CVE-2021-27861: IEEE 802.2 Auth Bypass Vulnerability

CVE-2021-27861 Overview

CVE-2021-27861 is a network protocol vulnerability that allows attackers to bypass Layer 2 network filtering capabilities, including IPv6 Router Advertisement (RA) guard protections. The vulnerability exploits a flaw in how network devices process LLC/SNAP headers with invalid length fields, optionally combined with VLAN0 headers, enabling malicious traffic to evade security controls.

Critical Impact

Attackers on the same network segment can bypass IPv6 RA guard and other Layer 2 filtering mechanisms, potentially enabling man-in-the-middle attacks, rogue router advertisements, and network infrastructure compromise.

Affected Products

• IEEE 802.2 (LLC/SNAP protocol implementations)
• IETF P802.1Q (VLAN tagging implementations)
• Network devices implementing Layer 2 filtering based on these standards

Discovery Timeline

• 2022-09-27 - CVE CVE-2021-27861 published to NVD
• 2025-11-04 - Last updated in NVD database

Technical Details for CVE-2021-27861

Vulnerability Analysis

This vulnerability affects the fundamental Layer 2 network protocols defined in IEEE 802.2 (Logical Link Control) and IEEE 802.1Q (VLAN tagging). The core issue lies in how network security devices parse and validate LLC/SNAP (Subnetwork Access Protocol) headers when performing deep packet inspection for features like IPv6 RA guard.

When a network device receives a frame with an LLC/SNAP header containing an invalid length field, the security filtering logic may fail to properly inspect the encapsulated payload. This parsing discrepancy between security devices and end hosts creates a window for evasion—the security device may not recognize the malicious content, while the destination host processes it normally.

The attack can be enhanced by combining the malformed LLC/SNAP headers with VLAN0 (native VLAN) tagging, further confusing security inspection mechanisms. This technique exploits the complexity of Layer 2 protocol stacking and the assumptions made by filtering implementations.

Root Cause

The root cause stems from two related weaknesses identified as CWE-130 (Improper Handling of Length Parameter Inconsistency) and CWE-290 (Authentication Bypass by Spoofing). Network filtering devices do not properly validate or handle LLC/SNAP headers with inconsistent or invalid length values. When the length field does not match the actual frame content, security devices may:

1. Skip inspection of the encapsulated payload entirely
2. Misinterpret frame boundaries and miss malicious content
3. Apply incorrect parsing logic that differs from the receiving host

This implementation disparity between security devices and network hosts allows crafted frames to bypass filtering while still being processed by target systems.

Attack Vector

The attack requires adjacent network access (same Layer 2 broadcast domain) and leverages the protocol-level vulnerability to evade security controls. An attacker constructs specially crafted Ethernet frames with the following characteristics:

1. LLC/SNAP Header Manipulation: The attacker creates frames using LLC/SNAP encapsulation with deliberately invalid length fields that cause parsing inconsistencies
2. Optional VLAN0 Stacking: Adding VLAN0 (priority-tagged) headers can further complicate frame parsing
3. Malicious Payload Delivery: The actual attack payload (such as rogue IPv6 Router Advertisements) is encapsulated within the malformed frame structure

The technique described in the Champtar Blog Post demonstrates how these header manipulations can be combined to bypass RA guard implementations. The attack does not require authentication and can be performed by any device on the local network segment.

Detection Methods for CVE-2021-27861

Indicators of Compromise

• Unusual LLC/SNAP encapsulated traffic on network segments that typically use Ethernet II framing
• IPv6 Router Advertisements originating from unexpected MAC addresses after RA guard is enabled
• Network frames with VLAN0 tags combined with LLC/SNAP headers
• Length field mismatches in captured network traffic between LLC header values and actual payload sizes

Detection Strategies

• Deploy network capture tools to analyze raw Ethernet frames for LLC/SNAP encapsulation anomalies
• Monitor for IPv6 RA messages that bypass configured RA guard policies
• Implement deep packet inspection at multiple network layers to detect framing inconsistencies
• Use IDS/IPS signatures that specifically look for malformed LLC/SNAP length fields

Monitoring Recommendations

• Enable verbose logging on network switches and routers for Layer 2 security feature violations
• Capture and analyze traffic on security-sensitive VLANs for non-standard frame encapsulation
• Monitor for changes in IPv6 default gateway or DNS configurations that may indicate successful RA attacks
• Alert on any RA guard bypass attempts detected by updated switch firmware

How to Mitigate CVE-2021-27861

Immediate Actions Required

• Contact your network equipment vendor to determine if firmware updates addressing this vulnerability are available
• Review the CERT Vulnerability ID 855201 for vendor-specific guidance and affected product lists
• Audit network segments for critical assets and prioritize mitigation on high-value targets
• Consider implementing additional IPv6 security controls beyond RA guard as defense-in-depth

Patch Information

This vulnerability affects protocol implementations across multiple vendors. Mitigation requires firmware or software updates from individual network equipment manufacturers. Consult the CERT Vulnerability ID 855201 advisory for a comprehensive list of affected vendors and their patch status. The IEEE 802.1Q Standard and IEEE 802.2 Standard documentation provides additional technical context.

Workarounds

• Implement strict port security to limit which devices can connect to network segments
• Use 802.1X network access control to authenticate devices before granting network access
• Deploy additional IPv6 security mechanisms such as DHCPv6 guard and SAVI (Source Address Validation Improvement)
• Consider disabling LLC/SNAP processing on switches if not required for legacy protocols
• Segment critical network infrastructure using physically separate networks where RA guard bypass would be most impactful

# Example: Enable additional IPv6 security features on Cisco IOS
# These measures provide defense-in-depth alongside RA guard

# Enable DHCPv6 guard on VLAN interface
ipv6 dhcp guard policy DHCP_GUARD_POLICY
  device-role server
  
# Apply to interface
interface Vlan100
  ipv6 dhcp guard attach-policy DHCP_GUARD_POLICY
  
# Enable source guard for IPv6
ipv6 source-guard policy SOURCE_GUARD_POLICY
  deny global-autoconf
  permit link-local