CVE-2024-56509: Path Traversal Vulnerability in Changedetection.io

Executive Summary

CVE-2024-56509 is a critical vulnerability affecting changedetection.io, a popular open-source tool for monitoring webpage changes, restock notifications, and web content updates. This vulnerability arises from improper input validation, which allows attackers to exploit Local File Read (LFR) and Path Traversal attacks. By crafting malicious file paths, such as file:../../../etc/passwd, an attacker can bypass weak validation mechanisms and gain unauthorized access to sensitive files on the server. The vulnerability is rated High (CVSS 8.6) due to its ease of exploitation and significant confidentiality impact. A patch addressing this issue was released in version 0.48.05.

Technical Details

Affected Systems

• Software: Changedetection.io
• Versions: All versions prior to 0.48.05
• Components:
  • URL handling logic in changedetectionio/processors/__init__.py
  • Test cases in changedetectionio/tests/test_security.py

Vulnerability Description

The vulnerability stems from insufficient sanitization of user-supplied input when constructing file paths. Specifically, the application fails to properly validate URLs beginning with file: or its variations (file:/, file://). This oversight allows attackers to perform path traversal attacks, potentially accessing sensitive files such as /etc/passwd or application configuration files.

The vulnerability is classified under:

• CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
• CWE-200: Exposure of Sensitive Information to an Unauthorized Actor

Root Cause Analysis

The root cause lies in the improper use of regular expressions to validate file URLs. The original implementation only partially checked for file:/ patterns, leaving other variations (e.g., file:, file://) unfiltered. Below is the vulnerable code snippet from changedetectionio/processors/__init__.py:

# Vulnerable Code
url = self.watch.link

# Protect against file://, file:/ access
if re.search(r'^file:/', url.strip(), re.IGNORECASE):
    if not strtobool(os.getenv('ALLOW_FILE_URI', 'false')):
        raise Exception(
            "file:// type access is denied for security reasons."
        )

Why This Fails

• The regular expression ^file:/ only matches URLs starting with file:/, but not file: or file://.
• An attacker can bypass this validation by using alternative formats like file:../../../etc/passwd.

Exploit Example

An attacker could exploit this vulnerability by submitting a malicious URL:

file:../../../etc/passwd

This would allow the attacker to read sensitive files on the server.

Patch Analysis

The patch for this vulnerability was introduced in commit f7e9846c9b40a229813d19cdb66bf60fbe5e6a2a. Below is a line-by-line analysis of the changes:

File: changedetectionio/processors/__init__.py

-        if re.search(r'^file:/', url.strip(), re.IGNORECASE):
+        if re.search(r'^file:', url.strip(), re.IGNORECASE):

• The updated regular expression now matches all variations of file: URLs (file:, file:/, file://), closing the validation gap.

File: changedetectionio/tests/test_security.py

The patch also refactored and expanded test cases to ensure comprehensive coverage of all file: URL variations.

+def _runner_test_various_file_slash(client, file_uri):
+    client.post(
+        url_for("form_quick_watch_add"),
+        data={"url": file_uri, "tags": ''},
+        follow_redirects=True
+    )
+    wait_for_all_checks(client)
+    res = client.get(url_for("index"))
+
+    if strtobool(os.getenv('ALLOW_FILE_URI', 'false')):
+        assert b"URLs with hostname components are not permitted" in res.data
+    else:
+        assert b'file:// type access is denied for security reasons.' in res.data

• A new helper function _runner_test_various_file_slash was added to test multiple file: URL variations.
• The test cases now verify that the application correctly blocks unauthorized file access.

Exploitation Techniques

Proof of Concept (PoC)

Below is a step-by-step PoC for exploiting this vulnerability:

1. Setup: Deploy a vulnerable version of changedetection.io (e.g., 0.48.04).
2. Craft Malicious Input:
   Submit a URL like:

   file:../../../etc/passwd
   

3. Observe the Output:
   If successful, the contents of /etc/passwd will be displayed or logged.

Real-World Impact

• Data Breach: Attackers can access sensitive files, including configuration files, logs, and credentials.
• Reconnaissance: Gaining access to server files can provide attackers with valuable information for further exploitation.

Mitigation Strategies

1. Upgrade:

   • Update to changedetection.io version 0.48.05 or later.

2. Environment Hardening:

   • Set the environment variable ALLOW_FILE_URI to false to disable file URL access entirely:

     export ALLOW_FILE_URI=false
     

3. Input Validation:

   • Implement strict input validation to sanitize user-supplied URLs.
   • Use allowlists to restrict acceptable URL schemes (e.g., http, https).

4. Monitoring and Logging:

   • Enable logging to detect unauthorized access attempts.
   • Monitor application logs for suspicious file access patterns.

5. Web Application Firewall (WAF):

   • Deploy a WAF to block malicious requests containing file: URLs.

Timeline of Discovery and Disclosure

References

• NVD Entry for CVE-2024-56509
• GitHub Commit Fixing the Vulnerability
• GitHub Security Advisory
• Technical Blog on LinkedIn

By addressing CVE-2024-56509, organizations can safeguard their systems against critical path traversal attacks. Stay vigilant and ensure your applications are up-to-date with the latest security patches.