# CVE-2026-74743: macvlan: inherit needed_headroom and needed_tailroom from lowerdev

> A Linux kernel macvlan bug causes use-after-free crashes when lower devices require extra headroom, leading to denial of service.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 26 Aug 2026
- **Updated:** 16 Oct 2026
- **Also known as:** AZL-97782, BELL-CVE-2026-74743, DEBIAN-CVE-2026-74743, GHSA-5qfp-9h7g-rcpm, UBUNTU-CVE-2026-74743

## What is CVE-2026-74743?

CVE-2026-74743 is a critical use after free vulnerability in Linux, published on 26 Aug 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:

macvlan: inherit needed_headroom and needed_tailroom from lowerdev

macvlan devices inherit hard_header_len from lowerdev during macvlan_init(),
but leave needed_headroom and needed_tailroom set to 0.

When the underlying lowerdev requires extra headroom or tailroom for
headers/trailers (e.g. macsec, ipsec, wireguard, tunnels, or veth with rx
headroom), upper layers calculating packet headroom and tailroom fail to
reserve sufficient space.

This can result in reallocation overhead, skb headroom underflows, or KASAN
slab-use-after-free crashes when dev_hard_header() / macvlan_hard_header()
prepends header data or when lower devices append tailroom.

Fix this by:
1. Inheriting needed_headroom and needed_tailroom from lowerdev in macvlan_init().
2. Propagating needed_headroom and needed_tailroom updates to attached macvlans
   in macvlan_device_event() when receiving NETDEV_FEAT_CHANGE events.

## Which versions are affected by CVE-2026-74743?


## How do I fix CVE-2026-74743?

Apply the vendor's patch.

## Is CVE-2026-74743 being exploited?

There is no known exploitation of CVE-2026-74743 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-74743 and fix it if it is affected.

What it is: CVE-2026-74743 is a critical use after free vulnerability in Linux, published on 26 Aug 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-2026-74743 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-2026-74743/ (Markdown: https://insights.sbomify.com/CVE-2026-74743/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-74743
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-74743
- OSV: https://osv.dev/vulnerability/CVE-2026-74743

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-74743/
Summaries are generated and may contain mistakes; the official description above is the record.