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

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

Published: 2022-09-27 Last update: 2026-06-17 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-27861 is rated Low Risk (34.3/100): CVSS Medium severity, with low exploitation likelihood (EPSS 0.58%). Monitor for updates and reassess as exploit intelligence or EPSS changes.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-27861

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 0.08% 0.58% +0.50%
2 2025-11-21 0.15% 0.08% -0.07%
3 2025-11-18 — 0.15% —

Full EPSS history (9 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-27861

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
4.7 3.1 MEDIUM
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N Click to expand
Attack vector (AV:A)
Attacker has to be nearby on the network—same office, same link, that vibe—not the whole wide internet.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 2.8 1.4 [email protected]
4.7 3.1 MEDIUM
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N Click to expand
Attack vector (AV:A)
Attacker has to be nearby on the network—same office, same link, that vibe—not the whole wide internet.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:N)
Service keeps running; no real outage angle.
 2.8 1.4 134c704f-9b21-4f2e-91b3-4a467353bcc0

Weakness enumeration for CVE-2021-27861

OS Trackers for CVE-2021-27861

vendor priority summary link
redhat medium — https://access.redhat.com/security/cve/CVE-2021-27861
suse medium CVE-2021-27861 severity moderate: SUSE including 13 source package names (kernel-default, kernel-default-base, …), 234 product×package rows across 56 product lines (SUSE CaaS Platform 4.0, SUSE Enterprise Storage 6, … (56 product lines)): Known Not Affected 234. https://www.suse.com/security/cve/CVE-2021-27861/

Affected software / configurations for CVE-2021-27861

Vendor Product Version Raw CPE
ieee ieee_802.2 <= 802.2h-1997 cpe:2.3:a:ieee:ieee_802.2:*:*:*:*:*:*:*:*
ietf p802.1q <= d1.0 cpe:2.3:a:ietf:p802.1q:*:*:*:*:*:*:*:*

References for CVE-2021-27861

cvelogic Threat Intelligence