A straightforward NULL-pointer bug in the Linux NFC stack — fixed upstream in the 6.5.9 stable release — created a local denial‑of‑service risk that could crash kernels handling Near‑Field Communication traffic; the defect was tracked as CVE‑2023‑46343 and closed by a one‑line defensive check in the NFC NCI SPI code.
The vulnerability lives in the Linux kernel’s NFC (Near‑Field Communication) Controller Interface implementation, specifically in the SPI transport path for NCI devices. The problematic code was located in net/nfc/nci/spi.c inside the function named send_acknowledge, where a failure to handle an allocation function returning NULL could allow the kernel to dereference a NULL pointer and crash the host. This exact problem is described in the authoritative CVE entry and was remedied in the 6.5.9 stable kernel release.
What changed in the upstream patch is deliberately small: maintainers added a defensive check to handle allocation failure from the NCI helper (nci_skb_alloc / alloc_skb), avoiding the subsequent dereference when memory allocation fails. The change was credited to the reporter and merged as part of the 6.5.9 changelog. (cdn.kernel.org)
Source: MSRC Security Update Guide - Microsoft Security Response Center
Background / Overview
The vulnerability lives in the Linux kernel’s NFC (Near‑Field Communication) Controller Interface implementation, specifically in the SPI transport path for NCI devices. The problematic code was located in net/nfc/nci/spi.c inside the function named send_acknowledge, where a failure to handle an allocation function returning NULL could allow the kernel to dereference a NULL pointer and crash the host. This exact problem is described in the authoritative CVE entry and was remedied in the 6.5.9 stable kernel release.What changed in the upstream patch is deliberately small: maintainers added a defensive check to handle allocation failure from the NCI helper (nci_skb_alloc / alloc_skb), avoiding the subsequent dereference when memory allocation fails. The change was credited to the reporter and merged as part of the 6.5.9 changelog. (cdn.kernel.org)
What the bug is — technical summary
How the fault is triggered
- The NCI over SPI receive/send path creates transient kernel networking buffers using an internal helper (nci_skb_alloc that in turn calls alloc_skb()).
- If that allocation fails and returns NULL, the existing send_acknowledge implementation proceeded to dereference the returned pointer without checking it.
- The dereference of NULL inside kernel space produces a kernel oops or panic, yielding an immediate denial‑of‑service condition for the affected host.
Why this is important
- The bug is a classic NULL pointer dereference (CWE‑476): conceptually simple, often trivial to trigger under certain conditions, and reliably produces host instability when hit.
- While the defect requires a path that reaches the NFC SPI driver — meaning it is scoped to machines with NFC hardware or emulation — on systems where the NFC stack is present it gives a local or adjacent attacker the ability to crash the kernel. Multiple vulnerability trackers and vendor advisories classify this as a medium‑severity problem (CVSS v3.1 base score ~5.5) with high availability impact.
Timeline and upstream patch details
- Reporter: The issue was reported by a security researcher (credited in the upstream commit). The kernel changelog notes the reporter and the specific commit that patches the problem. (cdn.kernel.org)
- Upstream patch: A targeted change was merged into the net/nfc/nci tree to add a NULL check after the allocation helper and return an appropriate error code on allocation failure. The commit message is explicit: “Handle memory allocation failure from nci_skb_alloc() (calling alloc_skb()) to avoid possible NULL pointer dereference.” (cdn.kernel.org)
- Stable release: The fix was shipped in Linux kernel 6.5.9; the stable changelog and the kernel commit history record the change. (cdn.kernel.org)
- Vendor advisories and downstream coverage: Several vendors and security trackers recorded the CVE, produced advisories or backports, and incorporated detection into scanners shortly after public disclosure. Distribution security trackers list patched packages or kernel updates for their images.
