CVE-2022-23971: ASUS RT-AX56U Path Traversal Vulnerability

CVE-2022-23971 Overview

CVE-2022-23971 is a path traversal vulnerability affecting ASUS RT-AX56U routers. The vulnerability exists in the update_PLC/PORT file due to insufficient filtering for special characters in the URL parameter. An unauthenticated attacker with LAN access can exploit this flaw to overwrite system files by uploading malicious PLC/PORT files with the same filename, leading to service disruption.

Critical Impact

Unauthenticated LAN attackers can overwrite critical system files, causing service disruption and potentially compromising router integrity without any authentication requirements.

Affected Products

• ASUS RT-AX56U (Hardware)
• ASUS RT-AX56U Firmware version 3.0.0.4.386.45898

Discovery Timeline

• 2022-04-07 - CVE-2022-23971 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-23971

Vulnerability Analysis

This path traversal vulnerability (CWE-22) stems from improper input validation in the ASUS RT-AX56U router's update_PLC/PORT functionality. The firmware fails to properly sanitize special characters within URL parameters, allowing attackers to escape the intended directory structure and access arbitrary file system locations.

The vulnerability is exploitable from adjacent networks without requiring any authentication or user interaction. Successful exploitation enables attackers to overwrite system files, which can result in denial of service conditions. The attack impacts both the integrity and availability of the affected router, though confidentiality remains unaffected as the vulnerability is primarily used for file overwriting rather than data exfiltration.

Root Cause

The root cause is insufficient input validation and filtering of special path characters (such as ../ sequences) in the URL parameter processed by the update_PLC/PORT file. The firmware does not properly sanitize or validate user-supplied input before using it in file system operations, allowing directory traversal sequences to escape the intended upload directory.

Attack Vector

The attack requires adjacent network access, meaning the attacker must be on the same local network segment as the vulnerable router. The attack flow involves:

1. An attacker on the LAN identifies a vulnerable ASUS RT-AX56U router
2. The attacker crafts a malicious request to the update_PLC/PORT endpoint containing path traversal sequences in the URL parameter
3. The insufficient filtering allows the attacker to specify arbitrary file paths
4. By uploading a file with a name matching a critical system file, the attacker can overwrite it
5. The system file overwrite causes service disruption or router malfunction

The vulnerability does not require authentication, making any device on the local network a potential attacker. This is particularly concerning in environments where guest networks share the same LAN segment or where untrusted devices may be present.

Detection Methods for CVE-2022-23971

Indicators of Compromise

• Unexpected modifications to system files on the router
• Router service disruptions or unexpected reboots following network activity
• Suspicious HTTP requests to the update_PLC/PORT endpoint containing ../ sequences or other path traversal patterns
• Anomalous file upload activity from LAN devices to the router's management interface

Detection Strategies

• Monitor network traffic for requests containing path traversal sequences (e.g., ../, ..%2f, %2e%2e/) targeting router management endpoints
• Implement intrusion detection rules to flag suspicious requests to PLC/PORT update functionality
• Deploy network monitoring to detect unusual file upload patterns to router management interfaces
• Review router logs for failed or successful file operations involving unexpected file paths

Monitoring Recommendations

• Enable verbose logging on the ASUS RT-AX56U router if available
• Deploy network-level monitoring solutions to inspect traffic destined for router management interfaces
• Implement file integrity monitoring on critical router system files if accessible
• Regularly audit network segments for unauthorized devices that could launch adjacent network attacks

How to Mitigate CVE-2022-23971

Immediate Actions Required

• Update ASUS RT-AX56U firmware to the latest available version from ASUS
• Restrict management interface access to trusted devices only
• Segment the network to isolate the router's management interface from untrusted LAN devices
• Disable remote management features if not required
• Monitor for suspicious network activity targeting router endpoints

Patch Information

ASUS users should check for firmware updates through the router's administration interface or download the latest firmware from the ASUS support website. The TWCERT advisory provides additional details about this vulnerability at the TWCERT Security Advisory.

Workarounds

• Implement network segmentation to isolate the router from potentially malicious LAN devices
• Use VLAN configurations to separate management traffic from general network traffic
• Deploy a firewall or access control lists to restrict which devices can communicate with the router's management interface
• Consider temporarily disabling the PLC functionality if not required until a patch can be applied
• Monitor and restrict physical access to the network to prevent unauthorized device connections

# Example: Network segmentation using VLAN (implementation varies by network equipment)
# Isolate router management interface from untrusted network segments
# Consult your network equipment documentation for specific VLAN configuration

# Restrict access to management interface via firewall rules
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP