# CVE-2026-43037: ip6_tunnel: clear skb2->cb[] in ip4ip6_err()

> A memory corruption vulnerability in the Linux kernel's IPv6 tunnel error handling allows attackers to execute arbitrary code or cause a denial of service.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 1 May 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-43037, DEBIAN-CVE-2026-43037, GHSA-p2wf-qj6m-x5fg, RHSA-2026:25534, RHSA-2026:28738, RHSA-2026:28740, RHSA-2026:28741, RHSA-2026:28742, RHSA-2026:28748, RHSA-2026:28749, RHSA-2026:28750, ROOT-OS-DEBIAN-11-CVE-2026-43037, ROOT-OS-UBUNTU-2204-CVE-2026-43037, ROOT-OS-UBUNTU-2404-CVE-2026-43037, UBUNTU-CVE-2026-43037

## What is CVE-2026-43037?

CVE-2026-43037 is a critical memory corruption vulnerability in Linux kernel, published on 1 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

ip6_tunnel: clear skb2->cb[] in ip4ip6_err()

Oskar Kjos reported the following problem.

ip4ip6_err() calls icmp_send() on a cloned skb whose cb[] was written
by the IPv6 receive path as struct inet6_skb_parm. icmp_send() passes
IPCB(skb2) to __ip_options_echo(), which interprets that cb[] region
as struct inet_skb_parm (IPv4). The layouts differ: inet6_skb_parm.nhoff
at offset 14 overlaps inet_skb_parm.opt.rr, producing a non-zero rr
value. __ip_options_echo() then reads optlen from attacker-controlled
packet data at sptr[rr+1] and copies that many bytes into dopt->__data,
a fixed 40-byte stack buffer (IP_OPTIONS_DATA_FIXED_SIZE).

To fix this we clear skb2->cb[], as suggested by Oskar Kjos.

Also add minimal IPv4 header validation (version == 4, ihl >= 5).

Weakness: CWE-787 (Out-of-bounds Write), CWE-843 (Access of Resource Using Incompatible Type), CWE-121 (Stack-based Buffer Overflow)

## Which versions are affected by CVE-2026-43037?

It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `kpatch-patch-6_12_0-211_16_1` | Red Hat:enterprise_linux:10.2 | All versions | 0:1-1.el10_2 |
| `kpatch-patch-6_12_0-211_16_1-debuginfo` | Red Hat:enterprise_linux:10.2 | All versions | 0:1-1.el10_2 |
| `kpatch-patch-6_12_0-211_16_1-debugsource` | Red Hat:enterprise_linux:10.2 | All versions | 0:1-1.el10_2 |
| `kpatch-patch-4_18_0-553_109_1` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-4.el8_10 |
| `kpatch-patch-4_18_0-553_109_1-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-4.el8_10 |
| `kpatch-patch-4_18_0-553_109_1-debugsource` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-4.el8_10 |
| `kpatch-patch-4_18_0-553_125_1` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-1.el8_10 |
| `kpatch-patch-4_18_0-553_125_1-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-1.el8_10 |
| `kpatch-patch-4_18_0-553_125_1-debugsource` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-1.el8_10 |
| `kpatch-patch-4_18_0-553_53_1` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-11.el8_10 |
| `kpatch-patch-4_18_0-553_53_1-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-11.el8_10 |
| `kpatch-patch-4_18_0-553_53_1-debugsource` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-11.el8_10 |
| `kpatch-patch-4_18_0-553_72_1` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-8.el8_10 |
| `kpatch-patch-4_18_0-553_72_1-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-8.el8_10 |
| `kpatch-patch-4_18_0-553_72_1-debugsource` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-8.el8_10 |
| `kpatch-patch-4_18_0-553_85_1` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-6.el8_10 |
| `kpatch-patch-4_18_0-553_85_1-debuginfo` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-6.el8_10 |
| `kpatch-patch-4_18_0-553_85_1-debugsource` | Red Hat:enterprise_linux:8::baseos | All versions | 0:1-6.el8_10 |
| `kpatch-patch-5_14_0-687_10_1` | Red Hat:enterprise_linux:9::baseos | All versions | 0:1-1.el9_8 |
| `kpatch-patch-5_14_0-687_10_1-debuginfo` | Red Hat:enterprise_linux:9::baseos | All versions | 0:1-1.el9_8 |
| `kpatch-patch-5_14_0-687_10_1-debugsource` | Red Hat:enterprise_linux:9::baseos | All versions | 0:1-1.el9_8 |
| `kernel` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-core` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-debug` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-debug-core` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-debug-debuginfo` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-debug-modules` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-debug-modules-core` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-debug-modules-extra` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-debuginfo` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-modules` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-modules-core` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-64k-modules-extra` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-abi-stablelists` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-core` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-debug` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-debug-core` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-debug-debuginfo` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |
| `kernel-debug-devel` | Red Hat:enterprise_linux_nvidia:10::el10 | All versions | 0:6.12.0-212.10.el10nv |

Products: Linux kernel.

## How do I fix CVE-2026-43037?

Apply the vendor's patch.

## Is CVE-2026-43037 being exploited?

There is no known exploitation of CVE-2026-43037 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-43037 and fix it if it is affected.

What it is: CVE-2026-43037 is a critical memory corruption vulnerability in Linux kernel, published on 1 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-43037 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-43037/ (Markdown: https://insights.sbomify.com/CVE-2026-43037/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-43037
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-43037
- OSV: https://osv.dev/vulnerability/CVE-2026-43037

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-43037/
Summaries are generated and may contain mistakes; the official description above is the record.