Anthropic-Cybersecurity-Skills/skills/analyzing-tls-certificate-transparency-logs/SKILL.md

name, description, domain, subdomain, tags, version, author, license, atlas_techniques, nist_csf, mitre_attack, mitre_f3

name	description	domain	subdomain	tags	version	author	license	atlas_techniques	nist_csf	mitre_attack	mitre_f3
analyzing-tls-certificate-transparency-logs	Queries Certificate Transparency logs via crt.sh and pycrtsh to detect phishing domains, unauthorized certificate issuance, and shadow IT. Monitors newly issued certificates for typosquatting and brand impersonation using Levenshtein distance. Use for proactive phishing domain detection and certificate monitoring.	cybersecurity	security-operations	certificate-transparency ct-logs crt-sh phishing-detection tls-monitoring security-operations	1.0	mahipal	Apache-2.0	AML.T0073 AML.T0052	DE.CM-01 RS.MA-01 GV.OV-01 DE.AE-02	T1583.001 T1566.002 T1598.003 T1583.006	version tactics techniques 1.1 reconnaissance resource-development initial-access id name tactic source T1598 Phishing for Information reconnaissance attack id name tactic source T1593 Search Open Websites/Domains reconnaissance attack id name tactic source T1583.001 Acquire Infrastructure: Domains resource-development attack id name tactic source F1020.002 Create Fake Materials: Fake Website resource-development f3 id name tactic source T1660 Phishing initial-access attack

Analyzing TLS Certificate Transparency Logs

When to Use

• When investigating security incidents that require analyzing tls certificate transparency logs
• When building detection rules or threat hunting queries for this domain
• When SOC analysts need structured procedures for this analysis type
• When validating security monitoring coverage for related attack techniques

Prerequisites

• Familiarity with security operations concepts and tools
• Access to a test or lab environment for safe execution
• Python 3.8+ with required dependencies installed
• Appropriate authorization for any testing activities

Instructions

Query crt.sh Certificate Transparency database to find certificates issued for domains similar to your organization's brand, detecting phishing infrastructure.

from pycrtsh import Crtsh

c = Crtsh()
# Search for certificates matching a domain
certs = c.search("example.com")
for cert in certs:
    print(cert["id"], cert["name_value"])

# Get full certificate details
details = c.get(certs[0]["id"], type="id")

Key analysis steps:

1. Query crt.sh for all certificates matching your domain pattern
2. Identify certificates with typosquatting variations (Levenshtein distance)
3. Flag certificates from unexpected CAs
4. Monitor for wildcard certificates on suspicious subdomains
5. Cross-reference with known phishing infrastructure

Examples

from pycrtsh import Crtsh
c = Crtsh()
certs = c.search("%.example.com")
for cert in certs:
    print(f"Issuer: {cert.get('issuer_name')}, Domain: {cert.get('name_value')}")