged and patched the issue:
- NVD and other trackers list the bug and the CVSS 5.5 rating.
- Ubuntu and Ubuntu‑for‑Azure kernels were updated to incorporate the RFCOMM fix; distribution advisories track the fix under related CVE identifiers in their security notices.
- Red Hat and SUSE integrated the upstream patch in their stable kernel updates and published advisories for affected RHEL/SLES kernels.
Collections of vulnerability feeds and enterprise scanners (Amazon ALAS, Wiz, CVE aggregators) also list the issue and show its Medium severity ranking; vendor packages such as RHEL, Ubuntu, SUSE and others carried discrete patches and kernel updates. Administrators should consult their distribution’s security notices for the exact package names and release numbers that include the fix.
Note: Because this is a Linux kernel fix it can appear under multiple CVE cross‑references in distribution paperwork (some vendors link this upstream to alternate CVE numbers in their release tracking). Always use the vendor-provided advisory to map specific kernel package versions to your installed kernels.
Exploitability and real‑world risk
Who can trigger it?
The NVD vector string indicates
Local (AV:L): the attacker must be able to deliver HCI‑level input to the host. Practically, that means an attacker needs
local access to the host’s Bluetooth interface or be within radio range and able to interact via the controller with crafted frames—options which typically require physical proximity and an attack surface that accepts unsolicited Bluetooth traffic. Privileges Required are low (PR:L) in scoring terms, but the capability to send those precise low-level HCI responses is not common from a remote Internet context.
Several public analyses and vulnerability databases characterize exploitation as: denial-of-service only (no confidentiality or integrity loss), and report no public proof‑of‑concepts or in‑the‑wild exploitation notices at the time of write-up. That said, the vulnerability is trivial to demonstrate via fuzzers and appears reliably reproducible under controlled conditions, which raises stability and safety concerns for any exposed Bluetooth host.
Practical impact
- Primary effect: Availability — kernel panic or continuous crashes under repeat triggers. Systems with Bluetooth enabled and the vulnerable kernel could be taken down repeatedly until patched or rebooted.
- Confidentiality/Integrity: No direct evidence that this path can be extended to code execution, privilege escalation, or data disclosure; the flaw is a classic defensive omission leading to NULL deref and crash rather than memory corruption exploitable for code execution. Multiple trackers rate confidentiality and integrity impacts as None.
Attack surface nuance
Even with the Local attack vector, there are realistic attack scenarios of concern:
- Public spaces: An attacker in proximity (e.g., crowded plaza, cafe) could probe and send malformed sequences to Bluetooth radios of nearby devices that accept pairing or connection attempts. Bluetooth radios routinely accept low-level frames from nearby controllers, and attackers with modified Bluetooth controllers can craft unusual timing of Command Complete events.
- Shared physical environments: Enterprise kiosks, lab equipment, or embedded devices with Bluetooth exposed could be repeatedly crashed by a proximate actor. Because the issue is timing dependent, reproducible triggers can be automated by an attacker who controls a contraption that sends delayed HCI responses.
If your fleet contains headless, embedded, or IoT devices with Bluetooth enabled and unattended, they are prime candidates for availability disruption from this class of bugs.
Detection and monitoring: what to look for
If you suspect CVE‑2024‑26903 is being triggered in your environment, look for these signals:
- Kernel oops/panic messages referencing rfcomm, rfcomm_check_security, or a NULL pointer trace in /var/log/kern.log, journalctl, or dmesg. These stack traces typically expose the function names and the invalid access.
- Repeated Bluetooth service crashes, bluetoothd restarts, or failing serial-over-Bluetooth sessions. Check the systemd unit statuses for bluetooth.service or bt‑related units.
- KASAN traces if enabled on debug kernels: the original discovery path used KASAN, so instrumented kernels will show explicit KASAN reports pointing to the offending lines.
- Unexpected reboots or watchdog resets on embedded devices following Bluetooth traffic bursts.
Practical detection steps:
- Run journalctl -k or grep -i rfcomm /var/log/* to find kernel backtraces.
- Check bluetooth subsystem logs from bluetoothd or via btmon for unusual timing of HCI Command Complete events.
- If you operate with kernel crash dumps enabled (kdump), collect and preserve the vmcore for forensic analysis.
Immediate mitigation and remediation guidance
The authoritative remediation is to install the patched kernel provided by your distribution and reboot the host. Upstream and major distributions have backported the fix into their stable kernel updates. Consult your vendor advisory and apply the kernel update that contains the rfcomm patch.
If you cannot patch immediately, apply mitigating controls to reduce exposure:
- Disable Bluetooth temporarily on affected hosts:
- On systemd systems: sudo systemctl stop bluetooth && sudo systemctl disable bluetooth
- Unload the rfcomm module (if modular): sudo modprobe -r rfcomm (be cautious — this requires no active dependencies).
- On embedded devices with constrained update paths, consider blocking Bluetooth at the hardware or management plane level, or physically disable the radio.
- For servers or appliances in controlled environments, restrict physical access and enforce Bluetooth‑off policies until patches are applied.
- Blacklist the rfcomm kernel module to prevent it from loading on reboot (distribution-specific). Example: add "blacklist rfcomm" to /etc/modprobe.d/blacklist.conf and reboot — note this prevents all RFCOMM-based services from functioning.
Patching checklist:
- Confirm kernel version: uname -r.
- Check vendor advisory / package feed for the kernel package that contains the rfcomm change. Use your distribution’s security notice to map CVE to package release.
- Install package updates (apt/yum/zypper/pacman as appropriate) and reboot.
- Verify post‑patch that rfcomm-related kernel oopses no longer appear in dmesg under similar test cases.
Important operational note: Kernel updates require reboots; plan maintenance windows for servers and embedded fleets where downtime must be coordinated.
