Recently, a new vulnerability designated as CVE-2023-5255 has been discovered in Puppet Server, specifically affecting its auto-renew certificate feature. This flaw allows attackers to exploit unrevoked certificates, potentially gaining unauthorized access to sensitive resources and systems. In this post, we will provide an overview of the vulnerability, code snippets highlighting the weakness, and details on how to successfully exploit the flaw. Furthermore, we will link to original references and resources for security researchers and system administrators interested in mitigating this risk.

Vulnerability Overview

Puppet Server is a widely used software for automating configuration management and deployment processes across IT infrastructure. The issue at hand exists within the auto-renew certificate functionality, which helps manage certificates' lifecycle by automating their renewal upon expiry. Unfortunately, a logic error prevents revoked certificates from being purged or removed from the server, so attackers can continue to use an expired certificate to access protected resources.

Code Snippets

The following code snippet (written in Ruby) represents a simplified version of Puppet Server certificate handling to illustrate the vulnerability.

# Check certificate structure for valid expiry
def check_certificate_expiration(certificate)
  return certificate.not_after > Time.now
end

# Revoked certificates are added to revoked_certs list
def revoke_certificate(certificate)
  revoked_certs << certificate
end

# Auto-Renew Process: Only considers expiry and does not check for revoked status
def auto_renew_certificates(certificates)
  certificates.each do |certificate|
    if !check_certificate_expiration(certificate)
      renew_certificate(certificate)
      # The logic error lies here as it does not remove the revoked certificate
    end
  end
end

As seen above, the auto_renew_certificates function renews the certificate after checking for expiration, but does not remove revoked certificates (as it should). This allows expired, revoked certificates to remain valid for malicious use.

Knowledge of the target system's expected Certificate Authority (CA) signing chain.

3. A compatible SSL/TLS client to make requests using the revoked certificate.

With the revoked certificate, the attacker can establish a secure connection to the affected Puppet Server using an SSL/TLS client while impersonating a trusted puppet agent. The server will fail to identify the revoked certificate and grant unauthorized access, compromising the system's security.

- Vulnerability Details: CVE-2023-5255
- Puppet Server Project: https://puppet.com/docs/puppetserver/latest/
- Puppet Security Advisory: https://puppet.com/security/cve/CVE-2023-5255/

Mitigation

There is currently no official patch available for this vulnerability, but we recommend the following temporary mitigation steps:

1. Disable auto-renew certificate functionality in Puppet Server by modifying corresponding configuration settings.

Regularly audit and prune any expired certificates, especially those that have been revoked.

3. Implement strict access control and monitoring solutions to detect and prevent unauthorized access to Puppet Server.

Conclusion

CVE-2023-5255 is a significant security flaw in the auto-renewal certificate feature of Puppet Server that allows attackers to exploit unrevoked certificates. It is vital for administrators to address this vulnerability urgently and apply appropriate mitigation steps to secure their Puppet infrastructure. Keep an eye out for future updates and ensure you stay protected - and always be sure to practice secure certificate management among your IT systems.

Timeline

Published on: 10/03/2023 18:15:00 UTC
Last modified on: 10/05/2023 16:48:00 UTC