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CVE-2021-27853

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

MEDIUM
4.7
CVSS Score
Published: September 27, 2022 at 05:55 PM UTC
Updated: June 17, 2026 at 03:45 AM UTC
309 products affected across 3 vendorsIOS XeSg500-28p FirmwareSg500-28 Firmware

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Severity
MEDIUM4.7
Exploit prediction
EPSS 0.8%
SSVC
No known exploit

Description

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.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Catalyst 6503-e Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6503-e (same device)
Catalyst 6504-e Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6504-e (same device)
Catalyst 6506-e Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6506-e (same device)
Catalyst 6509-e Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6509-e (same device)
Catalyst 6509-neb a Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6509-neb-a (same device)
Catalyst 6509-v E Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6509-v-e (same device)
Catalyst 6513-e Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6513-e (same device)
Catalyst 6800IA Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6800ia (same device)
Catalyst 6807-xl Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6807-xl (same device)
Catalyst 6840-x Firmware· Cisco
95%
15.5(1.01.85)sy7
cisco-catalyst-6840-x (same device)

Remediation

No guide yet

No remediation guidance has been written for this CVE yet. What the record does show:

Patch and advisory references

References (7)

ExploitThird Party Advisory
Source: [email protected]
https://blog.champtar.fr/VLAN0_LLC_SNAP/
Technical DescriptionThird Party Advisory
Source: [email protected]
https://datatracker.ietf.org/doc/draft-ietf-v6ops-ra-guard/08/
Third Party AdvisoryUS Government Resource
Source: [email protected]
https://kb.cert.org/vuls/id/855201
Vendor Advisory
Source: [email protected]
https://standards.ieee.org/ieee/802.1Q/10323/
Vendor Advisory
Source: [email protected]
https://standards.ieee.org/ieee/802.2/1048/
Vendor AdvisoryThird Party Advisory
Source: [email protected]
https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-VU855201-J3z8CKTX
Source: af854a3a-2127-422b-91ae-364da2661108
https://www.kb.cert.org/vuls/id/855201

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

4.7MEDIUMCISA-ADP
Attack Vector
ADJACENT_NETWORK
Complexity
LOW
Privileges
NONE
User Interaction
NONE
Scope
CHANGED
Confidentiality
NONE
Integrity
LOW
Availability
NONE
Vector?

Quick Info

StatusMODIFIED
CWE
CWE-290

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