Certificate Transparency: How It Protects Your Domain

Certificate Transparency (CT) is a revolutionary security mechanism that brings accountability to the certificate ecosystem. It helps domain owners detect unauthorized certificates and prevents certificate authorities from secretly issuing certificates for domains they shouldn't. Here's how it works and why it matters.

The Problem CT Solves

The Certificate Trust Model

HTTPS security relies on Certificate Authorities (CAs) only issuing certificates to legitimate domain owners. But with hundreds of trusted CAs, and any CA able to issue certificates for any domain, the system is vulnerable to:

• Compromised CAs: Attackers gaining control of a CA's signing keys
• Malicious insiders: CA employees issuing fraudulent certificates
• Government coercion: CAs forced to issue surveillance certificates
• Validation failures: CAs mistakenly issuing certificates to the wrong party

Real-World Incidents

Before CT, major incidents occurred:

• DigiNotar (2011): Attackers issued certificates for google.com, used for Iranian surveillance
• Comodo (2011): Fraudulent certificates for major sites including Gmail
• CNNIC (2015): Unauthorized Google certificates issued
• Symantec (2015-2017): Thousands of improperly issued certificates

These incidents were often discovered by accident. CT ensures every certificate is publicly logged and auditable.

How Certificate Transparency Works

The CT Ecosystem

1. CA submits certificate: Before issuing, the CA submits the certificate to CT logs
2. Log returns SCT: The log adds the certificate and returns a Signed Certificate Timestamp (SCT)
3. Certificate includes SCT: The CA embeds the SCT in the certificate or delivers it separately
4. Browser verifies SCT: When connecting, browsers verify the certificate has valid SCTs
5. Public monitoring: Anyone can search CT logs for certificates issued for their domain

CT Log Structure

CT logs are append-only, cryptographically verifiable data structures (Merkle trees). Key properties:

• Append-only: Certificates can only be added, never removed or modified
• Publicly auditable: Anyone can verify the log's integrity
• Consistent: All observers see the same log contents

SCT Delivery Methods

Servers can provide SCTs in three ways:

• X.509v3 extension: Embedded directly in the certificate (most common)
• TLS extension: Delivered during the TLS handshake
• OCSP stapling: Included in the OCSP response

Browser Requirements

Chrome's CT Policy

Chrome requires all publicly-trusted certificates to have valid SCTs. The number required depends on certificate validity:

• < 15 months: 2 SCTs from different logs
• 15-27 months: 3 SCTs
• 27-39 months: 4 SCTs
• > 39 months: 5 SCTs

Safari's CT Policy

Safari requires at least 2 SCTs from approved logs, with additional requirements for longer-lived certificates.

Firefox

Firefox doesn't currently enforce CT but plans to in the future.

Monitoring CT Logs

Why Monitor?

CT monitoring alerts you to:

• Certificates issued without your authorization
• Potential phishing sites using similar domain names
• Shadow IT using unauthorized subdomains
• Certificate misconfigurations by your team

Free Monitoring Services

• crt.sh: Search and subscribe to certificate issuance
• Google Certificate Transparency: Search Google's CT logs
• Facebook Certificate Transparency Monitoring: Free monitoring with alerts
• Cert Spotter: Open-source monitoring tool

Setting Up Monitoring

# Search for certificates issued for your domain on crt.sh
curl "https://crt.sh/?q=%.example.com&output=json" | jq '.[0:5]'

# Using certspotter
certspotter -domain example.com -follow

What to Look For

When reviewing CT logs, investigate:

• Certificates for subdomains you don't recognize
• Certificates from CAs you don't use
• Multiple certificates issued in a short time
• Certificates with unusual validity periods
• Wildcard certificates you didn't request

Responding to Suspicious Certificates

Verification Steps

1. Check ownership: Verify if the certificate was legitimately requested
2. Identify the CA: Contact them about the certificate's origin
3. Review validation: Understand how domain ownership was verified
4. Check for compromise: If unauthorized, investigate how validation was bypassed

Revocation

If you find an unauthorized certificate:

1. Contact the issuing CA immediately
2. Provide proof of domain ownership
3. Request certificate revocation
4. Monitor for additional unauthorized certificates
5. Consider implementing CAA records

CAA Records: Preventing Unauthorized Issuance

Certificate Authority Authorization (CAA) DNS records specify which CAs can issue certificates for your domain:

# Only allow Let's Encrypt and DigiCert to issue certificates
example.com.  CAA  0 issue "letsencrypt.org"
example.com.  CAA  0 issue "digicert.com"
example.com.  CAA  0 issuewild ";"  # Prevent wildcard certificates

# Send reports of policy violations
example.com.  CAA  0 iodef "mailto:security@example.com"

CAA Record Types

• issue: CAs allowed to issue certificates
• issuewild: CAs allowed to issue wildcard certificates
• iodef: Where to report policy violations

CAA Best Practices

• Always set CAA records for your domains
• Be explicit about wildcard policies
• Include an iodef record for notifications
• Review CAA records when changing CAs

CT for Security Teams

Asset Discovery

CT logs reveal your certificate footprint, including:

• All subdomains with certificates
• Shadow IT and forgotten systems
• Third-party services using your domain
• Historical certificate information

Incident Detection

CT monitoring can detect:

• Phishing infrastructure being set up
• Compromised DNS leading to certificate issuance
• Insider threats requesting unauthorized certificates
• Supply chain attacks on your vendors

Compliance

Many security frameworks now recommend or require CT monitoring:

• PCI DSS for payment processing
• SOC 2 for service organizations
• Industry-specific requirements (financial, healthcare)

CT Implementation Checklist

• Ensure all your certificates include valid SCTs
• Set up CT log monitoring for your domains
• Configure CAA records to restrict certificate issuance
• Establish a process to investigate CT alerts
• Document your approved CAs and subdomains
• Include CT monitoring in your security program
• Use SecScanner to verify CT compliance
• Review CT logs during security assessments
• Train teams on CT alert handling
• Test your incident response for unauthorized certificates

Certificate Transparency has fundamentally improved web security by making certificate issuance auditable. Take advantage of this visibility to protect your domains and users.