CVE-2021-27862: IEEE 802.2 Auth Bypass Vulnerability

CVE-2021-27862 Overview

CVE-2021-27862 is a network protocol vulnerability that allows attackers to bypass Layer 2 network filtering capabilities such as IPv6 RA guard. The bypass is achieved using LLC/SNAP headers with invalid length combined with Ethernet to WiFi frame conversion, and optionally VLAN0 headers. This vulnerability affects fundamental network protocol implementations, making it particularly concerning for enterprise network security.

Critical Impact

Attackers on adjacent networks can bypass IPv6 Router Advertisement (RA) guard protections, potentially enabling rogue router advertisements, network traffic interception, and man-in-the-middle attacks on the local network segment.

Affected Products

• IEEE 802.2 (All versions)
• IETF P802.1Q (All versions)
• Network devices implementing IEEE 802.2 and 802.1Q standards

Discovery Timeline

• 2022-09-27 - CVE CVE-2021-27862 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2021-27862

Vulnerability Analysis

This vulnerability exploits weaknesses in how network devices handle LLC/SNAP (Logical Link Control/Subnetwork Access Protocol) headers during frame processing. The core issue lies in improper length validation of LLC/SNAP headers (CWE-130) combined with authentication bypass through spoofing (CWE-290).

When network filtering mechanisms such as IPv6 RA guard inspect frames, they typically parse the Ethernet frame to identify the payload type. However, by crafting packets with LLC/SNAP headers containing invalid length values, attackers can cause the filtering mechanism to misinterpret the frame structure. The frame conversion process between Ethernet and WiFi further compounds this issue, as the translation may not preserve the security context that would normally trigger filtering rules.

The optional use of VLAN0 headers adds another layer of obfuscation, as VLAN0 (priority tagging) is often handled differently by network devices compared to standard VLAN-tagged frames.

Root Cause

The root cause of CVE-2021-27862 is twofold: improper handling of length inconsistencies in LLC/SNAP headers (CWE-130) and the failure to properly validate the authenticity of network frames during Ethernet-to-WiFi conversion (CWE-290). Network filtering mechanisms trust the header length fields without adequate validation, allowing malformed packets to evade inspection.

Attack Vector

The attack requires adjacent network access, meaning the attacker must be on the same Layer 2 network segment as the target. The attack does not require authentication or user interaction, and can affect systems beyond the vulnerable component (changed scope). The attacker crafts specially formatted Ethernet frames with LLC/SNAP headers containing invalid length values, which are then converted to WiFi frames. This conversion process, combined with the malformed headers, causes RA guard implementations to fail to detect malicious Router Advertisements.

The attack flow involves:

1. Attacker crafts Ethernet frames with LLC/SNAP headers containing deliberately invalid length values
2. Optional VLAN0 tagging is applied to further obfuscate the packet
3. Frames are transmitted on the local network and converted between Ethernet and WiFi
4. IPv6 RA guard filtering mechanisms fail to properly parse the malformed frames
5. Malicious Router Advertisements bypass security controls and reach target hosts

Detection Methods for CVE-2021-27862

Indicators of Compromise

• Unexpected IPv6 Router Advertisements from non-authorized sources on the network
• Network frames with LLC/SNAP headers exhibiting length field anomalies
• Increased VLAN0-tagged traffic or unusual Ethernet-to-WiFi frame conversion patterns
• Client devices showing unexpected IPv6 default gateway changes

Detection Strategies

• Implement deep packet inspection rules that validate LLC/SNAP header length fields against actual payload sizes
• Monitor for Router Advertisement messages from MAC addresses not associated with authorized routers
• Configure network monitoring tools to alert on VLAN0-tagged frames in environments where priority tagging is not expected
• Deploy IPv6 monitoring solutions that track RA source addresses and flag unauthorized senders

Monitoring Recommendations

• Enable logging on network switches and wireless access points for LLC/SNAP frame processing anomalies
• Implement network behavior analysis to detect unexpected routing changes on client devices
• Review CERT Vulnerability Advisory for updated detection guidance
• Monitor for multiple RA messages with conflicting information from different sources

How to Mitigate CVE-2021-27862

Immediate Actions Required

• Review network infrastructure for devices implementing IPv6 RA guard and identify those requiring updates
• Implement additional network segmentation to limit the impact of potential Layer 2 attacks
• Configure static IPv6 router addresses on critical systems where possible to reduce reliance on RA
• Consult vendor documentation for specific RA guard hardening recommendations

Patch Information

As this vulnerability affects fundamental protocol specifications (IEEE 802.2 and IETF P802.1Q), remediation depends on individual vendor implementations. Network administrators should consult the IEEE 802.2 Standard and IETF Draft Document for guidance on proper implementation. Contact your network equipment vendors for specific patches or firmware updates that address LLC/SNAP header validation issues.

Workarounds

• Implement additional Layer 3 filtering rules to complement RA guard functionality
• Use IEEE 802.1X port-based authentication to restrict which devices can send traffic on protected network segments
• Deploy separate VLANs for wired and wireless networks to reduce Ethernet-to-WiFi conversion attack surface
• Consider disabling VLAN0 priority tagging if not required in your environment

# Example: Enhanced RA Guard configuration concepts (vendor-specific syntax varies)
# Consult your vendor documentation for exact commands

# 1. Enable strict RA validation on switch ports
# interface ethernet1/1
#   ipv6 nd ra-guard enable
#   ipv6 nd ra-guard policy strict-validation

# 2. Configure allowed RA sources explicitly
# ipv6 nd ra-guard-list authorized-routers
#   permit ipv6 2001:db8::1/128

# 3. Enable LLC/SNAP validation if supported
# Refer to vendor-specific documentation for implementation