CVE-2020-26139: NetBSD Kernel Wi-Fi DoS Vulnerability

CVE-2020-26139 Overview

CVE-2020-26139 is an authentication bypass vulnerability discovered in the NetBSD 7.1 kernel that affects Wi-Fi Access Point (AP) implementations. The flaw allows an Access Point to forward EAPOL (Extensible Authentication Protocol over LAN) frames to other connected clients even when the sender has not yet successfully authenticated to the AP. This vulnerability is part of the broader "FragAttacks" (fragmentation and aggregation attacks) family of Wi-Fi security vulnerabilities that affect numerous wireless networking implementations across multiple vendors.

The vulnerability can be abused in protected Wi-Fi networks to launch denial-of-service attacks against legitimately connected clients. Additionally, it facilitates the exploitation of other vulnerabilities present in connected client devices by allowing unauthenticated attackers to inject malicious EAPOL frames into the network.

Critical Impact

Unauthenticated attackers within wireless range can forward EAPOL frames to connected clients, enabling denial-of-service attacks and potentially facilitating exploitation of additional vulnerabilities in client devices on protected Wi-Fi networks.

Affected Products

• NetBSD 7.1
• Debian Linux 9.0
• Cisco Aironet Series Access Points (1532, 1542, 1552, 1572, 1702, 1800, 2702, 2800, 3702, 3800, 4800 series)
• Cisco Catalyst 9100 Series Access Points (9105, 9115, 9117, 9120, 9124, 9130)
• Cisco Meraki MR/MX/Z Series Wireless Devices
• Cisco IP Phones (6861, 8821, 8832, 8861, 8865)
• Cisco Webex Board and Room Devices
• Arista C-Series and O-Series Access Points
• Intel Wi-Fi 6 AX200/AX201 Adapters
• Intel Killer Wi-Fi 6/6E Adapters
• Intel ProSet AC/Wi-Fi 6 Series Adapters

Discovery Timeline

• May 11, 2021 - CVE-2020-26139 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-26139

Vulnerability Analysis

This vulnerability relates to improper authentication handling (CWE-287) in the Wi-Fi frame forwarding logic within the kernel. The flaw specifically impacts how Access Points process and forward EAPOL frames during the authentication handshake phase.

In a properly secured Wi-Fi network, the AP should only forward frames between clients that have completed the 802.1X authentication process. However, affected implementations forward EAPOL frames from unauthenticated sources to authenticated clients, breaking this security boundary. This allows an attacker who is within wireless range but has not authenticated to the network to inject EAPOL frames that will be delivered to legitimate, authenticated clients.

The attack requires adjacent network access (the attacker must be within wireless range of the target AP) and has high attack complexity due to specific timing and protocol requirements. While the vulnerability does not directly compromise confidentiality or integrity, it can cause complete denial of service to connected clients.

Root Cause

The root cause of CVE-2020-26139 lies in inadequate validation of sender authentication state before forwarding EAPOL frames. The kernel's wireless networking stack fails to properly verify that the source of an EAPOL frame has completed the 802.1X authentication handshake before relaying the frame to other connected clients. This missing authentication check violates the security assumptions of protected Wi-Fi networks, where only authenticated devices should be able to communicate with other network participants.

Attack Vector

An attacker exploiting this vulnerability must be within wireless range of the target Access Point. The attack proceeds as follows:

1. The attacker associates with the target AP but does not complete the authentication process
2. The attacker crafts malicious EAPOL frames targeting specific authenticated clients
3. Due to the vulnerability, the AP forwards these unauthenticated EAPOL frames to the target clients
4. The malicious frames can cause denial-of-service conditions or be used to facilitate exploitation of other client-side vulnerabilities

The vulnerability is particularly concerning in enterprise environments where Wi-Fi Protected Access (WPA2/WPA3) is expected to provide isolation between authenticated and unauthenticated devices. By bypassing this isolation, attackers can target devices that are otherwise protected by network authentication.

Detection Methods for CVE-2020-26139

Indicators of Compromise

• Unusual volume of EAPOL frames from non-associated or unauthenticated wireless sources
• Client devices experiencing unexpected disconnections or authentication failures
• Wireless IDS/IPS alerts for abnormal EAPOL frame patterns
• Log entries indicating EAPOL frame processing from unauthenticated MAC addresses

Detection Strategies

• Deploy wireless intrusion detection systems (WIDS) to monitor for anomalous EAPOL frame activity
• Configure AP logging to capture detailed information about EAPOL frame sources and authentication states
• Monitor for clients reporting authentication failures or unexpected deauthentication events
• Implement network-level monitoring to detect patterns consistent with FragAttacks exploitation

Monitoring Recommendations

• Enable verbose logging on wireless controllers and access points to track EAPOL frame handling
• Establish baseline metrics for normal EAPOL traffic patterns to identify anomalies
• Configure alerts for high volumes of EAPOL frames from devices that have not completed authentication
• Review wireless client health metrics for signs of denial-of-service impact

How to Mitigate CVE-2020-26139

Immediate Actions Required

• Identify all affected wireless access points, controllers, and client devices in your environment
• Prioritize patching of access points that service critical or high-security network segments
• Review vendor security advisories from Cisco, Intel, Arista, and other affected manufacturers
• Consider implementing additional network segmentation to limit the impact of potential exploitation

Patch Information

Multiple vendors have released firmware and driver updates to address this vulnerability. Organizations should apply patches according to their vendor's recommendations:

• Cisco: See the Cisco Security Advisory for affected products and patch availability
• Intel: Updated wireless drivers are available through Intel's driver download center
• Arista: Review the Arista Security Advisory 12602 for firmware updates
• Debian: Security updates are available through Debian LTS announcements
• Siemens: Refer to the Siemens Security Advisory SSA-913875 for industrial device updates

For comprehensive technical details on the FragAttacks vulnerability family, see the FragAttacks research summary on GitHub or the official FragAttacks website.

Workarounds

• Enable client isolation features on access points where available to limit frame forwarding between clients
• Consider using wired connections for critical devices until patches can be applied
• Implement MAC address filtering as an additional layer of defense (though not a complete mitigation)
• Monitor wireless networks closely for signs of exploitation while awaiting patch deployment

# Example: Enable client isolation on Cisco Catalyst 9800 Wireless Controller
# This limits direct client-to-client communication through the AP

configure terminal
wlan YOUR_WLAN_NAME
peer-blocking drop
end
write memory