CVE-2020-26147: Linux Kernel Fragment Injection Vulnerability

CVE-2020-26147 Overview

CVE-2020-26147 is a Wi-Fi security vulnerability discovered in the Linux kernel 5.8.9 that affects the WEP, WPA, WPA2, and WPA3 protocol implementations. The flaw exists in how these implementations handle fragmented frames, allowing reassembly of fragments even when some of them were sent in plaintext while the connection uses encrypted protocols (WEP, CCMP, or GCMP). This vulnerability is part of the broader "FragAttacks" (fragmentation and aggregation attacks) family of Wi-Fi vulnerabilities.

Critical Impact

An attacker within adjacent network range can exploit this vulnerability to inject packets and/or exfiltrate selected fragments when another device sends fragmented frames, potentially compromising data confidentiality and integrity on encrypted Wi-Fi networks.

Affected Products

• Linux Kernel (multiple versions through 5.8.9)
• Debian Linux 9.0
• Arista C-75, C-65, O-90, and W-68 wireless access points (firmware)
• Siemens SCALANCE W700 IEEE 802.11n and W1700 IEEE 802.11ac (firmware)

Discovery Timeline

• 2021-05-11 - CVE-2020-26147 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-26147

Vulnerability Analysis

This vulnerability targets a fundamental flaw in how Wi-Fi implementations handle frame fragmentation. When a large frame needs to be transmitted over Wi-Fi, it can be split into smaller fragments. The receiving device is expected to reassemble these fragments before processing the complete frame. The vulnerability arises because the Linux kernel's Wi-Fi stack does not properly validate whether all fragments in a sequence were encrypted consistently.

When an encrypted Wi-Fi connection (using WEP, CCMP, or GCMP) is established, all fragments should be encrypted. However, the vulnerable implementation accepts a mix of plaintext and encrypted fragments during reassembly. This creates an opportunity for an attacker positioned within Wi-Fi range to inject malicious plaintext fragments that get reassembled together with legitimate encrypted fragments from another device.

Root Cause

The root cause lies in insufficient validation during the fragment reassembly process in the Linux kernel's wireless networking subsystem. The implementation fails to enforce that all fragments belonging to a single frame must be either all plaintext or all encrypted. When using CCMP or GCMP encryption, the code does not verify the encryption state consistency across fragments, allowing mixed plaintext/encrypted fragment sequences to be processed and reassembled.

Attack Vector

This vulnerability requires the attacker to be within adjacent network range (Wi-Fi proximity) of the target device. The attack involves:

1. Monitoring fragmented frame transmissions from a victim device on the network
2. Injecting carefully crafted plaintext fragments that match the expected sequence numbers
3. The vulnerable implementation reassembles the attacker's plaintext fragments with legitimate encrypted fragments
4. This allows packet injection or selective exfiltration of fragment data

The attack exploits the Wi-Fi fragmentation mechanism where frames are identified by sequence numbers and fragment numbers. By injecting fragments with matching identifiers, an attacker can manipulate the reassembly process. The vulnerability requires user interaction in that the victim device must be actively sending fragmented frames.

Detection Methods for CVE-2020-26147

Indicators of Compromise

• Unusual fragmented frame patterns in wireless network traffic with inconsistent encryption states
• Detection of plaintext fragments on networks configured for WPA2/WPA3 encryption
• Anomalous reassembly behavior in wireless driver logs indicating mixed fragment types
• Unexpected packet injection attempts from unauthenticated sources within Wi-Fi range

Detection Strategies

• Deploy wireless intrusion detection systems (WIDS) capable of monitoring fragment-level traffic anomalies
• Implement network monitoring for inconsistent encryption states within fragmented frame sequences
• Review kernel and wireless driver logs for reassembly errors or warnings related to fragment processing
• Monitor for unexpected traffic patterns that may indicate fragment injection attempts

Monitoring Recommendations

• Enable verbose logging in wireless drivers to capture fragment reassembly events
• Deploy SentinelOne agents on Linux endpoints to monitor for exploitation attempts targeting network stack vulnerabilities
• Implement continuous monitoring of Wi-Fi management and data frames for protocol violations
• Establish baseline fragmentation patterns to detect anomalous fragment sequences

How to Mitigate CVE-2020-26147

Immediate Actions Required

• Update the Linux kernel to a patched version that addresses the FragAttacks vulnerabilities
• Apply firmware updates from Arista, Siemens, and other affected device vendors as documented in their respective security advisories
• Review network architecture to minimize exposure of vulnerable devices to untrusted adjacent networks
• Consider disabling frame fragmentation where operationally feasible

Patch Information

Patches for this vulnerability have been released by multiple vendors. For Linux systems, kernel updates addressing the FragAttacks family of vulnerabilities should be applied. Debian has released updates as documented in Debian LTS Announcement #19 and Debian LTS Announcement #20. Additional vendor-specific patches are available from:

• Siemens Security Advisory SSA-913875
• Arista Security Advisory #12602
• Cisco Security Advisory

For comprehensive technical details on the FragAttacks vulnerabilities, refer to the FragAttacks Official Site and the GitHub FragAttacks Summary.

Workarounds

• Disable Wi-Fi fragmentation support if the environment permits, though this may impact connectivity for some legacy devices
• Implement network segmentation to isolate critical systems from potential adjacent network attackers
• Use wired Ethernet connections for sensitive systems where Wi-Fi vulnerability exposure is a concern
• Deploy additional network monitoring at the wireless access point level to detect fragment injection attempts

# Example: Check current kernel version for patch status
uname -r

# Verify wireless driver module information
modinfo <wireless_driver_name>

# Review available kernel updates (Debian/Ubuntu)
apt update && apt list --upgradable | grep linux-image