Impact analysis — availability, exploitability, and scope
Availability impact
The defect produces a direct availability impact: a kernel null‑dereference typically results in an oops or panic that crashes the host or triggers recovery mechanisms. For systems that load the NFC NCI SPI driver, the attack can be sustained while the attacker continues to send malformed NFC traffic that exercises the vulnerable code path. The practical consequence is an immediate and repeatable denial of service. Multiple vulnerability repositories list Availability: High as the principal impact.Exploitability and access vector
- Primary attack vector is local or adjacent: the NFC subsystem commonly exposes an adjacent attack surface (physical proximity or a local process that controls NFC frames). The NVD lists the vector as local with low privileges required, and vendors have different nuance — some label the vector as adjacent network depending on how NFC hardware is reachable. In short, exploitation is feasible but requires access to the NFC transport path.
- Ease: Because the issue is a missing NULL check, triggering it does not require complicated state manipulation beyond causing nci_skb_alloc to fail in the path used by send_acknowledge. That can be achieved with crafted input or resource exhaustion under the right conditions, which is why the fix is simple and prioritized. (cdn.kernel.org)
Confidentiality and integrity
Public CVE records and vendor write‑ups attribute no confidentiality or integrity loss to this bug beyond availability. There is no confirmed public evidence that this NULL dereference can be leveraged directly for code execution or privilege escalation; theoretical escalation beyond DoS is sometimes discussed in secondary write‑ups, but such claims are speculative unless a separate memory corruption path is present. Where commentators note a possible escalation, they label it cautionary rather than demonstrated. Treat code‑execution claims as unverified unless exploit proof is published.Who is affected
- Any Linux system running a kernel version earlier than 6.5.9 that includes the NCI NFC SPI code path is potentially affected. This is primarily systems with NFC controllers using the NCI over SPI transport, common in some mobile and embedded platforms and in certain laptops/tablets that expose NFC hardware.
- Many mainstream Linux distributions recorded the CVE in their trackers and provided patched kernel packages or backports; operators running vendor kernels should consult vendor advisories and apply distribution updates as appropriate. Vendor trackers and security lists show fixed package versions or that the change is included in the 6.5.9 upstream release from which backports can be derived.
Detection, confirmation, and triage
How to confirm whether your host is vulnerable
- Check the running kernel version:
- Run uname -r and verify whether the kernel is older than 6.5.9.
- Verify whether the NCI SPI driver is present or loaded:
- Inspect loaded modules for nci or related NFC modules (for example, look for modules named nci, nci_spi, or other vendor-specific nfc drivers).
- Search dmesg or journal logs for NFC-related oops messages:
- Kernel oops or WARN_ON traces that reference net/nfc/nci/spi.c, send_acknowledge, or an oops in the NFC stack indicate the symptom of this issue if it occurs in the field.
- 1.) uname -r
- 2.) lsmod | grep -i nci
- 3.) journalctl -k -n 200 | grep -i nfc
Mitigation and remediation — practical steps
The safest and recommended course is to apply the upstream fix or vendor-supplied backport. If immediate kernel upgrades are not possible, temporary mitigations are available.Primary remediation (recommended)
- Upgrade the kernel to version 6.5.9 or later or apply your distribution’s security update that contains the patched commit. Most major distributions produced advisory entries and kernel updates shortly after the CVE was published. (cdn.kernel.org)
Interim workarounds (when patching is delayed)
- Unload or blacklist the NFC NCI/SPI driver if the platform does not require NFC functionality. For systems where NFC is unused, removing the driver eliminates the vulnerable code path until a patch can be deployed.
- Example steps (run as root):
- Identify modules: lsmod | grep -i nci
- Remove module: modprobe -r nci_spi (repeat for nci, nfc, or vendor modules as appropriate)
- Persistently blacklist module: create a modprobe.d blacklist entry (echo "blacklist nci_spi" > /etc/modprobe.d/disable-nci.conf)
- Disable NFC in firmware/UEFI or OS configuration where vendors expose such a toggle (some laptop OEMs provide an NFC enable/disable in firmware or platform software).
