CVE-2023-31447: Draytek Vigor2620 Firmware RCE Vulnerability

CVE-2023-31447 Overview

CVE-2023-31447 is a critical remote code execution vulnerability affecting DrayTek Vigor2620 and Vigor2625 series routers. The vulnerability exists in the user_login.cgi component, which allows unauthenticated attackers to send a specially crafted payload that modifies the content of the code segment, enables shellcode insertion, and ultimately achieves arbitrary code execution on the affected device.

Critical Impact

This vulnerability allows unauthenticated remote attackers to achieve complete device compromise through code injection, potentially leading to network infrastructure takeover, traffic interception, and lateral movement within corporate environments.

Affected Products

• DrayTek Vigor2620 Firmware (versions before 3.9.8.4)
• DrayTek Vigor2620 Hardware
• DrayTek Vigor2625 Firmware (all versions)
• DrayTek Vigor2625 Hardware

Discovery Timeline

• 2023-08-21 - CVE-2023-31447 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2023-31447

Vulnerability Analysis

The vulnerability resides in the user_login.cgi handler on DrayTek Vigor routers. This CGI script processes authentication requests but fails to properly validate user-supplied input before processing. The lack of input validation allows attackers to inject malicious payloads that can modify the device's code segment at runtime.

The attack does not require any authentication and can be executed remotely over the network. When exploited successfully, an attacker can inject shellcode into the device's memory and execute arbitrary commands with the privileges of the web server process, which typically runs with root or elevated permissions on embedded devices like routers.

CWE-94 (Improper Control of Generation of Code) applies to this vulnerability, as the root cause involves insufficient sanitization of input that ultimately gets interpreted as executable code. This classification of code injection vulnerabilities is particularly dangerous in network infrastructure devices.

Root Cause

The root cause of CVE-2023-31447 is improper input validation within the user_login.cgi script. The CGI handler processes user-supplied data without adequate boundary checks or sanitization, allowing specially crafted input to overflow buffers or otherwise manipulate memory in a way that enables code segment modification. This design flaw permits attackers to bypass security controls and insert executable shellcode directly into the device's runtime environment.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can target the vulnerable CGI endpoint remotely by sending malformed HTTP requests to the device's web management interface. The attack flow typically involves:

1. Identifying an exposed DrayTek Vigor2620 or Vigor2625 device with web management enabled
2. Crafting a malicious payload targeting the user_login.cgi endpoint
3. Sending the payload to modify the code segment and inject shellcode
4. Achieving arbitrary code execution with elevated privileges on the router

The vulnerability is particularly concerning because many routers have their web management interfaces exposed to the internet, either intentionally or through misconfiguration. Technical details and proof-of-concept information can be found in the GitHub Gist published by security researchers.

Detection Methods for CVE-2023-31447

Indicators of Compromise

• Unexpected outbound connections from router management IP addresses to unknown external hosts
• Anomalous HTTP POST requests to /user_login.cgi containing unusual payload sizes or malformed parameters
• Evidence of unauthorized configuration changes on DrayTek Vigor devices
• Unexpected process execution or memory consumption patterns on affected routers

Detection Strategies

• Implement network intrusion detection rules to monitor for suspicious requests targeting user_login.cgi on DrayTek devices
• Deploy web application firewall (WAF) rules to block requests with abnormally large payloads or shellcode patterns to router management interfaces
• Monitor authentication logs on DrayTek devices for unusual login attempts or failed authentication events
• Utilize SentinelOne Singularity for network traffic analysis to identify exploitation attempts against IoT and network infrastructure devices

Monitoring Recommendations

• Enable detailed logging on DrayTek router web management interfaces and forward logs to a centralized SIEM solution
• Implement network segmentation to isolate router management interfaces from untrusted networks
• Conduct regular firmware integrity checks on DrayTek devices to detect unauthorized modifications
• Set up alerts for any access attempts to user_login.cgi from external IP addresses

How to Mitigate CVE-2023-31447

Immediate Actions Required

• Update DrayTek Vigor2620 firmware to version 3.9.8.4 or later immediately
• For Vigor2625 devices, note that all versions are vulnerable; contact DrayTek support for mitigation guidance
• Disable remote web management access if not strictly required
• Restrict access to the router management interface using IP whitelisting or VPN-only access

Patch Information

DrayTek has released firmware version 3.9.8.4 for Vigor2620 devices that addresses this vulnerability. Users should download the latest firmware from the DrayTek Official Website and apply it according to the vendor's upgrade procedures. For Vigor2625 devices, the vulnerability affects all firmware versions, and users should consult DrayTek's official support channels for remediation options or consider device replacement with a supported model.

Workarounds

• Disable the web management interface entirely if firmware updates cannot be immediately applied
• Implement strict firewall rules to block external access to ports 80 and 443 on the router management interface
• Deploy a reverse proxy with strong input validation in front of the router management interface
• Consider network segmentation to place affected devices in isolated management VLANs accessible only from trusted jump hosts

# Example firewall rule to restrict management access (adjust for your environment)
# Block external access to DrayTek management interface
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.1.0/24 -j DROP