analyzing-certificate-transparency-for-phishing

name	analyzing-certificate-transparency-for-phishing
description	Monitor Certificate Transparency logs using crt.sh and Certstream to detect phishing domains, lookalike certificates, and unauthorized certificate issuance targeting your organization.
domain	cybersecurity
subdomain	threat-intelligence
tags	["certificate-transparency","ct-logs","phishing","crt-sh","certstream","ssl","domain-monitoring","threat-intelligence"]
version	1.0
author	mahipal
license	Apache-2.0
atlas_techniques	["AML.T0052"]
nist_csf	["ID.RA-01","ID.RA-05","DE.CM-01","DE.AE-02"]
mitre_attack	["T1583.001","T1583.004","T1566.002","T1608.005","T1596.003"]
source	https://github.com/mukul975/Anthropic-Cybersecurity-Skills
source_commit	04450304b12645cb2b974ab96d28c0664758a88d
note	Vendored verbatim from an external Apache-2.0 security-skill library, pinned by commit. Exceeds the internal 300-line skill guideline (agent-code-constraints.md) -- kept as-is because this is vendored reference material (forensics/threat-intel procedure), not Yana AI-authored content, and trimming would damage technical accuracy.

Analyzing Certificate Transparency for Phishing

Overview

Certificate Transparency (CT) is an Internet security standard that creates a public, append-only log of all issued SSL/TLS certificates. Monitoring CT logs enables early detection of phishing domains that register certificates mimicking legitimate brands, unauthorized certificate issuance for owned domains, and certificate-based attack infrastructure. This skill covers querying CT logs via crt.sh, real-time monitoring with Certstream, building automated alerting for suspicious certificates, and integrating findings into threat intelligence workflows.

When to Use

• When investigating security incidents that require analyzing certificate transparency for phishing
• 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

• Python 3.9+ with requests, certstream, tldextract, Levenshtein libraries
• Access to crt.sh (https://crt.sh/) for historical CT log queries
• Certstream (https://certstream.calidog.io/) for real-time monitoring
• List of organization domains and brand keywords to monitor
• Understanding of SSL/TLS certificate structure and issuance process

Key Concepts

Certificate Transparency Logs

CT logs are cryptographically assured, publicly auditable, append-only records of TLS certificate issuance. Major CAs (Let's Encrypt, DigiCert, Sectigo, Google Trust Services) submit all issued certificates to multiple CT logs. As of 2025, Chrome and Safari require CT for all publicly trusted certificates.

Phishing Detection via CT

Attackers register lookalike domains and obtain free certificates (often from Let's Encrypt) to make phishing sites appear legitimate with HTTPS. CT monitoring detects these early because the certificate appears in logs before the phishing campaign launches, providing a window for proactive blocking.

crt.sh Database

crt.sh is a free web interface and PostgreSQL database operated by Sectigo that indexes CT logs. It supports wildcard searches (%.example.com), direct SQL queries, and JSON API responses. It tracks certificate issuance, expiration, and revocation across all major CT logs.

Workflow

Step 1: Query crt.sh for Certificate History

import requests
 json
 datetime  datetime
 tldextract

 :
    CRT_SH_URL = 

     ():
        .monitored_domains = monitored_domains
        .brand_keywords = [k.lower()  k  brand_keywords]

     ():
        
        params = {
            : ,
            : ,
        }
          include_expired:
            params[] = 

        resp = requests.get(.CRT_SH_URL, params=params, timeout=)
         resp.status_code == :
            certs = resp.json()
            ()
             certs
         []

     ():
        
        certs = .query_crt_sh(domain)
        suspicious = []

         cert  certs:
            common_name = cert.get(, ).lower()
            name_value = cert.get(, ).lower()
            issuer = cert.get(, )
            not_before = cert.get(, )
            not_after = cert.get(, )

            
            extracted = tldextract.extract(common_name)
            cert_domain = 
             cert_domain == domain:
                  

            
            flags = []
             domain.replace(, )  common_name.replace(, ):
                flags.append()
             (kw  common_name  kw  .brand_keywords):
                flags.append()
               issuer.lower():
                flags.append()

             flags:
                suspicious.append({
                    : cert.get(, ),
                    : cert.get(, ),
                    : issuer,
                    : not_before,
                    : not_after,
                    : cert.get(, ),
                    : flags,
                    : cert.get(, ),
                    : ,
                })

        ()
         suspicious

monitor = CTLogMonitor(
    monitored_domains=[, ],
    brand_keywords=[, , ],
)
suspicious = monitor.find_suspicious_certs()
 cert  suspicious[:]:
    ()