L2 network filtering can be bypassed using stacked VLAN0 and LLC/SNAP headers

published 2022-09-27 4 y ago modified 2026-06-17 4 mo ago also EUVD-2021-14591

cvss: 4.7; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.8%; EPSS 0.8% chance of exploitation in 30 days, percentile 57%
public exploits: 0; none known

Layer 2 network filtering capabilities such as IPv6 RA guard or ARP inspection can be bypassed using combinations of VLAN 0 headers and LLC/SNAP headers.

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-27853 updated 2025-11-21 cvss 5.3

kernel: layer 2 network filtering capabilities bypass

Red Hat Enterprise Linux 6 perf, kernel, python-perf, kernel-kdump, kernel-firmware, kernel-bootwrapper, +1 more Red Hat Enterprise Linux 7 perf, kernel, bpftool, kernel-rt, python-perf, kernel-kdump, +8 more Red Hat Enterprise Linux 8 perf, kernel, bpftool, kernel-rt, kernel-core, kernel-tools, +29 more Red Hat Enterprise Linux 9 rv, perf, rtla, kernel, bpftool, libperf, +81 more
not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation: none
Automatable: no
Technical impact: partial
Assessed: 17 mo ago (2025-05-21)

Exploit prediction

0.81% chance of exploitation in the next 30 days, higher than 57% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

blog.champtar.fr https://blog.champtar.fr/VLAN0_LLC_SNAP/ referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CISA ADP	3.1	4.7	CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
NVD	3.1	4.7	CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
CNA	3.1	4.7	CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N

Affected products

IEEE 802.2
IETF draft-ietf-v6ops-ra-guard
IETF P802.1Q

Classification

Weakness

CWE-290 Authentication Bypass by Spoofing

Technology feeds

Cisco IOS rss critical rss

References

blog.champtar.fr /VLAN0_LLC_SNAP/
exploit, third-party-advisory
tools.cisco.com /security/center/content/CiscoSecurityAdvisory/cisco-sa-VU855201-J3z8CKTX
vendor-advisory, third-party-advisory
standards.ieee.org /ieee/802.2/1048/
vendor-advisory
standards.ieee.org /ieee/802.1Q/10323/
vendor-advisory
datatracker.ietf.org /doc/draft-ietf-v6ops-ra-guard/08/
technical-description, third-party-advisory
kb.cert.org /vuls/id/855201
third-party-advisory, us-government-resource
kb.cert.org /vuls/id/855201

Sources

CVE List CVE-2021-27853
NVD CVE-2021-27853

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary