CVE-2021-27861Public exploit

Layer 2 filtering bypass via invalid LLC/SNAP lengths

Published Sep 27, 2022 · Updated Nov 4, 2025

Layer 2 filtering bypass in IEEE 802.2 and IETF IPv6 RA Guard allows attackers to redirect traffic with crafted frames. Implementations inconsistently validate the 802.3 length field and then forward invalid LLC/SNAP frames without applying configured Layer 2 inspection. Attackers must reach the same Layer 2 path; accepted rogue router advertisements can blackhole traffic or place attackers between hosts and their destinations.

CVSS severity4.7
Medium
EPSS probability0.62%
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 RA Guard allows attackers to redirect traffic with crafted frames. Implementations inconsistently validate the 802.3 length field and then forward invalid LLC/SNAP frames without applying configured Layer 2 inspection. Attackers must reach the same Layer 2 path; accepted rogue router advertisements can blackhole traffic or place attackers between hosts and their destinations.

The record

CVE
CVE-2021-27861
Published
Sep 27, 2022
Updated
Nov 4, 2025
Vendor
IEEE
Product
IEEE 802.2
Classifications
CWE-290, CWE-130, T1557
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

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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

4.7CISA-ADPCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
4.7Cisco PSIRTCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
4.7NIST NVDCVSS: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
CWE-130Improper Handling of Length Parameter Inconsistency
T1557Adversary-in-the-Middle

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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