What is Certificate Transparency?

Understanding Certificate Transparency

Certificate Transparency addresses a critical vulnerability in the web's trust model—the fact that compromised Certificate Authorities could issue fraudulent certificates allowing attackers to impersonate legitimate websites, often without detection. This framework creates public, append-only logs where all issued SSL/TLS certificates must be recorded, enabling domain owners to monitor for unauthorized certificates issued for their domains. When Certificate Authorities add each newly issued certificate to multiple independent, cryptographically verifiable logs, it becomes virtually impossible to issue certificates secretly. Modern browsers check these logs to verify certificate legitimacy, rejecting those not properly logged. Beyond improving security, this transparency creates accountability throughout the certificate ecosystem. Organizations leverage CT logs for various security purposes: monitoring for unauthorized certificates that might indicate attempted attacks, identifying forgotten or shadow IT systems when unexpected legitimate certificates appear, and verifying their certificate inventory for expiration management. While primarily designed to address PKI vulnerabilities, Certificate Transparency has evolved into a valuable resource for attack surface management and digital asset discovery.