A subtle NULL‑pointer bug in the Linux kernel’s IPv4 routing code — tracked as CVE‑2024‑36008 — was patched in mid‑2024 after syzbot triggered a NULL dereference in fib_validate_source() that can crash a system processing IPv4 packets; the issue is real, reproducible in some configurations, and important to networks and cloud operators even though it is limited to availability impact and requires local or low‑privilege action to trigger. (
nvd.nist.gov)
Background / Overview
The vulnerability CVE‑2024‑36008 affects the Linux kernel’s IPv4 routing path. At a high level, the problem is a missing NULL check: the kernel calls __in_dev_get_rcu() to obtain an in_dev pointer and then proceeds to use that pointer in the ip_route_use_hint() / fib_validate_source() logic without ensuring it is non‑NULL. Under specific test conditions syzbot produced a KASAN‑detected null‑pointer dereference and general protection fault, with the crash trace leading back to fib_validate_source() and ip_route_use_hint(). (
nvd.nist.gov)
Distributions and vendors subsequently published advisories, and patched kernel releases were published for stable branches. The NVD and major Linux distributions list the issue as medium severity with a CVSS v3.1 base score of
5.5 and classify the weakness as a
NULL‑pointer dereference (CWE‑476); the practical impact is denial of service (availability) rather than data disclosure or privilege escalation. (
nvd.nist.gov)
What exactly went wrong: a concise technical explanation
The vulnerable path, in plain terms
- The IPv4 routing helper ip_route_use_hint() consults device‑specific route hints when deciding whether to accept or reject traffic from certain sources.
- As part of that path, the kernel uses __in_dev_get_rcu() to fetch an in_dev structure associated with a device.
- If __in_dev_get_rcu() returns NULL and the result is used without a check, subsequent dereferencing inside fib_validate_source() can access address 0x0 and crash the kernel. The syzbot reproduction included a KASAN null‑ptr‑deref and an associated general protection fault. (nvd.nist.gov)
Where the crash was observed in the call trace
The publicly recorded call trace (from the test reproduction) shows the crash originating in fib_validate_source(), propagating through ip_route_use_hint, and ultimately being reachable from IPv4 packet receive paths and BPF/XDP test harnesses (bpf_prog_test_run / xdp_test_run). This mapping explains why a specially crafted runtime sequence or unprivileged BPF activity — in environments that permit it — may hit the defect. (
nvd.nist.gov)
Severity and exploitability — parsing the technical scores
- CVSS v3.1: 5.5 (Medium) with vector CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H. That vector encodes local attack vector (AV:L), low privileges required (PR:L), and availability impact only (A:H). In short: the bug is locally triggerable and can crash a host, but it does not provide code execution or data theft in known public reports. (nvd.nist.gov)
- Why local? The NVD call trace includes bpf_test_run/xdp paths and shows the kernel was driven by syzkaller’s local test executor; public coverage converges on the conclusion that local stimuli (for example, BPF programs or privileged syscall interactions) are the realistic trigger vectors in practice. That reduces the risk of remote unauthenticated exploitation over a network stack compared with a remotely triggered heap corruption bug, but it does not eliminate risk in multi‑tenant or shared environments. (nvd.nist.gov)
- Public exploit status: as of February 18, 2026, there are no widely reported remote exploits weaponizing this CVE in the wild; vendors treated this as a kernel stability/availability fix and issued patches for stable kernel trees. Operators should still take it seriously because a local crash on critical infrastructure or cloud hypervisor guests can be disruptive.
Who is affected
- Any system running Linux kernel versions that include the vulnerable code path prior to the stable patches is potentially affected. NVD and vendor trackers list affected ranges and fixed versions: the fixes were backported into the stable branches and appear in releases such as 5.10.216, 5.15.158, 6.1.90, 6.6.30, 6.8.9 and later (vendor granularities vary). Operators should consult their distribution's advisory for precise package names and release numbers. (nvd.nist.gov)
- Particular environments of concern:
- Multi‑tenant cloud hosts and virtual machine hosts where unprivileged or low‑privilege users can exercise BPF/XDP or other packet processing facilities.
- Containers and orchestration platforms that expose networking hooks (e.g., dataplane BPF, XDP) to non‑root tenants.
