CVE-2020-14472: Draytek Vigor300b RCE Vulnerability

CVE-2020-14472 Overview

CVE-2020-14472 is a command injection vulnerability affecting DrayTek Vigor3900, Vigor2960, and Vigor300B enterprise routers running firmware versions prior to 1.5.1.1. The vulnerability exists within the mainfunction.cgi file, which fails to properly sanitize user-supplied input before passing it to system commands. This allows unauthenticated remote attackers to execute arbitrary operating system commands with root privileges on affected devices.

Critical Impact

Unauthenticated remote attackers can achieve complete system compromise on affected DrayTek routers by injecting arbitrary commands through the vulnerable CGI interface, potentially leading to full network infrastructure takeover.

Affected Products

• DrayTek Vigor3900 (firmware versions before 1.5.1.1)
• DrayTek Vigor2960 (firmware versions before 1.5.1.1)
• DrayTek Vigor300B (firmware versions before 1.5.1.1)

Discovery Timeline

• 2020-06-24 - CVE-2020-14472 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-14472

Vulnerability Analysis

This vulnerability is classified as CWE-77 (Command Injection), where user-controlled input is incorporated into system commands without adequate validation or sanitization. The affected DrayTek devices expose a web management interface that processes requests through CGI scripts. The mainfunction.cgi script contains multiple code paths where attacker-supplied parameters are directly concatenated into shell commands executed by the underlying Linux operating system.

The vulnerability is particularly severe because it requires no authentication to exploit. An attacker with network access to the router's management interface can submit specially crafted HTTP requests containing shell metacharacters and malicious command sequences. These injected commands execute with the privileges of the web server process, which typically runs as root on embedded network devices.

Successful exploitation grants attackers complete control over the router, enabling them to intercept network traffic, modify routing configurations, establish persistent backdoors, pivot to internal network segments, or use the compromised device as part of a botnet infrastructure.

Root Cause

The root cause of this vulnerability is insufficient input validation within the mainfunction.cgi script. The CGI handler accepts parameters from HTTP requests and incorporates them into system shell commands without properly escaping or sanitizing special characters. This allows attackers to break out of the intended command context using shell metacharacters such as semicolons (;), pipes (|), backticks (`), or command substitution syntax ($()).

Attack Vector

The attack vector is network-based, requiring the attacker to reach the router's web management interface. This interface is often exposed on the WAN interface for remote administration purposes, significantly increasing the attack surface. The exploitation flow involves:

1. Identifying a vulnerable DrayTek device with an accessible management interface
2. Crafting an HTTP request to mainfunction.cgi with command injection payloads in vulnerable parameters
3. The malicious payload bypasses input validation and is executed by the system shell
4. The attacker receives command execution output or establishes a reverse shell connection

Technical details and proof-of-concept implementations are available through the GitHub Gist PoC by Cossack9989 and the GitHub Gist Exploit by WinMin.

Detection Methods for CVE-2020-14472

Indicators of Compromise

• Unusual HTTP requests to mainfunction.cgi containing shell metacharacters (;, |, $(), backticks)
• Unexpected outbound connections from the router to external IP addresses
• Unexplained processes running on the device or abnormal CPU/memory utilization
• Modified configuration files or new user accounts on the router
• Network traffic anomalies such as DNS tunneling or connections to known C2 infrastructure

Detection Strategies

• Deploy network intrusion detection systems (IDS) with signatures targeting command injection patterns in HTTP requests to DrayTek management interfaces
• Monitor web server logs on DrayTek devices for requests containing encoded shell metacharacters or common injection payloads
• Implement web application firewalls (WAF) to inspect and block malicious requests targeting CGI endpoints
• Use SentinelOne Singularity to monitor for behavioral indicators of compromise on network segments containing vulnerable devices

Monitoring Recommendations

• Establish baseline network behavior for DrayTek devices and alert on deviations
• Configure SIEM rules to correlate authentication failures with subsequent successful management interface access
• Monitor for firmware integrity changes using file integrity monitoring where supported
• Track outbound connections from router management interfaces to unexpected destinations

How to Mitigate CVE-2020-14472

Immediate Actions Required

• Upgrade all affected DrayTek Vigor3900, Vigor2960, and Vigor300B devices to firmware version 1.5.1.1 or later immediately
• Restrict management interface access to trusted IP addresses using access control lists (ACLs)
• Disable remote management access from WAN interfaces if not strictly required
• Implement network segmentation to isolate management interfaces from untrusted networks
• Review device configurations and logs for signs of prior compromise

Patch Information

DrayTek has addressed this vulnerability in firmware version 1.5.1.1 and later releases. Organizations should obtain the latest firmware from DrayTek's official support channels and apply updates following the vendor's documented procedures. Prior to updating, backup current device configurations to enable recovery in case of update failures.

Workarounds

• Disable the web management interface entirely if remote management is not required
• Configure firewall rules to block external access to the management interface (typically ports 80, 443)
• Place management interfaces on a dedicated, isolated VLAN with strict access controls
• Use VPN connections to access management interfaces rather than direct exposure

# Example: Restrict management access via ACL (syntax may vary by firmware version)
# Access management interface only from trusted subnet
ip access-list extended MGMT-ACCESS
 permit tcp 192.168.1.0 0.0.0.255 any eq 443
 deny tcp any any eq 443
 deny tcp any any eq 80