CVE-2025-27726

Executive Summary

CVE-2025-27726 is a path traversal vulnerability in the USB storage file-sharing function of the HGW-BL1500HM router, affecting firmware versions Ver 002.002.003 and earlier. The flaw allows an attacker on the local LAN to craft HTTP requests that bypass directory restrictions, enabling unauthorized reading or modification of files stored on the device. With a CVSS score of 2.1 (Low), the risk is limited to LAN-side attackers with no authentication required, but the impact is confined to file access and alteration. No public exploit code or patch is currently available, and the EPSS score is 0.0%, indicating low exploitation likelihood at this time.

Supply Chain & Package Security

This vulnerability is not related to NPM, PyPI, Go, or Ruby packages. It affects embedded firmware for a specific hardware device (HGW-BL1500HM router). No supply chain package security implications apply.

CVSS Breakdown & Threat Level

• CVSS Score: 2.1 (Low)
• Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N
• Attack Vector (AV): Adjacent Network (A) – attacker must be on the same LAN.
• 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) – vulnerability does not affect other components.
• Confidentiality (C): Low (L) – limited file disclosure.
• Integrity (I): None (N) – no data modification.
• Availability (A): None (N) – no denial of service.

Threat Level: Low. The attack surface is restricted to LAN-side devices, and the impact is limited to file access. However, if sensitive files (e.g., configuration, credentials) are stored on USB, the risk increases.

Deep Dive & Mechanics

The vulnerability resides in the USB storage file-sharing function of the HGW-BL1500HM router. The device exposes an HTTP endpoint for downloading files from attached USB storage. Due to improper input validation, the endpoint fails to sanitize path traversal sequences (e.g., ../), allowing an attacker to escape the intended USB root directory and access arbitrary files on the device's filesystem.

Root Cause: The file download handler does not restrict the path parameter to the USB mount point. For example, a request like GET /download?file=../../../etc/passwd would traverse directories and read system files.

Attack Flow:

1. Attacker connects to the same LAN as the vulnerable router.
2. Attacker sends a crafted HTTP GET request to the file download endpoint with path traversal sequences.
3. The router processes the request without proper validation, returning the contents of the requested file.
4. Attacker can read sensitive files (e.g., /etc/shadow, configuration files) or potentially overwrite files if the endpoint also supports upload.

Limitations: The attack is limited to LAN-side access, and the device's filesystem may have restricted permissions. No authentication is required, making it trivial for any LAN user to exploit.

Vulnerable vs. Patched Code

No patch is currently available. The vulnerable code likely resembles:

# Vulnerable pseudo-code
@app.route('/download')
def download_file():
    filename = request.args.get('file')
    # No path validation
    return send_file('/mnt/usb/' + filename)

A patched version would sanitize the path:

# Patched pseudo-code
import os
@app.route('/download')
def download_file():
    filename = request.args.get('file')
    # Validate path
    safe_path = os.path.normpath('/mnt/usb/' + filename)
    if not safe_path.startswith('/mnt/usb/'):
        return "Access denied", 403
    return send_file(safe_path)

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

• Target: HGW-BL1500HM router with firmware Ver 002.002.003 (or earlier) on a local LAN.
• Attacker: Any device on the same LAN (e.g., laptop with curl or browser).
• USB storage: Attached to the router (optional, but required for the function to be active).

Exploitation Steps:

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

   curl "http://192.168.1.1/download?file=../../../etc/passwd"
   

3. If successful, the response will contain the contents of /etc/passwd.
4. To alter files (if the endpoint supports write), use PUT or POST requests with path traversal.

Note: No public exploit code is available. The above is a conceptual demonstration.

Blue Team (Defensive)

Detection:

• Monitor HTTP requests to the file download endpoint for path traversal patterns (e.g., ../, %2e%2e%2f).
• Enable logging on the router (if supported) to capture anomalous requests.
• Use network segmentation to limit LAN access to trusted devices only.

Mitigation:

• Disable USB file-sharing if not required.
• Apply firmware updates when available.
• Implement firewall rules to restrict access to the router's management interface.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP GET requests containing ../ or URL-encoded variants (%2e%2e%2f) in the file parameter.
  • Requests to /download or similar endpoints from unexpected LAN IPs.
• File IOCs:
  • Unauthorized access to /etc/passwd, /etc/shadow, or configuration files.
  • Modified files in the USB storage directory.

Detection Engineering

YARA Rule

rule CVE_2025_27726_Path_Traversal {
    meta:
        description = "Detects path traversal attempts targeting HGW-BL1500HM USB file-sharing"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-27726"
    strings:
        $path_traversal = /(\.\.\/|%2e%2e%2f|%2e%2e\/)/
        $endpoint = "/download" nocase
    condition:
        $endpoint and $path_traversal
}

Sigma Rule

title: CVE-2025-27726 Path Traversal on HGW-BL1500HM
id: 8a3b2c1d-4e5f-6a7b-8c9d-0e1f2a3b4c5d
status: experimental
description: Detects path traversal attempts in HTTP requests to HGW-BL1500HM USB file-sharing endpoint
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: hgw-bl1500hm
detection:
    selection:
        cs-uri-query|contains:
            - "../"
            - "%2e%2e%2f"
            - "%2e%2e/"
        cs-uri-stem|contains: "/download"
    condition: selection
falsepositives:
    - Legitimate use of "../" in file names (unlikely)
level: high

Suricata Rule

alert http any any -> $HOME_NET any (
    msg:"CVE-2025-27726 Path Traversal Attempt on HGW-BL1500HM";
    flow:to_server,established;
    content:"/download"; http_uri;
    pcre:"/(\.\.\/|%2e%2e%2f|%2e%2e\/)/i";
    sid:1000001;
    rev:1;
    metadata:created_at 2025-04-10, cve CVE-2025-27726;
)

Remediation & Mitigation

Immediate Actions:

1. Disable the USB storage file-sharing function on the router if not needed.
2. Restrict LAN access to trusted devices only (e.g., via MAC filtering or VLAN segmentation).
3. Monitor logs for suspicious HTTP requests containing path traversal patterns.

Long-term Fix:

• Apply firmware updates from the vendor when a patch is released.
• If no patch is available, consider replacing the device with a supported alternative.

Workaround:

• Implement a reverse proxy or WAF in front of the router to filter path traversal attempts (e.g., using ModSecurity rules).

Valters IT
DevSecOps & Cyber Threat Intelligence