| name | auditing-tls-certificate-transparency-logs |
| description | Monitors Certificate Transparency (CT) logs to detect unauthorized certificate issuance, discover subdomains via CT data, and alert on suspicious certificate activity for owned domains. Uses the crt.sh API and direct CT log querying based on RFC 6962 to build continuous monitoring pipelines that catch rogue certificates, track CA behavior, and map the external attack surface. Activates for requests involving certificate transparency monitoring, CT log auditing, subdomain discovery via certificates, or certificate issuance alerting.
|
| domain | cybersecurity |
| subdomain | threat-intelligence |
| tags | ["certificate-transparency","CT-logs","crt-sh","subdomain-discovery","TLS-monitoring","RFC-6962"] |
| version | 1.0.0 |
| author | mukul975 |
| license | Apache-2.0 |
| nist_csf | ["ID.RA-01","ID.RA-05","DE.CM-01","DE.AE-02"] |
Auditing TLS Certificate Transparency Logs
When to Use
- Monitoring owned domains for unauthorized or unexpected certificate issuance by unknown Certificate Authorities
- Discovering subdomains and hidden services through certificates logged in public CT logs
- Detecting phishing infrastructure that uses look-alike domain certificates (typosquatting, homograph attacks)
- Auditing Certificate Authority compliance by verifying all issued certificates appear in CT logs as required by browser policies
- Building continuous certificate monitoring into a security operations pipeline with alerting for new issuances
Do not use for attacking or disrupting Certificate Authorities, for scraping CT logs in violation of rate limits or terms of service, or as the sole method of subdomain enumeration without corroborating results through DNS verification.
Detection Gaps & Validation
- MMD / crt.sh latency: an SCT only promises inclusion within the 24h Maximum Merge Delay, and crt.sh polls logs on its own schedule -- a 15-minute poll can miss a freshly issued cert. Do not treat "not in crt.sh yet" as "not issued."
- Coverage gaps: crt.sh omits logs it does not ingest, and querying only
%.example.com misses certs that list your domain in a SAN of an unrelated CN. Query by name_value/SAN, include subsidiary brands, and corroborate with certspotter or a direct RFC 6962 get-entries pull.
- Precertificate vs leaf: ignoring precerts (poison OID 1.3.6.1.4.1.11129.2.4.3) forfeits the early-warning window -- track both.
- Baseline drift: an un-updated baseline DB re-alerts on every legitimate renewal (alert fatigue) while a too-broad authorized-CA set hides a rogue issuance.
To validate: confirm a known recent cert for your domain appears via the API before trusting the monitor, and rate-limit-test with backoff so HTTP 429s do not create silent gaps. Verify unauthorized-CA logic by injecting a test issuer not in the allowlist and confirming the alert fires; confirm STH signatures and get-sth-consistency proofs actually validate (append-only) rather than being skipped. After any authorized change, update the baseline so renewals do not generate false positives.
Prerequisites
- Python 3.10+ with
requests, cryptography, and pyOpenSSL libraries installed
- Network access to crt.sh (HTTPS) and public CT log servers
- A list of domains to monitor (owned domains, brand variations, typosquat candidates)
- SMTP credentials or webhook URL for alerting on new certificate discoveries
