CVE-2021-27853: IEEE 802.2 Auth Bypass Vulnerability

CVE-2021-27853 Overview

CVE-2021-27853 is a network protocol vulnerability affecting Layer 2 network filtering mechanisms in devices implementing IEEE 802.2 and 802.1Q standards. The vulnerability allows attackers to bypass critical network security controls such as IPv6 RA (Router Advertisement) guard and ARP inspection by crafting malicious Ethernet frames using combinations of VLAN 0 headers and LLC/SNAP (Logical Link Control/Subnetwork Access Protocol) headers.

This authentication spoofing vulnerability (CWE-290) enables adjacent network attackers to circumvent Layer 2 security mechanisms that are designed to protect networks from spoofing attacks, rogue router advertisements, and ARP-based attacks. The flaw stems from improper handling of stacked VLAN tags and LLC/SNAP encapsulation, which can cause security inspection mechanisms to fail to properly analyze malicious traffic.

Critical Impact
Attackers on an adjacent network can bypass IPv6 RA guard and ARP inspection protections, enabling man-in-the-middle attacks, network traffic interception, and potential compromise of network integrity without authentication.

Affected Products

IEEE 802.2 and IEEE 802.1Q protocol implementations
Cisco Catalyst 3650, 3850, 6500-E, 6800, 9200, 9300, 9400, 9500, and 9600 series switches
Cisco Nexus 9000 series switches (firmware version 9.3(5))
Cisco Meraki MS210, MS225, MS250, MS350, MS355, MS390, MS410, MS420, MS425, and MS450 series
Cisco SF500 and SG500 series switches (firmware version 3.0.0.61)
Cisco IOS XE versions 15.2(07)E02, 15.2(07)E03, 17.3.3, 17.4.1, and 17.6.1

Discovery Timeline

September 27, 2022 - CVE-2021-27853 published to NVD
November 4, 2025 - Last updated in NVD database

Technical Details for CVE-2021-27853

Vulnerability Analysis

This vulnerability exploits a fundamental weakness in how network devices process IEEE 802.1Q VLAN tags and LLC/SNAP headers when performing Layer 2 security inspections. The attack requires the attacker to be on an adjacent network (same Layer 2 broadcast domain) and does not require any authentication or user interaction.

The vulnerability affects the integrity of network traffic by allowing attackers to craft frames that appear legitimate to security controls but actually contain malicious payloads. While the vulnerability does not directly lead to data confidentiality breaches, successful exploitation enables various secondary attacks including ARP spoofing and rogue IPv6 router advertisements.

The impact is particularly concerning for enterprise environments relying on these Layer 2 protections as a security boundary, as the bypass renders these controls ineffective against a motivated attacker with adjacent network access.

Root Cause

The root cause lies in the improper validation and inspection of Ethernet frames containing VLAN 0 tags (priority tagging without VLAN assignment) combined with LLC/SNAP encapsulation. When network devices encounter these specially crafted frame combinations, the security inspection logic fails to properly parse and analyze the encapsulated payload.

Specifically, the VLAN 0 header causes some implementations to strip the tag and process the remaining frame differently, while the LLC/SNAP headers provide an alternate encapsulation method that bypasses the normal EtherType-based filtering mechanisms. The combination of these two techniques creates a parsing differential between the security control and the actual frame processing, allowing malicious traffic to pass inspection.

Attack Vector

An attacker positioned on the same Layer 2 network segment as the target can construct Ethernet frames that exploit this parsing vulnerability. The attack proceeds by:

Crafting Ethernet frames with VLAN ID 0 (802.1Q priority tagging)
Encapsulating malicious payloads using LLC/SNAP headers instead of standard EtherType encoding
Embedding ARP spoofing packets or malicious IPv6 Router Advertisements within the LLC/SNAP encapsulated payload
Sending these frames on the local network segment

The security inspection mechanisms (RA guard, ARP inspection) fail to properly decode and inspect the LLC/SNAP encapsulated content, allowing the malicious traffic to reach its intended targets. The exploitation mechanism involves manipulating how the frame headers are interpreted, causing a mismatch between what the security control inspects and what the receiving host processes.

For detailed technical analysis of the VLAN 0 and LLC/SNAP bypass techniques, see the security researcher's technical writeup and the CERT vulnerability note.

Detection Methods for CVE-2021-27853

Indicators of Compromise

Unusual Ethernet frames containing VLAN ID 0 (priority-tagged frames) on network segments where priority tagging is not expected
LLC/SNAP encapsulated frames carrying IPv6 or ARP payloads rather than standard EtherType-encoded frames
Unexpected ARP responses or IPv6 Router Advertisements from unauthorized MAC addresses
Network traffic patterns indicating man-in-the-middle positioning after RA guard or ARP inspection bypass

Detection Strategies

Implement deep packet inspection at network monitoring points to identify frames using VLAN 0 tags with LLC/SNAP encapsulation
Deploy network behavior analysis to detect anomalous ARP or IPv6 RA patterns that may indicate successful bypass attempts
Configure SIEM rules to alert on Layer 2 security control violations or bypasses reported by switch telemetry
Monitor for gratuitous ARP or duplicate address detection failures that may indicate ARP spoofing following bypass

Monitoring Recommendations

Enable comprehensive logging on affected Cisco switches to capture security control bypass attempts
Implement port-based traffic mirroring to capture and analyze suspicious Layer 2 traffic patterns
Deploy network detection systems capable of inspecting Layer 2 frame structures beyond standard EtherType processing
Establish baseline traffic patterns for ARP and IPv6 RA traffic to identify deviations indicative of attack activity

How to Mitigate CVE-2021-27853

Immediate Actions Required

Review the Cisco Security Advisory for vendor-specific mitigation guidance and available patches
Identify all affected Cisco Catalyst, Nexus, Meraki, and SF/SG500 series devices in your environment
Prioritize patching on network segments with sensitive traffic or where Layer 2 security controls are critical
Consider implementing additional network segmentation to limit the attack surface for adjacent network attacks

Patch Information

Cisco has released security advisories addressing this vulnerability for affected product lines. Organizations should consult the Cisco Security Advisory cisco-sa-VU855201-J3z8CKTX for specific firmware versions that address CVE-2021-27853.

For Cisco IOS XE devices, upgrade to patched versions beyond 17.6.1 as specified in vendor documentation. For Nexus 9000 series, upgrade firmware beyond version 9.3(5). Cisco Meraki devices receive automatic cloud-managed updates when fixes are available.

Additional context on the protocol-level issues can be found in the IEEE 802.1Q Standard and IEEE 802.2 Standard documentation.

Workarounds

Configure port security and DHCP snooping to limit the effectiveness of ARP spoofing even if inspection is bypassed
Implement static ARP entries for critical infrastructure devices where feasible to reduce reliance on dynamic ARP inspection
Consider deploying out-of-band network monitoring that does not rely on the same Layer 2 security controls being bypassed
Enable 802.1X port-based network access control to authenticate devices before granting Layer 2 network access

bash

# Example: Enable additional port security on Cisco IOS switches
# This provides defense-in-depth while awaiting patches

interface GigabitEthernet1/0/1
  switchport port-security
  switchport port-security maximum 2
  switchport port-security violation restrict
  switchport port-security aging time 2
  switchport port-security aging type inactivity