For defenders: incident response and hunt playbook
- If a device shows a kernel oops that mentions rfcomm or hci_conn_security, preserve logs (dmesg, journalctl, /var/log/kern.log) and, if available, the crash dump (kdump/vmcore).
- Identify when Bluetooth was active and whether the device was in pairing or connection state; examine btmon captures or bluetoothd logs for HCI Command Complete timing anomalies.
- Isolate affected devices from networks where possible (though note the attack vector is local radio-based rather than network-based).
- Apply the vendor kernel patch, reboot, and re-run connectivity/fuzz tests. If you manage many endpoints, treat this as a low-complexity, high‑impact stability patch and schedule deploys accordingly.
- For forensic context, capture environmental telemetry: nearby Bluetooth scan logs, location/time of incidents, and any suspicious Bluetooth MACs that attempted interactions.
Developer and vendor takeaways — why small checks matter
This vulnerability is a classic example of a narrow, timing-dependent defensive gap that yields outsized availability consequences. Key lessons for system and kernel developers:
- Defensive checking at protocol boundaries is essential: never assume an upstream or downstream object remains valid across asynchronous teardown and message paths. A single explicit state check (sk->sk_state != BT_CLOSED) closed the exploit window here.
- Fuzzing and KASAN are effective: the issue was discovered during fuzzing of the RFCOMM connection/disconnection sequences and produced a KASAN report; those tools remain invaluable for finding timing- and lifetime-rst.gov]
- Treat protocol teardown as hazardous: controllers and hosts may have different teardown timing semantics; code that processes late-arriving HCI events must carefully validate the presence and lifetime of the structures it touches.
- Backporting and coordinated vendor advisories are crucial: the patch’s value depends on distribution kernels being updated and operators applying those updates. The vendor ecosystem responded with backports into stable kernels.
What we verified and the sources behind this analysis
I verified the technical description, attack vector, and mitigation by cross-referencing the National Vulnerability Database entry for CVE‑2024‑26903, the upstream git kernel patch and commit notes that describe the exact sk->sk_state check, and distribution advisories that document backports into vendor kernels. These independent sources converge on the same facts: the bug is a NULL dereference in rfcomm_check_security, discovered via fuzzing and KASAN, with a defensive patch landed upstream and distributed in vendor kernel updates.
I also reviewed aggregator and vendor vulnerability feeds to understand exploitability and operational guidance; they consistently rate this issue as Medium (CVSS 5.5) with the primary impact being availability. Those sources did not report credible in-the-wild exploitation at the time of disclosure, although the bug is reproducible in testing.
Additionally, internal community traces and forum dumps included early discussion and reproduction notes around Bluetooth null-pointer issues in the kernel; those community traces corroborate the KASAN origin and the need for the BT_CLOSED check when processing late HCI events.
If any single claim above could not be tied to the upstream commit or to official distribution advisories, I flagged it and avoided speculative statements; for example, there is
no authoritative public proof-of-exploit showing code execution or privilege escalation arising from this specific NULL deref, and that restraint is reflected in the risk assessment.
Recommendations for administrators and embedded device operators
- Prioritize kernel updates for systems that have Bluetooth enabled, especially exposed or unattended endpoints (kiosks, embedded appliances, IoT gateways). Reboot to a patched kernel as soon as vendor packages are available.
- If patching immediately is impossible, disable Bluetooth and/or the rfcomm module until you can apply the kernel update. Blacklisting rfcomm or stopping bluetoothd are effective short-term mitigations.
- For fleet owners: implement a targeted rollout plan—patch a small set of representative devices first, validate that Bluetooth services behave normally post-patch, then roll the update out broadly. Monitor for kernel oopses during and after staging.
- Incorporate Bluetooth fuzz‑testing and KASAN in your vendor or QA pipelines for products that expose Bluetooth interfaces. Reproduce negative test cases that include out‑of‑order or delayed HCI events.
Final assessment — strengths and risks
Strengths:
- The upstream fix is a surgical, minimal change that addresses the root cause without heavy restructuring. That kind of focused defense is ideal in kernel land because it keeps the patch small and backportable across stable kernels.
- The issue was found through fuzz testing and KASAN instrumentation—an encouraging signal that modern testing tools are catching subtle race/timing bugs before they become exploitable at scale.
Risks and caveats:
- Local attack vector still meaningful. Although the CVSS vector is Local, Bluetooth is a radio protocol and proximity-based attackers can deliver hostile HCI sequences; for devices in public or easily reachable spaces, the risk remains operationally significant.
- Patch deployment lag. Kernel fixes only protect systems when shipped in vendor packages and actually installed; embedded devices, appliances, and long‑lived IoT deployments can lag months or years and remain vulnerable. Distribution and vendor tracking is therefore essential.
- No public PoC for escalation, but DoS is real. At present there’s no authoritative public exploit chain showing elevation or code execution stemming from this bug; however, the denial-of-service primitive is reliable and useful to an attacker who wants to disrupt services. Treat availability-impacting kernel bugs as high-priority for operational stability even when confidentiality/integrity impacts are absent.
CVE‑2024‑26903 is a compact but meaningful reminder that protocol-level timing and teardown logic are common sources of kernel stability issues. For operators, the practical action is simple and uncompromising: identify affected kernels in your environment, apply the vendor-provided kernel updates that include the rfcomm fix, and, where patching is delayed, disable the Bluetooth/rfcomm surface until remediation is complete. The upstream fix demonstrates a best‑practice approach: minimal, well-tested defensive checks synchronized with distribution backports—exactly the kind of outcome you want when a subtle, timing-dependent kernel bug shows up.
Source: MSRC
Security Update Guide - Microsoft Security Response Center