What it does. 'Queries Certificate Transparency logs via crt.sh and pycrtsh to detect Part of mukul975/Anthropic-Cybersecurity-Skills (817 security skills) (mukul975/Anthropic-Cybersecurity-Skills).

Install

• npx skills add mukul975/Anthropic-Cybersecurity-Skills --skill analyzing-tls-certificate-transparency-logs, or copy the skill folder into ~/.claude/skills/analyzing-tls-certificate-transparency-logs/.
• Raw file: curl -sL https://raw.githubusercontent.com/mukul975/Anthropic-Cybersecurity-Skills/HEAD/skills/analyzing-tls-certificate-transparency-logs/SKILL.md

SKILL.md (verbatim)

name: analyzing-tls-certificate-transparency-logs
description: '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.

  '
domain: cybersecurity
subdomain: security-operations
tags:
- certificate-transparency
- ct-logs
- crt-sh
- phishing-detection
- tls-monitoring
- security-operations
version: '1.0'
author: mahipal
license: Apache-2.0
atlas_techniques:
- AML.T0073
- AML.T0052
nist_csf:
- DE.CM-01
- RS.MA-01
- GV.OV-01
- DE.AE-02
mitre_attack:
- T1583.001
- T1566.002
- T1598.003
- T1583.006
mitre_f3:
  version: '1.1'
  tactics:
  - reconnaissance
  - resource-development
  - initial-access
  techniques:
  - id: T1598
    name: Phishing for Information
    tactic: reconnaissance
    source: attack
  - id: T1593
    name: Search Open Websites/Domains
    tactic: reconnaissance
    source: attack
  - id: T1583.001
    name: 'Acquire Infrastructure: Domains'
    tactic: resource-development
    source: attack
  - id: F1020.002
    name: 'Create Fake Materials: Fake Website'
    tactic: resource-development
    source: f3
  - id: T1660
    name: Phishing
    tactic: initial-access
    source: 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')}")

Other files in this skill

• LICENSE
• references/api-reference.md
• scripts/agent.py

references/api-reference.md (verbatim)

API Reference: Analyzing TLS Certificate Transparency Logs

pycrtsh

from pycrtsh import Crtsh
c = Crtsh()

# Search certificates by domain
certs = c.search("example.com")      # exact match
certs = c.search("%.example.com")    # wildcard subdomains

# Get certificate details by ID
details = c.get(cert_id, type="id")
details = c.get(sha1_hash, type="sha1")
details = c.get(sha256_hash, type="sha256")

crt.sh REST API (Direct)

import requests

# JSON output
resp = requests.get("https://crt.sh/?q=%.example.com&output=json")
records = resp.json()
# Fields: id, issuer_ca_id, issuer_name, common_name,
#          name_value, not_before, not_after, serial_number

certstream (Real-Time CT Monitoring)

import certstream

def callback(message, context):
    if message["message_type"] == "certificate_update":
        all_domains = message["data"]["leaf_cert"]["all_domains"]
        print(all_domains)

certstream.listen_for_events(callback, url="wss://certstream.calidog.io/")

Key Certificate Fields

References

• pycrtsh: https://pypi.org/project/pycrtsh/
• crt.sh: https://crt.sh/
• certstream: https://certstream.calidog.io/
• CT RFC 6962: https://datatracker.ietf.org/doc/html/rfc6962

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