Vulnerabilities in Layer 2 Network Security Controls Affecting Cisco Products: September 2022

On September 27, 2022, the following vulnerabilities affecting Cisco products were disclosed by Cert/CC as part of VU855201, titled L2 network security controls can be bypassed using VLAN 0 stacking and/or 802.3 headers [“https://kb.cert.org/vuls/id/855201”]:

CVE-2021-27853: Layer 2 network filtering capabilities such as IPv6 RA guard or ARP inspection can be bypassed using a combination of VLAN 0 headers and LLC/SNAP headers.
CVE-2021-27854: Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using a combination of VLAN 0 headers, LLC/SNAP headers in Ethernet to Wifi frame translation, and in the reverse—Wifi to Ethernet.
CVE-2021-27861: Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using LLC/SNAP headers with invalid length (and optionally VLAN0 headers).
CVE-2021-27862: Layer 2 network filtering capabilities such as IPv6 RA guard can be bypassed using LLC/SNAP headers with invalid length and Ethernet to Wifi frame conversion (and optionally VLAN0 headers).

Exploitation of these vulnerabilities could allow an adjacent attacker to bypass configured first-hop security (FHS) features on the affected Cisco products.

For more information about these vulnerabilities, see the Details [“#details”] section of this advisory.

This advisory is available at the following link: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-VU855201-J3z8CKTX [“https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-VU855201-J3z8CKTX”]