Certificate Transparency Logs

Certificate Transparency Logs#

What Are They?#

Certificate Transparency (CT) Logs are public, append-only logs that track the issuance of SSL/TLS certificates.

These logs are maintained by independent organizations and are open for anyone to inspect.

Why Do They Matter?#

SSL/TLS certificates are crucial for securing communication between your browser and websites.

But if a rogue or misissued certificate is created, it could allow attackers to:

Impersonate legitimate websites
Intercept sensitive data
Spread malware

Think of CT logs as public registries where every certificate ever issued is recorded for transparency and accountability.

How Do CT Logs Help?#

1. Early Detection of Rogue Certificates#

CT logs allow website owners and researchers to:

Monitor for unauthorized or suspicious certificates
Detect fraud early and revoke compromised certificates

2. Holding Certificate Authorities Accountable#

If a Certificate Authority (CA) issues a certificate improperly, it will be visible in CT logs, promoting:

Transparency
Public scrutiny
Sanctions or revocation of trust if misused

3. Strengthening Web PKI#

CT logs help reinforce the Public Key Infrastructure (PKI) that underpins secure web communications.

CT Logs in Web Reconnaissance#

CT Logs are an amazing tool for subdomain enumeration and intelligence gathering.

Advantages over Traditional Methods#

Method	Limitations

Wordlists	Limited by pre-known subdomains

Brute-forcing	Time-consuming & unreliable

CT Logs	Access to historical, real subdomains

What You Can Discover:#

Hidden subdomains from old/expired certificates
Subdomains not exposed via DNS or site links
Legacy systems potentially vulnerable to attack

CT logs give a historical record of all subdomains associated with SSL certificates.

Tools to Search CT Logs#

Tool	Features	Pros	Cons
crt.sh	- Web UI for querying certificates - Shows SANs (Subject Alternative Names)	Free, easy to use, no login	Limited filtering
Censys	- Advanced search filters - Can query by IP, certificate fields, etc.	In-depth data, API available	Requires registration (free tier)

Using crt.sh for Subdomain Enumeration#

Web Interface#

Visit: https://crt.sh

Search for a domain (e.g., facebook.com) and find all SSL/TLS certificates related to it.

Using the Terminal#

To find all ‘dev’ subdomains on facebook.com:

curl -s "https://crt.sh/?q=facebook.com&output=json" \
| jq -r '.[] | select(.name_value | contains("dev")) | .name_value' \
| sort -u

Breakdown#

curl -s: Fetch JSON data silently from crt.sh
jq -r: Filter JSON where name_value contains “dev”
sort -u: Sort results alphabetically and remove duplicates

Sample Output#

.dev.facebook.com
*.newdev.facebook.com
*.secure.dev.facebook.com
[dev.facebook.com](http://dev.facebook.com/)[devvm1958.ftw3.facebook.com](http://devvm1958.ftw3.facebook.com/)[facebook-amex-dev.facebook.com](http://facebook-amex-dev.facebook.com/)[facebook-amex-sign-enc-dev.facebook.com](http://facebook-amex-sign-enc-dev.facebook.com/)[newdev.facebook.com](http://newdev.facebook.com/)[secure.dev.facebook.com](http://secure.dev.facebook.com/)

Summary#

Certificate Transparency Logs increase visibility and trust in SSL/TLS certificates.
They are crucial for security monitoring, subdomain discovery, and CA accountability.
Tools like crt.sh and Censys make these logs accessible for anyone doing recon or monitoring.