What’s Vulnerable › Vulnerabilities

CVE-2021-27861: L2 network filtering bypass using stacked VLAN0 and LLC/SNAP headers with invalid lengths

• Severity: medium (CVSS 4.7 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2022-09-27 · updated 2026-06-17
• Weakness: CWE-130

Description

Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using LLC/SNAP headers with invalid length (and optionally VLAN0 headers)

Affected products

• Ieee 802.2: through 802.2h-1997
• IETF P802.1Q: through d1.0

References

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

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