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Battling Certificate Errors with Entrust + DISA

By Danny Gershman

In the complex landscape of enterprise security and government IT infrastructure, certificate management presents ongoing challenges that can disrupt critical automated processes and system integrations. A recent incident involving the Defense Information Systems Agency (DISA) and Entrust certificate renewals illustrates the intricate nature of modern PKI management and the importance of robust certificate chain validation in enterprise environments.

The Incident: When Automation Breaks

The issue emerged when a colleague discovered that an automated process could no longer retrieve files from a CloudFront-hosted DISA site. What had been functioning seamlessly for months suddenly began failing with TLS-related errors, creating operational disruptions that required immediate investigation and resolution.

The affected endpoint, https://dl.dod.cyber.mil, remained accessible through modern web browsers, creating a puzzling situation where manual access worked perfectly while automated systems failed. This discrepancy immediately suggested that the issue lay not with the website itself, but with subtle changes in certificate validation that browsers could handle but command-line tools could not.

Timing and Root Cause Analysis

The timing of the incident provided crucial clues to its root cause. The automated process had functioned correctly until five days prior, when the SAN (Subject Alternative Name) certificate was renewed by Entrust. This timing correlation strongly suggested that the certificate renewal process had introduced changes that affected certificate chain validation for certain clients.

Initial investigation revealed that while the certificate appeared valid and functional for browser-based access, command-line tools like curl on both Ubuntu and RHEL systems were failing to validate the certificate chain properly. This behavior pattern indicated that the issue was not simply a matter of certificate expiration or corruption, but rather a more subtle problem with certificate chain construction or intermediate certificate availability.

Diagnostic Process and Tools

The diagnostic process required leveraging multiple tools and perspectives to understand the full scope of the certificate chain issue:

Command-Line Validation Failures

Testing with curl revealed consistent failures across different Linux distributions:

# Failing command
curl https://dl.dod.cyber.mil/some-file

# Error: SSL certificate problem: unable to get local issuer certificate

This error specifically indicated that the certificate chain was incomplete or that intermediate certificates were not being properly presented by the server.

Browser Compatibility Analysis

The fact that modern web browsers could successfully access the site while command-line tools failed suggested that browsers were performing additional certificate chain resolution that command-line tools were not. Modern browsers often cache intermediate certificates and can perform online certificate chain building, capabilities that simple HTTP clients typically lack.

Qualys SSL Labs Analysis

Using external certificate analysis tools provided objective validation of the certificate chain issues. Qualys SSL Labs' certificate checking tool revealed a broken certificate chain, confirming that the problem was indeed with certificate chain construction rather than client-side configuration issues.

Understanding Certificate Chain Architecture

The Entrust certificate renewal introduced changes to the certificate chain structure that affected validation in automated tools:

Root and Intermediate Certificates

The renewed certificate was issued with a different chain of roots and intermediate certificates compared to the previous certificate. This change required clients to have access to the appropriate intermediate certificates to build a complete chain of trust from the server certificate to a trusted root certificate.

G2 and L1K Certificate Components

The solution involved downloading and configuring specific root and intermediate certificates:

  • G2 Root Certificate: The trusted root certificate in the certificate hierarchy
  • L1K Intermediate Certificate: The intermediate certificate that bridges the gap between the server certificate and the root certificate

Resolution Strategy

The resolution process required constructing a complete certificate chain that command-line tools could use for validation:

Certificate Collection

The first step involved obtaining the necessary certificate components:

  1. Download the G2 root certificate from Entrust's certificate repository
  2. Download the L1K intermediate certificate
  3. Verify the authenticity and integrity of both certificates

Certificate Chain Construction

The certificates needed to be combined into a single PEM file that contained the complete certificate chain:

# Concatenate certificates into a single PEM file
cat entrust-g2-root.crt entrust-l1k-intermediate.crt > entrust-complete-chain.pem

The order of certificates in the PEM file is crucial—the chain must be constructed from the intermediate certificate up to the root certificate for proper validation.