- Embedded/network appliances or router distributions that run older kernel trees and may not receive rapid updates.
- Vendors and distros that published advisories and fixes include major Linux distributors and cloud images — pattern of updates seen across Amazon Linux, Ubuntu, Debian, SUSE and others. Check distribution advisories for backport timelines and exact package names.
Patches and vendor guidance — what was changed
The upstream kernel fixes are surgical: every call site that relied on __in_dev_get_rcu() was updated to check the returned pointer for NULL before use, eliminating the dereference path. Kernel maintainers merged the fixes into the stable trees and vendor kernels were updated accordingly. The NVD entry links to the kernel.org patch commits that implement the checks and backports. (
nvd.nist.gov)
Vendor guidance followed the usual pattern:
- Publish a CVE advisory entry.
- Ship fixed kernel packages for affected stable branches.
- Recommend installation and reboot when kernel ABI changes or when the running kernel is replaced by the vendor package.
Major vendors produced security advisories and distribution‑specific package updates within weeks of the issue’s disclosure.
Remediation: prioritized, practical steps for operators
- Patch first, verify second
- Apply the vendor‑supplied kernel update that contains the fix for CVE‑2024‑36008. Confirm the kernel package version matches the distribution advisory (for example, the stable kernel numbers listed by NVD and vendor trackers). Reboot to load the updated kernel if necessary. (nvd.nist.gov)
- If you cannot patch immediately — temporary mitigations
- Restrict or disable unprivileged eBPF usage. Many distributions expose a sysctl knob, kernel.unprivileged_bpf_disabled, that can block unprivileged use of the bpf(2) syscall. Setting the sysctl to 1 or 2 limits or disables unprivileged BPF, reducing the risk of low‑privilege code triggering kernel network paths that hit the bug. Test carefully before changing system behavior in production and consult vendor docs because the sysctl semantics vary by kernel version.
- Harden container runtime and orchestration settings to prevent untrusted users from inserting BPF or loading XDP programs on host interfaces.
- If you operate a cloud environment, consider isolating workloads that require BPF into more tightly constrained tenancy or dedicated hosts until patches are applied.
- Validate fixes
- After patching and rebooting, validate by checking kernel version and vendor package metadata.
- Monitor dmesg and journald for new kernel oopses or KASAN messages; those are the primary runtime indicators you hit the vulnerable code path before patching. Search logs for traces referencing fib_validate_source and ip_route_use_hint as part of postpatch validation drills. (nvd.nist.gov)
- Long term
- Keep kernel trees and vendor packages current. For cloud images, use vendor‑maintained, patched images or orchestrate rolling host kernel updates.
- Enforce least privilege for kernel‑level functionality: if a workload does not need BPF/XDP privileges, do not grant them.
Detection and hunting guidance
- Kernel log signatures to search for (examples for SIEM/hunt rules):
- Kernel oops lines containing “KASAN: null‑ptr‑deref” or “general protection fault” linked with net/ipv4/fib_frontend.c or references to fib_validate_source or ip_route_use_hint. These substrings appear in the original reproduction traces and vendor advisories and are the most direct evidence of a triggered crash. (nvd.nist.gov)
- Unexpected kernel reboots or crashes on hosts that process high packet rates or run BPF/XDP workloads.
- Audit or runtime events showing unprivileged bpf() syscalls, or attempts to load BPF programs from unprivileged users (if auditing is available).
- Detection workflow (recommended):
- Query journald/syslog for last 30‑90 days for KASAN or “ip_route_use_hint” strings.
- Triage any hits to determine whether the kernel was already patched at the timestamp of the log.
- For cloud or host‑level incidents, cross‑reference with orchestration events to identify tenants that had recent BPF/XDP activity. (nvd.nist.gov)
Risk analysis — strengths of the fix, residual risks
What’s good
- The fix is small, targeted, and straightforward — a simple NULL check where it was missing. That makes the patch low‑risk to merge and backport, and it was accepted into stable trees quickly. The nature of the bug (NULL dereference) reduces chances of a secondary privilege escalation or arbitrary code execution vector from the same flaw. (nvd.nist.gov)
- Many vendors produced coordinated updates, and major distributions included the fixes in routine kernel security updates, so coverage for mainstream deployments is broad.
