# CVE-2026-98401: net: iptunnel: fix stale transport header during tunnel decapsulation

> A stale transport header offset in Linux kernel tunnel decapsulation causes a kernel crash, leading to denial of service.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 14 Oct 2026
- **Updated:** 24 Oct 2026
- **Also known as:** AZL-108141, BELL-CVE-2026-98401, DEBIAN-CVE-2026-98401, GHSA-c6hw-23wq-xx46, UBUNTU-CVE-2026-98401

## What is CVE-2026-98401?

CVE-2026-98401 is a high-severity vulnerability in Linux, published on 14 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

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

net: iptunnel: fix stale transport header during tunnel decapsulation

Syzbot reported a crash in qdisc_pkt_len_segs_init() caused by a stale
transport_header offset after tunnel decapsulation.

BUG: unable to handle page fault for address: ffffed102091a42e
Oops: Oops: 0000 [#1] SMP KASAN NOPTI
CPU: 0 UID: 0 PID: 340 Comm: qdisc_uaf_repro Not tainted 7.2.0-rc4-00061-g248951ddc14d #256 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:__asan_load2
<IRQ>
qdisc_pkt_len_segs_init (net/core/dev.c:4145)
__dev_queue_xmit (net/core/dev.c:4787)
br_dev_queue_push_xmit (net/bridge/br_forward.c:53)
br_handle_frame_finish (net/bridge/br_input.c:229)
br_handle_frame (net/bridge/br_input.c:315)
__netif_receive_skb_core.constprop.0 (net/core/dev.c:6099)
__netif_receive_skb_list_core (net/core/dev.c:6287)
netif_receive_skb_list_internal (net/core/dev.c:6445)
napi_complete_done (net/core/dev.c:6813)
gro_cell_poll (net/core/gro_cells.c:74)
__napi_poll (net/core/dev.c:7735)
net_rx_action (net/core/dev.c:7798 net/core/dev.c:7955)
handle_softirqs (kernel/softirq.c:622)
do_softirq (kernel/softirq.c:523  kernel/softirq.c:510 )
__local_bh_enable_ip (kernel/softirq.c:450)
tun_get_user (drivers/net/tun.c:1986 (discriminator 1))
tun_chr_write_iter (drivers/net/tun.c:2032)

The issue is completely latent until qdisc read transport header in
commit 7fb4c1967011 ("net: pull headers in qdisc_pkt_len_segs_init()").
The crash requires four conditions to line up:

1. The incoming packet is encapsulated and carries GSO metadata. The outer
   transport header offset is stored in skb->transport_header while the
   packet is still in the outer tunnel context.
2. The tunnel receiver strips the outer headers. skb->data is advanced to
   the inner frame, but skb->transport_header is left pointing to the
   now-removed outer L4 header, so it becomes a negative offset relative to
   the new data.
3. The inner frame is not delivered to the local IP stack. Instead, it
   is forwarded at L2 by a bridge or HSR, so ip_rcv_core() never runs and
   the transport header is not reset to the inner L4 offset.
4. The forwarding path calls __dev_queue_xmit(), which enters
   qdisc_pkt_len_segs_init(). That function computes the GSO header length
   from skb_transport_offset(skb). Because the offset is negative, the
   unsigned cast overflows and pskb_may_pull(skb, hdr_len +
   sizeof(struct tcphdr)) reads past the end of the skb, triggering a
   KASAN fault or page fault.

The issue specifically requires GSO packets (shinfo->gso_size != 0), which
are processed/aggregated through gro_cells. Fix this by clearing
transport_header to the ~0U sentinel in gro_cell for all tunnnel driver.
GTP does not support GRO/GSO, drop the evil GSO packets in GTP directly.

## Which versions are affected by CVE-2026-98401?


## How do I fix CVE-2026-98401?

Apply the vendor's patch.

## Is CVE-2026-98401 being exploited?

There is no known exploitation of CVE-2026-98401 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98401 and fix it if it is affected.

What it is: CVE-2026-98401 is a high-severity vulnerability in Linux, published on 14 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-98401 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-2026-98401/ (Markdown: https://insights.sbomify.com/CVE-2026-98401/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98401
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98401
- OSV: https://osv.dev/vulnerability/CVE-2026-98401

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98401/
Summaries are generated and may contain mistakes; the official description above is the record.