CVE-2025-27718: HGW-BL1500HM Path Traversal Vulnerability

CVE-2025-27718 Overview

CVE-2025-27718 is a path traversal vulnerability [CWE-22] affecting the HGW-BL1500HM home gateway, version 002.002.003 and earlier. The flaw resides in the file upload process of the USB storage file-sharing function. An authenticated attacker on the LAN side can send a crafted HTTP request to traverse outside the intended directory. Successful exploitation allows the attacker to read, modify, or replace product files and execute arbitrary code. The issue is tracked publicly through JVN Advisory JVN04278547 and analyzed in the KDDI Technical Document Analysis.

Critical Impact
A LAN-connected attacker with low privileges can execute arbitrary code on the HGW-BL1500HM gateway, compromising confidentiality, integrity, and availability of the device.

Affected Products

HGW-BL1500HM firmware version 002.002.003
HGW-BL1500HM firmware versions earlier than 002.002.003
USB storage file-sharing function on affected HGW-BL1500HM devices

Discovery Timeline

2025-03-28 - CVE-2025-27718 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-27718

Vulnerability Analysis

The HGW-BL1500HM gateway exposes a USB storage file-sharing feature accessible over the LAN. The file upload handler accepts a file name or destination path from the HTTP request without properly normalizing or validating directory traversal sequences. Attackers can supply path components such as ../ to escape the intended USB storage directory and write to arbitrary locations on the device file system. Because the gateway processes uploaded files with elevated privileges, an attacker can overwrite configuration files, scripts, or binaries used by the device. This conversion of an upload primitive into arbitrary file write enables arbitrary code execution on the embedded Linux system.

Root Cause

The root cause is improper limitation of a pathname to a restricted directory [CWE-22]. The file upload routine in the USB sharing function does not sanitize input paths, allowing .. segments and absolute path references to bypass the intended chroot or directory restriction. Input validation of file names before they reach file system APIs is absent.

Attack Vector

Exploitation requires network access to the LAN interface and low-privilege authentication. The attacker sends a crafted HTTP request to the USB storage file upload endpoint, embedding traversal sequences in the file name or path parameter. The gateway writes the attacker-controlled content outside the intended directory. No user interaction is required. See the JVN Advisory JVN04278547 for additional technical context on the upload endpoint behavior.

No verified public proof-of-concept code is available. The vulnerability mechanism follows the standard pattern for path traversal in HTTP file upload handlers on embedded gateways.

Detection Methods for CVE-2025-27718

Indicators of Compromise

HTTP POST or PUT requests to the HGW-BL1500HM USB file-sharing endpoint containing ../ or URL-encoded variants such as %2e%2e%2f in file name parameters
Unexpected modification timestamps on gateway configuration files or system binaries
New or modified files appearing in directories outside the USB storage mount point
Outbound connections from the gateway to unfamiliar hosts following an upload event

Detection Strategies

Inspect HTTP traffic to the gateway LAN management interface for path traversal sequences in upload requests
Baseline legitimate USB file-sharing usage and alert on deviations in request paths and file names
Monitor authentication logs on the gateway for low-privilege accounts performing upload operations at unusual times

Monitoring Recommendations

Capture and review gateway access logs for HTTP requests targeting USB storage upload functions
Deploy network sensors on the LAN segment to identify traversal patterns and encoded variants
Track firmware version inventory across deployed HGW-BL1500HM devices to identify unpatched units

How to Mitigate CVE-2025-27718

Immediate Actions Required

Update HGW-BL1500HM firmware to a version later than 002.002.003 as released by the vendor
Restrict access to the gateway management and USB file-sharing functions to trusted administrators only
Disable the USB storage file-sharing function if it is not required for business operations
Rotate credentials for all accounts that can authenticate to the gateway

Patch Information

Refer to the JVN Advisory JVN04278547 and the KDDI Technical Document Analysis for current vendor guidance and firmware availability for HGW-BL1500HM devices. Apply the corrected firmware to all affected units.

Workarounds

Disable the USB storage file-sharing service on the gateway until firmware can be applied
Segment the gateway management interface onto a restricted VLAN reachable only by administrative hosts
Enforce strong, unique credentials on all gateway accounts to limit the pool of attackers able to reach the authenticated upload function

bash

# Configuration example: restrict LAN access to gateway management interface
# Example iptables rule on an upstream device to limit who can reach the gateway admin UI
iptables -A FORWARD -d <gateway_ip> -p tcp --dport 80 -s <admin_subnet> -j ACCEPT
iptables -A FORWARD -d <gateway_ip> -p tcp --dport 80 -j DROP
iptables -A FORWARD -d <gateway_ip> -p tcp --dport 443 -s <admin_subnet> -j ACCEPT
iptables -A FORWARD -d <gateway_ip> -p tcp --dport 443 -j DROP