# CVE-2026-31429: net: skb: fix cross-cache free of KFENCE-allocated skb head

> A Linux kernel bug allows cross-cache memory corruption when KFENCE is enabled, potentially leading to system crashes or arbitrary code execution.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 20 Apr 2026
- **Updated:** 11 May 2026
- **Also known as:** BELL-CVE-2026-31429, DEBIAN-CVE-2026-31429, GHSA-vqjc-m5xh-jhx3, ROOT-OS-DEBIAN-13-CVE-2026-31429, ROOT-OS-UBUNTU-2404-CVE-2026-31429, UBUNTU-CVE-2026-31429

## What is CVE-2026-31429?

CVE-2026-31429 is a medium-severity memory corruption vulnerability in Linux kernel, published on 20 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

net: skb: fix cross-cache free of KFENCE-allocated skb head

SKB_SMALL_HEAD_CACHE_SIZE is intentionally set to a non-power-of-2
value (e.g. 704 on x86_64) to avoid collisions with generic kmalloc
bucket sizes. This ensures that skb_kfree_head() can reliably use
skb_end_offset to distinguish skb heads allocated from
skb_small_head_cache vs. generic kmalloc caches.

However, when KFENCE is enabled, kfence_ksize() returns the exact
requested allocation size instead of the slab bucket size. If a caller
(e.g. bpf_test_init) allocates skb head data via kzalloc() and the
requested size happens to equal SKB_SMALL_HEAD_CACHE_SIZE, then
slab_build_skb() -> ksize() returns that exact value. After subtracting
skb_shared_info overhead, skb_end_offset ends up matching
SKB_SMALL_HEAD_HEADROOM, causing skb_kfree_head() to incorrectly free
the object to skb_small_head_cache instead of back to the original
kmalloc cache, resulting in a slab cross-cache free:

  kmem_cache_free(skbuff_small_head): Wrong slab cache. Expected
  skbuff_small_head but got kmalloc-1k

Fix this by always calling kfree(head) in skb_kfree_head(). This keeps
the free path generic and avoids allocator-specific misclassification
for KFENCE objects.

Weakness: CWE-401 (Missing Release of Memory after Effective Lifetime)

## Which versions are affected by CVE-2026-31429?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-31429?

Apply the vendor's patch.

## Is CVE-2026-31429 being exploited?

There is no known exploitation of CVE-2026-31429 in the wild and no public exploit code.

Public exploit code:

- POC for CVE-2026-31429 (Linux Kernel >= 6.3 < 6.12.82 Slab Cross-Cache Confusion) - vulnerability discovered by Antonius - w1sdom - bluedragonsec.com (github_poc): https://github.com/bluedragonsecurity/CVE-2026-31429-POC

## 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-2026-31429 and fix it if it is affected.

What it is: CVE-2026-31429 is a medium-severity memory corruption vulnerability in Linux kernel, published on 20 Apr 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-31429 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-2026-31429/ (Markdown: https://insights.sbomify.com/CVE-2026-31429/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-31429
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-31429
- OSV: https://osv.dev/vulnerability/CVE-2026-31429

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-31429/
Summaries are generated and may contain mistakes; the official description above is the record.