CVE-2021-40109 : Exploit Details and Defense Strategies

A SSRF issue was discovered in Concrete CMS through 8.5.5 where users can access forbidden files on their local network by uploading URLs that redirect to internal resources.

Understanding CVE-2021-40109

This CVE identifies a Server-Side Request Forgery vulnerability in Concrete CMS allowing users to exploit file access permissions.

What is CVE-2021-40109?

SSRF issue in Concrete CMS through version 8.5.5
Users with file upload permissions can upload redirecting URLs
Allows access to forbidden files on the local network
Vulnerability exists due to improper handling of URL redirects

The Impact of CVE-2021-40109

Unauthorized access to restricted files on the local network
Possibility of sensitive data exposure

Technical Details of CVE-2021-40109

This section provides in-depth technical insights into the vulnerability.

Vulnerability Description

Users with file upload permissions can upload redirecting URLs leading to unauthorized file access.
An internal SSRF vulnerability allows loading forbidden files.

Affected Systems and Versions

Concrete CMS versions up to 8.5.5 are vulnerable.

Exploitation Mechanism

Upload URLs that redirect to internal resources to bypass file access restrictions.

Mitigation and Prevention

Protect your system from CVE-2021-40109 with the following actions:

Immediate Steps to Take

Update Concrete CMS to a patched version.
Implement network-level restrictions to prevent SSRF attacks.
Regularly monitor and audit file upload functionalities.

Long-Term Security Practices

Conduct security awareness training on SSRF attacks.
Enforce least privilege access for file uploads.
Maintain up-to-date security patches and configurations.

Patching and Updates

Concrete CMS users should upgrade to version 8.5.6 or the latest release.