Conducting a Security Assessment of Internal Certificate Authorities

Public Key Infrastructure (PKI) and Certificate Authorities (CAs) form the foundational trust layer of modern enterprise Zero Trust architectures. The landscape is currently undergoing massive disruption. Google’s push to reduce maximum public TLS certificate lifespans to 90 days is forcing organizations to abandon manual CA management, while the finalization of NIST’s Post-Quantum Cryptography (PQC) standards (FIPS 203, 204, and 205) requires immediate crypto-agility planning.

When major public CAs like Entrust face trust revocations from Google and Mozilla due to compliance failures, it serves as a stark warning: internal CAs must be held to equally rigorous standards. With machine identities now outnumbering human identities by a factor of 45 to 1, a robust CA security assessment is no longer a compliance checkbox—it is a critical business continuity requirement.

This guide details how to assess, secure, and automate your internal Certificate Authority infrastructure across four critical domains: Physical Security, Logical Access, Cryptographic Operations, and Lifecycle Automation.

Domain 1: Physical Security and Hardware Controls

The compromise of a Root CA private key is a catastrophic event that requires rebuilding the entire trust ecosystem from scratch. Physical security is your first and most critical line of defense.

Hardware Security Modules (HSMs)

Cryptographic keys should never reside in software or standard server memory. Assess whether your CA infrastructure utilizes Hardware Security Modules (HSMs) certified to FIPS 140-2 or FIPS 140-3 Level 3 or higher.

Whether you are using on-premise appliances like Thales Luna and Entrust nShield, or cloud-based solutions like AWS CloudHSM and Google Cloud HSM, the assessment criteria remain the same:
* Are the keys generated inside the HSM boundary?
* Is the HSM configured to prevent key extraction?
* Are tamper-evident seals used and regularly inspected on physical hardware?

The Offline Root CA

A Root CA should exist only to sign Subordinate (Issuing) CAs and Certificate Revocation Lists (CRLs). It should never be connected to a network.

During your assessment, verify that the Root CA is completely air-gapped. When not actively participating in a key ceremony, the Root CA server should be powered off, its hard drives encrypted, and stored in a physically secure, tiered-access vault (e.g., requiring biometrics, mantraps, and logged CCTV coverage).

Domain 2: Logical Security and Access Control

Logical security ensures that even if an attacker gains network access, they cannot issue unauthorized certificates or alter the CA configuration.

Multi-Person Control and Key Ceremonies

Critical CA operations, such as turning on the Root CA or signing a new Subordinate CA, must require Multi-Person Control (often referred to as M-of-N quorum). This relies on cryptographic mechanisms like Shamir's Secret Sharing, where a key is split into multiple parts.

Assess whether your "Key Ceremonies" require at least 3 out of 5 designated key custodians to authenticate using physical smart cards. Furthermore, digitize your runbooks. Manual key ceremonies are prone to human error; recording the entire ceremony and strictly auditing the logs ensures compliance with frameworks like WebTrust and ETSI EN 319 401.

Securing Microsoft AD CS (Mitigating ESC Vulnerabilities)

Many enterprises still rely on legacy on-premise Microsoft Active Directory Certificate Services (AD CS). AD CS is notoriously difficult to secure, with numerous escalation of privilege vulnerabilities (dubbed ESC1 through ESC14 by security researchers) discovered in recent years.

A primary assessment point for AD CS is auditing certificate templates. For example, the ESC1 vulnerability occurs when a template allows Client Authentication and permits the enrollee to supply the Subject Alternative Name (SAN). An attacker can use this to request a certificate as a Domain Admin and immediately take over the domain.

You can audit your AD CS templates using PowerShell to identify templates where CT_FLAG_ENROLLEE_SUPPLIES_SUBJECT is enabled:

# Import the AD PKI module
Import-Module ADCSAdministration

# Retrieve all certificate templates
$Templates = Get-CATemplate

foreach ($Template in $Templates) {
    $Flags = $Template.EnrollmentOptions
    # Check if the Enrollee Supplies Subject flag (0x00000001) is set
    if (($Flags -band 1) -eq 1) {
        Write-Warning "VULNERABILITY FOUND: Template '$($Template.Name)' allows the enrollee to supply the subject."
        Write-Host "Review this template immediately to ensure it does not also allow Client Authentication."
    }
}

Additionally, ensure that NTLM authentication is disabled on the CA web enrollment interfaces and that Extended Protection for Authentication (EPA) is strictly enforced to prevent relay attacks like PetitPotam.

Domain 3: Cryptographic and Operational Security

As the cryptographic landscape evolves, CAs are increasingly assessed on their "crypto-agility"—the ability to rapidly swap out underlying cryptographic algorithms without disrupting the infrastructure.

Algorithm and Key Size Compliance

Ensure your CA is enforcing modern cryptographic standards as defined by NIST SP 800-57.
* RSA: Minimum of 3072-bit keys (4096-bit recommended for Root CAs).
* ECC: NIST P-384 or higher.
* Prohibited: SHA-1 and MD5 must be strictly prohibited across the entire chain.

Furthermore, assess your CA's readiness to issue hybrid certificates that combine classical algorithms (ECC/RSA) with quantum-resistant algorithms (like ML-KEM/FIPS 203) to protect data against "harvest now, decrypt later" attacks.

Revocation Infrastructure High Availability

A CA is only as secure as its ability to revoke trust. Certificate Revocation Lists (CRLs) and Online Certificate Status Protocol (OCSP) responders must be highly available. If your OCSP responder goes offline, clients enforcing strict revocation checking will fail to connect to your services.

Assess your CRL publication intervals. Subordinate CAs should publish CRLs frequently (e.g., every 24 hours), with overlapping validity periods to account for replication delays.

Audit Logging and SIEM Integration

Every action taken by the CA must be immutably logged. This includes certificate issuance, revocation requests, template modifications, and administrator logins. These logs must be forwarded in real-time to a SIEM (like Splunk or Microsoft Sentinel) with alerting rules configured for anomalous issuance patterns.

Domain 4: Lifecycle Automation and Visibility

The era of manually provisioning 1-year certificates is over. The upcoming 90-day maximum lifespan for public certificates is establishing a new baseline that internal CAs are expected to follow. If you are not automating, you are courting outages.

Combating Shadow PKI with Developer-Friendly Automation

Developers often spin up unauthorized self-signed certificates or rogue CAs using OpenSSL to bypass slow IT ticketing processes. This "Shadow PKI" creates massive security blind spots.

To fix this, integrate a modern CA or Certificate Lifecycle Management (CLM) tool directly into your CI/CD pipelines. Tools like
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