CVE-2021-27853Public exploit

Filtering bypass via stacked VLAN and LLC headers

Published Sep 27, 2022 · Updated Nov 4, 2025

Layer 2 filtering bypass in IEEE 802.2 and IETF IPv6 Router Advertisement Guard allows adjacent attackers to evade configured controls. Devices forward frames after stacked VLAN 0 and LLC/SNAP headers prevent the filter from identifying the upper-layer protocol. A host on the same Layer 2 path can inject rogue advertisements, redirect traffic to observe unencrypted data, or blackhole traffic for a service interruption.

CVSS severity4.7
Medium
EPSS probability0.76%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Layer 2 filtering bypass in IEEE 802.2 and IETF IPv6 Router Advertisement Guard allows adjacent attackers to evade configured controls. Devices forward frames after stacked VLAN 0 and LLC/SNAP headers prevent the filter from identifying the upper-layer protocol. A host on the same Layer 2 path can inject rogue advertisements, redirect traffic to observe unencrypted data, or blackhole traffic for a service interruption.

The record

CVE
CVE-2021-27853
Published
Sep 27, 2022
Updated
Nov 4, 2025
Vendor
IEEE
Product
IEEE 802.2
Classifications
CWE-290, T1562
Attack vector
adjacent
Privileges
unauthenticated

Timeline

How it unfolded

  1. Sep 27, 2022CVE publishedPublication date reported by the CVE source.
  2. Nov 4, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=802.2h-1997 <=802.2h-1997

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesunauthenticated

What is affected?

IEEE · IEEE 802.2version=802.2h-1997 <=802.2h-1997
Internet Engineering Task Force (IETF) · IPv6 Router Advertisement Guardversion=08 <=08

Published CVSS scores

4.7CiscoCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N/E:X/RL:X/RC:X
4.7CISA-ADPCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
4.7Arista NetworksCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-290Authentication Bypass by Spoofing
T1562Impair Defenses

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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