- Apply network or access controls: restrict local or adjacent access to NFC interfaces where possible (physical controls, agent policies) so untrusted parties cannot interact with NFC hardware.
Recommended operational steps for security teams
- Inventory
- Find systems that run kernels older than 6.5.9 and that include NFC/NCI components in their images.
- Prioritize
- Prioritize patching systems that expose NFC to untrusted users or networks, or that operate in physically untrusted environments.
- Patch
- Apply vendor kernel updates or upstream 6.5.9-based patches. If you rely on long‑term or vendor kernels, coordinate with your vendor for backport availability.
- Temporary risk reduction
- Where patching cannot happen immediately, unload or blacklist the affected modules or disable NFC via firmware controls.
- Monitor
- Add detection signatures in your logging/host monitoring to catch kernel oopses referencing net/nfc/nci/spi.c and related stack traces.
- Record
- Document impacted endpoints, the applied mitigations, and the timeline for full remediation.
Why the fix was so small — and why that matters
The upstream correction illustrates a recurring theme in kernel security: many reliability and availability vulnerabilities are closed by focused defensive checks rather than architectural rewrites. The fix in this case amounted to verifying the return value of an allocation helper before use. That minimal change is effective because:- It eliminates a deterministic crash trigger without changing protocol behavior.
- It preserves functional behavior for valid allocations while making the code robust against transient resource constraints.
- It’s low‑risk for backporting into vendor kernels and stable branches, which accelerates distribution updates. (cdn.kernel.org)
Broader implications and lessons for administrators
- Availability is often the weak link. Even when a bug does not expose secrets or permit direct code execution, a kernel oops in a seldom‑used subsystem can still be a critical operational outage for affected hosts.
- Inventory matters. Many organizations are surprised to find rarely used hardware subsystems (like NFC or special SoC peripherals) present in images. Regularly inventorying kernel modules and hardware bindings helps prioritize low‑visibility risks like this one.
- Patch distribution lag can be the risk vector. The upstream fix is simple and obvious, but systems that ship long‑lived vendor kernels or conservative update schedules can remain exposed. Work with vendors to confirm backports for stable kernels where you cannot immediately upgrade to the upstream release.
What remains unverified / caveats
- Some secondary write‑ups speculate this NULL dereference might be chained into a more severe exploit on certain platforms; however, there is no public PoC that transforms this specific NULL pointer dereference into remote code execution as of the public advisories and kernel changelog entries consulted. Any claim that the bug enables code execution should be treated as speculative unless demonstrated by a trusted technical analysis or exploit publication. Always prefer vendor or upstream confirmation before treating speculative escalation scenarios as operational risk.
Quick reference — checklist for operators
- Confirm kernel version: uname -r — if older than 6.5.9, consider the system potentially vulnerable.
- Check for NFC NCI modules: lsmod | grep -i nci
- If NFC is not required: unload and blacklist the NFC/NCI SPI modules as an interim mitigation.
- If NFC is required: schedule kernel updates or apply vendor patches that include the 6.5.9 fix.
- Monitor kernel logs for oops messages referencing net/nfc/nci/spi.c or send_acknowledge.
Conclusion
CVE‑2023‑46343 is a reminder that even tiny omissions — a missing NULL check after a kernel allocation — can produce a high‑impact availability failure. The flaw was responsibly reported and promptly patched upstream in Linux kernel 6.5.9 by adding defensive handling around the NCI allocation helper. For administrators, the practical actions are straightforward: confirm exposure, apply vendor or upstream patches, or, if necessary, disable the NFC NCI SPI code until patched. The bug’s resolution reinforces the utility of simple defensive programming and the benefit of rapid, surgical fixes in kernel maintenance, but it also underscores the operational need for good inventory practices and timely kernel updates to eliminate simple yet disruptive stability risks. (cdn.kernel.org)Source: MSRC Security Update Guide - Microsoft Security Response Center
