Security readout for executives and security teams
Plain-English summary
This issue lets a device on the same local network bypass some Layer 2 filtering controls, including IPv6 RA Guard, when Ethernet frames are translated to Wi-Fi frames. The practical risk is network policy bypass, not direct data theft. The sources do not identify active exploitation or product-specific patches.
Executive priority
Prioritize assessment for enterprise Wi-Fi and bridged networks that depend on RA Guard or Layer 2 filtering for security boundaries. Urgency is moderate because exploitation requires adjacent access and sources do not report active exploitation.
Technical view
CVE-2021-27862 describes a filtering bypass involving LLC/SNAP headers with invalid length during Ethernet-to-Wi-Fi frame conversion, optionally with VLAN0 headers. It affects standards-level behavior in IEEE 802.2h-1997 and an IETF RA Guard draft. CVSS is 4.7 with adjacent-network attack vector and low integrity impact.
Likely exposure
Exposure is most likely where Wi-Fi, Ethernet bridging, or conversion paths rely on Layer 2 filtering such as IPv6 RA Guard. The provided sources do not name specific vendor devices, operating systems, or fixed versions.
Exploitation context
The CVE requires adjacent network access. The bundle provides public research references, but KEV is false and no cited source in the bundle states active exploitation. Treat it as a control-bypass risk requiring local network presence.
Researcher notes
Evidence is standards and research oriented, not product-specific. Focus analysis on parser differences during frame translation, handling of invalid LLC/SNAP length fields, optional VLAN0 handling, and whether filtering occurs before or after normalization.
Mitigation direction
- Check CERT VU#855201 and vendor guidance for affected network equipment.
- Do not rely solely on Layer 2 filtering for IPv6 RA protection.
- Review Wi-Fi-to-Ethernet bridging and filtering paths in managed networks.
- Segment untrusted wireless clients from sensitive Layer 2 domains.
- Monitor for unexpected IPv6 router advertisements or policy bypass indicators.
Validation and detection
- Inventory devices performing Ethernet-to-Wi-Fi frame translation or bridging.
- Identify where IPv6 RA Guard or similar Layer 2 filtering is enforced.
- Confirm vendor firmware or configuration guidance for each relevant platform.
- Validate filtering behavior in a controlled lab, not on production networks.
- Review logs for 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-130: Exact CWE lookup
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Open ATT&CK lookupCWE-290: Exact CWE lookup
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Open ATT&CK lookupCVE-2021-27862 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
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://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.
Improper Handling of Length Parameter Inconsistency
Improper Handling of Length Parameter Inconsistency represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
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.