Residual and operational risks
- The vulnerability remains operationally important because a kernel crash in a critical host (hypervisor, router, cloud node) causes real outages and can lead to follow‑on impact such as automated failover churn, container restarts, or SLA violations.
- The CVSS and vendor data indicate local exploitation is required. However, local in cloud and multi‑tenant contexts can be achieved by an unprivileged container tenant or by code the tenant runs inside their VM if host configuration allows it. In other words, the "local only" designation does not equate to "low concern" in shared environments.
- There’s always a chance of collateral or chained issues: a denial of service can be useful as part of a larger attack pattern (e.g., distract defenders during an intrusion), so defenders must treat availability failures seriously even if confidentiality/integrity are not directly at stake. (nvd.nist.gov)
Operational checklist (actionable, short)
- Immediately:
- Inventory hosts running affected kernels.
- Prioritize patching hosts in multi‑tenant, high‑availability, or network‑critical roles.
- If necessary, temporarily disable unprivileged BPF with kernel.unprivileged_bpf_disabled and lock down BPF/XDP privileges at the orchestration layer.
- Within 24–72 hours:
- Apply vendor kernel updates and schedule reboots in maintenance windows.
- Validate patch success by checking kernel versions and that pre‑patch crash signatures no longer appear in logs.
- Within 30 days:
- Review host hardening policies for kernel‑level features (BPF, XDP, XFRM) and ensure tenant isolation or capability scoping is appropriate.
- Revisit incident response runbooks to include kernel crash triage steps and automated alerting for KASAN/oops messages.
Why this matters to WindowsForum readers and the wider admin community
Thif two enduring truths in systems security:
- Kernel networking code is complex and tightly coupled to high‑performance datapath features (BPF, XDP, offloads). Small omissions — a missing NULL check — can produce a host‑taking impact.
- The threat model changes in cloud and containerized environments: “local” bugs can be escalated into meaningful outages when tenants share a kernel or host. Therefore, distribution‑level patches and kernel hardening knobs are critical defense layers.
WindowsForum readers who manage mixed environments or who operate Linux guests on Windows hypervisors should treat this CVE as a routine, but time‑sensitive, patch‑and‑validate operation: patch the kernel, confirm the patch, and reduce exposure by restricting unneeded capabilities. Community trackers and forum threads on kernel CVEs show similar patterns of immediate fixes followed by vendor advisories — vigilance and prompt patching work.
What I could not independently verify and where to be cautious
- Public exploit code: I found no reliable, public exploit that converts CVE‑2024‑36008 into a remote, unauthenticated compromise. That is consistent with the technical facts and vendor advisories, but the absence of a public exploit is not a guarantee that a targeted attacker will not develop a local exploit technique. Treat claims of "no exploit available" as time‑bound and check vendor advisories for changes. (This article’s factual cutoff is February 18, 2026; consult vendor feeds for newer evidence.) (nvd.nist.gov)
- Whether particular embedded devices or third‑party appliances will ever be patched depends on vendor support cycles. Many network appliance vendors ship older kernel trees and may not backport patches promptly; verify vendor statements rather than assuming coverage. Debian, Ubuntu, Red Hat and cloud OS vendors issued advisories, but smaller vendors may lag.
Final verdict — concise takeaway for sysadmins and security teams
CVE‑2024‑36008 is an important, real kernel bug that results in a null‑pointer dereference and host crash in IPv4 routing code. It is te code‑execution emergency, but it is a tangible availability risk — particularly in multi‑tenant cloud, container, and networking contexts where local actions can come from untrusted tenants.
Prioritize kernel updates from your distribution or cloud vendor, apply them in maintenance windows, and use temporary mitigations such as disabling unprivileged BPF where appropriate. Hunt your logs for KASAN/oops traces referencing fib_validate_source and ip_route_use_hint to confirm you were not previously impacted, and add this class of kernel‑level crash signature to your monitoring playbook. The fix is narrow, the mitigation straightforward, and the operational cost of ignoring it could be service disruption in critical infrastructure. (
nvd.nist.gov)
Conclusion: treat CVE‑2024‑36008 as a medium‑severity availability defect — patch promptly, harden BPF usage where feasible, and instrument your hosts for kernel crash detection so you can catch and contain any instances quickly.
Source: MSRC
Security Update Guide - Microsoft Security Response Center