# CVE-2026-90078: net/sched: act_skbmod: fix length calculations and avoid invalid header warnings

> A flaw in the Linux kernel's traffic control module causes invalid header warnings and drops valid short packets, leading to potential denial of service.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 17 Sep 2026
- **Updated:** 19 Oct 2026
- **Also known as:** AZL-102536, BELL-CVE-2026-90078, DEBIAN-CVE-2026-90078, GHSA-4hxp-vhhr-h77f, UBUNTU-CVE-2026-90078

## What is CVE-2026-90078?

CVE-2026-90078 is a denial of service vulnerability in Linux, published on 17 Sep 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/sched: act_skbmod: fix length calculations and avoid invalid header warnings

syzbot reported a warning in skb_network_header_len() triggered
by tcf_skbmod_act():

  !skb_transport_header_was_set(skb)
  WARNING: CPU: 0 PID: 14949 at include/linux/skbuff.h:3243 skb_network_header_len include/linux/skbuff.h:3243 [inline]
  WARNING: CPU: 0 PID: 14949 at net/sched/act_skbmod.c:55 tcf_skbmod_act+0xfe8/0x1810 net/sched/act_skbmod.c:55

There are a few issues in tcf_skbmod_act():

1. Calling skb_network_header_len() assumes skb->transport_header is set,
   which is not guaranteed when tcf_skbmod_act() runs at TC ingress.
2. Unconditionally calling skb_mac_header_len() at the beginning of
   tcf_skbmod_act() triggers a warning on L3 devices (e.g. TUN) where the
   MAC header is unset, evaluating to an underflowed garbage length.
3. On TC ingress, skb->data points to the network header. Adding the MAC
   header length to the IP header length causes skb_ensure_writable() to
   request more bytes than the actual IP packet length, dropping valid
   short packets (e.g. 28-byte UDP/IPv4 packets).

Fix these by:
- Using skb_network_offset(skb) + sizeof(struct iphdr/ipv6hdr) for
  SKBMOD_F_ECN so that the required length is correctly calculated on
  both ingress (offset == 0) and egress (offset == mac_len).
- Setting max_edit_len to ETH_HLEN for Ethernet header modifications
  after validating ARPHRD_ETHER.

## Which versions are affected by CVE-2026-90078?


## How do I fix CVE-2026-90078?

Apply the vendor's patch.

## Is CVE-2026-90078 being exploited?

There is no known exploitation of CVE-2026-90078 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-90078 and fix it if it is affected.

What it is: CVE-2026-90078 is a denial of service vulnerability in Linux, published on 17 Sep 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-2026-90078 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-2026-90078/ (Markdown: https://insights.sbomify.com/CVE-2026-90078/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-90078
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-90078
- OSV: https://osv.dev/vulnerability/CVE-2026-90078

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-90078/
Summaries are generated and may contain mistakes; the official description above is the record.