Security readout for executives and security teams
Plain-English summary
This CVE describes a way to evade some Layer 2 network filtering controls, including IPv6 RA Guard, by using unusual frame encapsulation and Ethernet-to-Wi-Fi translation behavior. The impact is limited to adjacent network attackers, but it can weaken controls that prevent rogue network advertisements or similar local-network manipulation.
Executive priority
Prioritize review for enterprise access networks, campus Wi-Fi, and environments depending on RA Guard for segmentation assurance. This is not a remote internet-facing critical issue, but it can undermine local network trust boundaries.
Technical view
The issue involves filtering gaps when frames combine VLAN 0 headers, LLC/SNAP headers, and Ethernet/Wi-Fi frame translation. Affected references include draft RA Guard material and IEEE 802.2/802.1Q-related specifications. CVSS 3.1 is 4.7, adjacent attack vector, no privileges, no user interaction, changed scope, and low integrity impact.
Likely exposure
Exposure is most relevant where switches, wireless bridges, access points, or controllers rely on RA Guard or similar Layer 2 filtering across wired and wireless boundaries. The source bundle identifies standards and drafts, not a complete vendor product list.
Exploitation context
CISA KEV status is false in the supplied bundle, and no provided source states active exploitation. Public references include CERT and a researcher write-up, so defenders should treat this as a known technique class rather than assume broad real-world exploitation.
Researcher notes
Evidence is strongest for a standards-level filtering bypass pattern, not for a specific patched product matrix. Focus research on parser normalization, encapsulation handling, and policy consistency before and after Ethernet/Wi-Fi translation.
Mitigation direction
- Check network vendor advisories for RA Guard and Layer 2 filtering updates.
- Update switch, access point, controller, and bridge firmware where vendors provide fixes.
- Do not rely on RA Guard as the only IPv6 first-hop protection control.
- Review wired-wireless bridging paths where frame translation may affect filtering.
- Monitor local network segments for unauthorized IPv6 router advertisements or control traffic.
Validation and detection
- Inventory devices enforcing RA Guard or comparable Layer 2 filtering.
- Identify segments where wired and wireless traffic is bridged or translated.
- Review vendor documentation for VLAN 0 and LLC/SNAP filtering behavior.
- Validate controls only in an approved lab or maintenance window.
- Confirm monitoring detects unauthorized IPv6 router advertisement activity.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
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CWE-290: Exact CWE lookup
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Open ATT&CK lookupCVE-2021-27854 mapping review
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Open ATT&CK lookup- Severity
- Medium
- CVSS
- 4.7 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N2.81.4Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
4.7MediumVector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:C/C:N/I:L/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://datatracker.ietf.org/doc/draft-ietf-v6ops-ra-guard/08/CVE reference
- https://standards.ieee.org/ieee/802.2/1048/CVE reference
- https://standards.ieee.org/ieee/802.1Q/10323/CVE reference
- https://kb.cert.org/vuls/id/855201CVE reference
- https://blog.champtar.fr/VLAN0_LLC_SNAP/CVE reference
- https://www.kb.cert.org/vuls/id/855201CVE reference
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Authentication Bypass by Spoofing
Authentication Bypass by Spoofing represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.