CVE-2023-31447: Draytek Vigor2620 Firmware RCE Vulnerability

CVE-2023-31447 Overview

CVE-2023-31447 is a code injection vulnerability in the user_login.cgi endpoint of DrayTek Vigor2620 routers running firmware prior to 3.9.8.4 and all firmware versions of DrayTek Vigor2925 devices. Attackers can send a crafted HTTP payload to the login handler to modify the contents of the code segment, insert shellcode, and execute arbitrary code on the device. The flaw is unauthenticated and exploitable over the network, giving remote attackers full control of affected routers. The issue is tracked under CWE-94: Improper Control of Generation of Code.

Critical Impact
Unauthenticated remote attackers can execute arbitrary code on perimeter routers, enabling persistent network compromise, traffic interception, and lateral movement into internal networks.

Affected Products

DrayTek Vigor2620 firmware versions prior to 3.9.8.4
DrayTek Vigor2620 hardware appliances
DrayTek Vigor2925 firmware (all versions) and Vigor2925 hardware appliances

Discovery Timeline

2023-08-21 - CVE-2023-31447 published to the National Vulnerability Database
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 exposed by the DrayTek web management interface. The handler processes attacker-controlled input from the login request without enforcing bounds or validating the structure of the supplied parameters. Because the routine writes attacker data into memory regions that overlap executable code, an attacker can overwrite instructions inside the code segment of the running process.

Once the code segment is modified, the inserted shellcode runs with the privileges of the web management daemon, which typically operates as root on embedded SOHO routers. This grants full control of the device, including configuration changes, credential theft, traffic redirection, and installation of persistent implants.

Root Cause

The root cause is improper control of code generation [CWE-94] combined with missing input validation in the login CGI. The handler treats request data as trusted and copies it into memory pages that should remain read-only. The lack of write protections on the code segment in the affected firmware permits direct modification of executable instructions.

Attack Vector

Exploitation requires only network reachability to the router's management interface. No authentication and no user interaction are needed. An attacker sends an HTTP request to user_login.cgi containing a crafted payload that triggers the memory write primitive and delivers shellcode. The vulnerability is exploitable across the WAN if remote management is enabled, and across the LAN in all default configurations.

A proof-of-concept describing the payload structure is published as a GitHub Gist. No entry exists in the CISA Known Exploited Vulnerabilities catalog at this time.

Detection Methods for CVE-2023-31447

Indicators of Compromise

Unexpected HTTP POST requests to /cgi-bin/user_login.cgi containing oversized or binary parameter values from untrusted source addresses
Outbound connections from the router to unknown command-and-control hosts shortly after a malformed login request
DNS configuration, static route, or firewall rule changes on the device that were not initiated by an administrator
New or modified administrative accounts in the DrayTek configuration export

Detection Strategies

Inspect HTTP traffic to the router management interface for requests targeting user_login.cgi with non-printable bytes or abnormally long fields
Correlate authentication failures and crash events in router syslog with subsequent outbound traffic from the device
Compare running firmware checksums and configuration baselines against known-good values

Monitoring Recommendations

Forward DrayTek syslog and SNMP trap data into a centralized analytics platform such as Singularity Data Lake for retention and behavioral analysis
Monitor north-south traffic for anomalous outbound sessions originating from the router management plane
Alert on any modification to the router's administrative account list, DNS servers, or VPN configuration

How to Mitigate CVE-2023-31447

Immediate Actions Required

Upgrade DrayTek Vigor2620 devices to firmware 3.9.8.4 or later, available from the DrayTek official site
Disable remote (WAN-side) access to the web management interface until patching is complete
Restrict LAN-side access to the management interface to a dedicated administrative VLAN or jump host
Rotate all administrative credentials and reissue VPN pre-shared keys after patching, in case of prior compromise

Patch Information

DrayTek released firmware 3.9.8.4 for the Vigor2620 line, which addresses the input validation flaw in user_login.cgi. The Vigor2925 product line is affected across all firmware versions; operators should consult DrayTek support for guidance and consider replacing end-of-life hardware with a supported model. Refer to the DrayTek official site for current firmware downloads and advisory updates.

Workarounds

Place an access control list in front of the router management interface so that only trusted internal addresses can reach TCP ports 80 and 443
Disable the Allow management from the Internet setting under System Maintenance in the DrayTek web UI
Use an upstream firewall to drop external traffic destined for the router's management ports until firmware is updated

bash

# Example: restrict router management to a single admin host on the LAN
# (run on an upstream firewall or layer-3 switch)
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -s <admin_host_ip> -j ACCEPT
iptables -A FORWARD -p tcp -d <router_ip> --dport 443 -j DROP
iptables -A FORWARD -p tcp -d <router_ip> --dport 80  -j DROP