---
id: "CVE-2026-98852"
kind: "vulnerability"
aliases: ["CVE-2026-98852"]
title: "Linux kernel: IPv6 tunneled packets with extension headers dropped by checksum path"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: "infrastructure"
products: []
ecosystems: []
tags: ["infrastructure", "ipv6", "kernel", "linux", "networking", "tunnels"]
not_affected_if: ["no-ipv6-tunnels"]
published_at: "2026-10-18T16:17:39+00:00"
first_seen_at: "2026-10-23T21:57:17+00:00"
updated_at: "2026-10-25T05:13:01+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-98852", "first_seen_at": "2026-10-23T21:57:17+00:00"}]
---

# Linux kernel: IPv6 tunneled packets with extension headers dropped by checksum path

The Linux kernel fails to check IPv6 extension headers on the outer and inner headers of tunneled packets during checksum offload. Affected GSO packets trigger a kernel warning and are dropped, which is a denial-of-service effect; the sources do not describe code execution.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- unknown

## Fix

- unknown

## Timeline

- 2026-10-23T21:57:17+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-98852

## From the sources

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

net: extend IPv6 exthdr detection of tunneled packets

Commit c4336a07eb6b ("net: correctly handle tunneled traffic on IPV6_CSUM
GSO fallback") split skb_gso_has_extension_hdr() into mutually exclusive
branches on skb->encapsulation. This did not yet address all paths:

1. With skb->encapsulation set, the outer header is not checked. IPv6
   tunnels such as ip6_gre and ip6_tunnel add an outer Destination
   Options header by default (encap_limit). Their GSO packets skip
   software GSO, then skb_csum_hwoffload_help() sees the outer extension
   header and calls skb_checksum_help() on the GSO skb, which warns and
   drops it.

2. Tunnels over IPv6 without an outer transport header, such as
   ip6_tunnel, leave skb->transport_header at the inner transport
   header. skb_network_header_len() then spans the outer IPv6, tunnel
   and inner IP headers, a false positive.

3. UDP tunnels without an inner network header, such as SCTP-in-UDP or
   PSP, have no inner IPv6 header to check. Decide on the outer header
   alone.

4. Directly dereferencing inner_ip_hdr(skb)->version without
   skb_header_pointer() is unsafe.

Instead, check ipv6_ext_hdr(nexthdr) on the outer IPv6 header and, if
set, on the inner IPv6 header. Read the headers with skb_header_pointer().
Keep the skb_network_header_len() check when !skb->encapsulation to also
catch encapsulated packets without skb->encapsulation (e.g., virtio).

Use the same helper in skb_csum_hwoffload_help(). Its open coded test
has the false positive of (2) and ignores the inner header.

Background: checksum offload of tunneled packets invariants:

Non-GSO skb:
- If the inner packet is CHECKSUM_PARTIAL, Local Checksum Offload computes
  the outer checksum in software and the device offloads only the inner L4
  checksum.
- If the inner packet is CHECKSUM_NONE (e.g., SCTP-in-UDP, ESP-in-UDP,
  Remote Checksum Offload), the device offloads the outer UDP or GRE
  checksum instead.

GSO skb:
- A device with NETIF_F_GSO_UDP_TUNNEL_CSUM or NETIF_F_GSO_GRE_CSUM
  computes both inner and outer checksums per segment.
  The outer checksum is seeded with only the pseudo-header checksum.

A NETIF_F_IPV6_CSUM device must parse through the outer headers to
reach the inner ones.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_