Validation Testing

Using the combined certificate file with curl resolved the validation issues:

# Successful command with certificate chain
curl --cacert entrust-complete-chain.pem https://dl.dod.cyber.mil/some-file

This approach enabled the automated process to successfully validate the certificate chain and resume normal operation.

Broader Implications and Lessons

This incident highlights several important considerations for enterprise certificate management:

Automated Process Resilience

Automated systems that depend on external services must be designed to handle certificate changes gracefully. This includes:

  • Regular monitoring of certificate status and expiration
  • Automated updating of certificate stores and trust bundles
  • Fallback mechanisms for certificate validation failures
  • Alerting systems for certificate-related issues

Certificate Chain Management

Organizations must maintain comprehensive understanding of their certificate chain dependencies:

  • Documentation of all certificate authorities and intermediate certificates in use
  • Regular auditing of certificate chains across all systems and applications
  • Proactive monitoring of certificate authority changes and updates
  • Testing of certificate validation across different client types and configurations

Vendor Communication and Change Management

Certificate authority changes can have widespread impact across enterprise systems:

  • Establishing communication channels with certificate vendors for advance notice of changes
  • Implementing change management processes for certificate updates
  • Testing certificate changes in staging environments before production deployment
  • Maintaining rollback procedures for certificate-related issues

Technical Best Practices

Several best practices emerge from this incident that can help organizations avoid similar issues:

Certificate Bundle Management

Maintaining comprehensive certificate bundles that include all necessary intermediate certificates:

  • Regular updates to system certificate stores
  • Custom certificate bundles for applications with specific requirements
  • Version control and change tracking for certificate configurations
  • Automated testing of certificate bundle completeness

Monitoring and Alerting

Implementing proactive monitoring for certificate-related issues:

  • Automated testing of certificate validation across different client types
  • Monitoring of certificate expiration dates and renewal schedules
  • Alerting for certificate chain validation failures
  • Integration with existing monitoring and alerting systems

Documentation and Knowledge Management

Maintaining comprehensive documentation of certificate dependencies and resolution procedures:

  • Inventory of all systems and applications that depend on specific certificates
  • Step-by-step procedures for certificate-related troubleshooting
  • Contact information for certificate authorities and support resources
  • Post-incident review and knowledge capture processes

Future Considerations

Organizations should consider several factors to improve their certificate management resilience:

Certificate Automation

Implementing automated certificate management systems that can handle:

  • Automated certificate renewal and deployment
  • Certificate chain validation and updating
  • Integration with existing deployment and configuration management systems
  • Rollback capabilities for failed certificate updates

Vendor Diversification

Reducing dependency on single certificate authorities through:

  • Multi-vendor certificate strategies for critical systems
  • Understanding of different certificate authority practices and procedures
  • Evaluation of certificate authority reliability and change management practices
  • Development of vendor-agnostic certificate management processes

Operational Preparedness

Developing organizational capabilities for certificate incident response:

  • Training for operations teams on certificate troubleshooting
  • Established escalation procedures for certificate-related issues
  • Regular testing of certificate incident response procedures
  • Integration with broader incident response and business continuity plans

Conclusion

The Entrust + DISA certificate incident serves as a valuable case study in the complexities of modern PKI management and the importance of robust certificate chain validation. While the immediate resolution involved downloading and configuring appropriate intermediate certificates, the broader lessons relate to organizational preparedness, process maturity, and the need for comprehensive certificate management strategies.

Organizations operating in complex enterprise environments must recognize that certificate management is not simply a technical issue but a critical operational capability that requires ongoing attention, monitoring, and improvement. The interconnected nature of modern systems means that certificate issues can have far-reaching impacts across multiple systems and processes.

By implementing comprehensive certificate management practices, organizations can minimize the risk of certificate-related disruptions while maintaining the security benefits that proper PKI implementation provides. The investment in robust certificate management processes pays dividends in reduced operational disruptions, improved security posture, and enhanced organizational resilience.

This incident reinforces the importance of treating certificate management as a critical operational discipline rather than a routine administrative task, requiring the same level of attention and sophistication as other critical infrastructure components.