# CVE-2026-98377: vlan: require the MAC header to be present in __vlan_insert_inner_tag()

> A local attacker can leak uninitialized kernel memory by sending crafted packets through VLAN interfaces, exposing sensitive slab data.

- **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:** 9 Oct 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-106896, BELL-CVE-2026-98377, DEBIAN-CVE-2026-98377, GHSA-jjqx-7ghh-p62x, UBUNTU-CVE-2026-98377

## What is CVE-2026-98377?

CVE-2026-98377 is a vulnerability in Linux, published on 9 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:

vlan: require the MAC header to be present in __vlan_insert_inner_tag()

__vlan_insert_inner_tag() only guarantees head room via skb_cow_head(),
never that mac_len bytes of MAC header are present.  Its ETH_HLEN
wrappers - __vlan_insert_tag() under skb_vlan_push(), and
vlan_insert_tag() under validate_xmit_vlan() on the generic transmit
path - therefore rewrite the first 16 bytes at skb->data: a 12-byte
memmove plus two 2-byte stores at +12 and +14.  No caller supplies the
bound, while the pop helpers use skb_ensure_writable()/pskb_may_pull().

An IFF_TUN device has hard_header_len == 0, so packet_snd() accepts a
one-byte AF_PACKET/SOCK_RAW frame.  The first vlan push only sets a
hwaccel tag; the next - clsact "action vlan push" or
bpf_skb_vlan_push() - enters the helper with skb->len still 1.  The
head comes from skbuff_small_head without __GFP_ZERO, so each push
drags bytes from beyond skb->tail into the frame.  After three the
one-byte send leaves as 13 bytes carrying 11 bytes of uninitialised
slab:

  0000: 5a b3 62 12 80 88 ff ff 00 b3 62 12 81
           `------------------------------'
  only 0x5a was sent; the rest is slab, here the top 56 bits of a
  linear-map address

Require the MAC header the helper rewrites to be present, so such a
frame is dropped rather than transmitted.

## Which versions are affected by CVE-2026-98377?


## How do I fix CVE-2026-98377?

Apply the vendor's patch.

## Is CVE-2026-98377 being exploited?

There is no known exploitation of CVE-2026-98377 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-98377 and fix it if it is affected.

What it is: CVE-2026-98377 is a vulnerability in Linux, published on 9 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-2026-98377 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-2026-98377/ (Markdown: https://insights.sbomify.com/CVE-2026-98377/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-98377
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-98377
- OSV: https://osv.dev/vulnerability/CVE-2026-98377

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-98377/
Summaries are generated and may contain mistakes; the official description above is the record.