# CVE-2026-98852: net: extend IPv6 exthdr detection of tunneled packets

> A flaw in Linux kernel IPv6 tunnel handling causes GSO packets to be dropped, leading to denial of service for network traffic.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 18 Oct 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-109070, BELL-CVE-2026-98852, DEBIAN-CVE-2026-98852, GHSA-jqxw-mh4h-wm57, UBUNTU-CVE-2026-98852

## What is CVE-2026-98852?

CVE-2026-98852 is a denial of service vulnerability in Linux, published on 18 Oct 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:

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.

## Which versions are affected by CVE-2026-98852?


## How do I fix CVE-2026-98852?

Apply the vendor's patch.

## Is CVE-2026-98852 being exploited?

There is no known exploitation of CVE-2026-98852 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98852 and fix it if it is affected.

What it is: CVE-2026-98852 is a denial of service vulnerability in Linux, published on 18 Oct 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-2026-98852 in the wild and no public exploit code.

Affected product: Linux. 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-2026-98852/ (Markdown: https://insights.sbomify.com/CVE-2026-98852/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98852
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98852
- OSV: https://osv.dev/vulnerability/CVE-2026-98852

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98852/
Summaries are generated and may contain mistakes; the official description above is the record.