CVE-2024-51139: Draytek Vigor Buffer Overflow Vulnerability

CVE-2024-51139 Overview

CVE-2024-51139 is a critical buffer overflow vulnerability affecting multiple DrayTek Vigor router firmware versions. The vulnerability exists in the CGI parser's handling of the Content-Length header in HTTP POST requests, allowing a remote attacker to execute arbitrary code on affected devices without authentication.

DrayTek Vigor routers are widely deployed enterprise and SOHO networking devices used for VPN, firewall, and routing functions. This vulnerability poses a severe risk to network infrastructure as successful exploitation could lead to complete device compromise, enabling attackers to intercept traffic, pivot to internal networks, or use compromised routers as part of a botnet.

Critical Impact

Remote attackers can achieve arbitrary code execution on DrayTek Vigor routers via malicious HTTP POST requests, potentially compromising network perimeter security without any user interaction or authentication.

Affected Products

• DrayTek Vigor2620/LTE200 firmware version 3.9.8.9 and earlier
• DrayTek Vigor2860/2925 firmware version 3.9.8 and earlier
• DrayTek Vigor2862/2926 firmware version 3.9.9.5 and earlier
• DrayTek Vigor2133/2762/2832 firmware version 3.9.9 and earlier
• DrayTek Vigor165/166 firmware version 4.2.7 and earlier
• DrayTek Vigor2135/2765/2766 firmware version 4.4.5.1 and earlier
• DrayTek Vigor2865/2866/2927 firmware version 4.4.5.3 and earlier
• DrayTek Vigor2962/3910 firmware version 4.3.2.8/4.4.3.1 and earlier
• DrayTek Vigor3912 firmware version 4.3.6.1 and earlier

Discovery Timeline

• 2025-02-27 - CVE-2024-51139 published to NVD
• 2025-05-28 - Last updated in NVD database

Technical Details for CVE-2024-51139

Vulnerability Analysis

This buffer overflow vulnerability (CWE-120: Buffer Copy without Checking Size of Input) resides in the CGI parser component responsible for processing HTTP POST requests on DrayTek Vigor routers. The vulnerability can be exploited remotely over the network without requiring authentication or user interaction.

When the CGI parser processes incoming HTTP POST requests, it reads the Content-Length header value to allocate memory and handle the request body. The vulnerable code fails to properly validate the Content-Length value before copying data into a fixed-size buffer, allowing an attacker to overflow the buffer with controlled data.

Successful exploitation enables attackers to overwrite critical memory regions, potentially including return addresses or function pointers, leading to arbitrary code execution with the privileges of the web server process running on the router.

Root Cause

The root cause is a classic buffer overflow condition (CWE-120) where the CGI parser does not properly validate or sanitize the Content-Length header value before using it in memory operations. When an attacker supplies a maliciously crafted Content-Length value or sends more data than the allocated buffer can accommodate, the excess data overflows into adjacent memory regions.

This type of vulnerability typically occurs when:

• Input length is not validated against buffer boundaries
• Memory allocation does not account for maximum input size
• Unsafe string or memory copy functions are used without bounds checking

Attack Vector

The attack vector is network-based, targeting the HTTP interface of DrayTek Vigor routers. An attacker can exploit this vulnerability by sending a specially crafted HTTP POST request to the router's web management interface. The attack does not require authentication, making any exposed DrayTek router with a reachable web interface vulnerable.

The exploitation involves crafting an HTTP POST request with a manipulated Content-Length header that causes the CGI parser to incorrectly handle buffer boundaries. This can be achieved by:

1. Sending a POST request with a Content-Length value that exceeds expected limits
2. Including payload data designed to overflow the buffer and overwrite memory
3. Crafting the overflow to redirect execution to attacker-controlled shellcode

For detailed technical analysis, refer to the Medium Advisory on DrayTek Vulnerabilities.

Detection Methods for CVE-2024-51139

Indicators of Compromise

• Unusual HTTP POST requests with abnormally large Content-Length headers targeting router management interfaces
• Unexpected crashes or reboots of DrayTek Vigor routers
• Unauthorized configuration changes or new administrative accounts on router devices
• Suspicious outbound connections from router IP addresses to unknown external hosts

Detection Strategies

• Monitor network traffic for HTTP POST requests with anomalous Content-Length values targeting DrayTek router management ports (typically port 80, 443, or 8080)
• Implement intrusion detection rules to flag requests to CGI endpoints with oversized headers or body content
• Enable logging on DrayTek devices and centralize logs for analysis of authentication attempts and configuration changes
• Deploy network segmentation to limit direct external access to router management interfaces

Monitoring Recommendations

• Configure SIEM alerts for repeated failed access attempts or unusual traffic patterns targeting network infrastructure devices
• Implement continuous vulnerability scanning to identify unpatched DrayTek routers in your environment
• Monitor for firmware integrity changes or unexpected process execution on router devices
• Establish baseline behavior for router management interface access and alert on deviations

How to Mitigate CVE-2024-51139

Immediate Actions Required

• Update all affected DrayTek Vigor router firmware to the latest available version from the DrayTek Official Website
• Disable remote management access to router web interfaces from untrusted networks
• Implement firewall rules to restrict access to router management interfaces to authorized IP addresses only
• Enable access control lists (ACLs) on the router to limit who can reach the web management interface

Patch Information

DrayTek has released firmware updates addressing this vulnerability for affected Vigor router models. Administrators should immediately download and apply the latest firmware versions from the official DrayTek support portal.

Refer to the following resources for firmware updates and security advisories:

• DrayTek Official Website
• Medium Advisory on DrayTek Vulnerabilities

Workarounds

• Disable web-based remote management if not required for operations
• Restrict management interface access to internal/trusted networks only using firewall rules
• Use VPN connections for remote administration instead of exposing the web interface directly
• Enable strong authentication and consider implementing IP whitelisting for management access

# Example: Restrict management access to specific IP using firewall/ACL
# (Consult DrayTek documentation for exact CLI syntax)
# Block external access to management ports
iptables -A INPUT -p tcp --dport 80 -s 0.0.0.0/0 -j DROP
iptables -A INPUT -p tcp --dport 443 -s 0.0.0.0/0 -j DROP
# Allow only trusted management IP
iptables -I INPUT -p tcp --dport 443 -s 192.168.1.100 -j ACCEPT