Certificate Transparency & Alternative Name Disclosure

Maybe you’ve heard of Certificate Transparency and its log. Citing Wikipedia: “Certificate Transparency (CT) is an Internet security standard and open source framework for monitoring and auditing digital certificates.” Basically, it gives you information about any public certificate that is issued. Besides its advantages, I thought of one possible problem as it leaks all FQDNs to the public when using TLS certificates, for example from Let’s Encrypt.

A similar problem might arise when using a single X.509 certificate with a couple of DNS names (subject alternative name SAN) from which one should be kept “private”. It will be publicly known as well.

Hence I made a self-experiment in which I generated two certificates with random names, monitoring the authoritative DNS servers as well as the IPv6 addresses of those names in order to check who is resolving/connecting to otherwise unknown hostnames. Here we go:

TL;DR: Over a test period of 8 months my (1) completely hidden FQDN was resolved by DNS queries about 700 times, while 73 IPv6 HTTPS connections came in. My (2) second FQDN (SAN on my blog) was queried about 500 times, while 273 IPv6 connections came in, split into port 80 and 443. That is: Both completely unused hostnames are “leaked” and scanned by the public, just by using X.509 certificates!

The Setup

I used randomly generated hostnames and IPv6 addresses:

  1. A Let’s Encrypt certificate with a single DNS name which I have never used anywhere. I didn’t even google for it or the like. Its name is  7qftpqiw5m.ib.weberdns.de having a single AAAA record pointing to 2001:470:765b:0:747d:36b6:0d74:f26a. Issued at 2019-10-30 14:49 UTC, CT log here: https://crt.sh/?id=2053680948.
  2. Another LE certificate with a DNS name of  xd524olksc.ib.weberdns.de (with AAAA to 2001:470:765b:0:d302:1962:0df9:91eb) along with weberblog.net and blog.webernetz.net (my old hostname for the blog). In fact, I used this certificate for 2,5 months on my blog itself, giving many entities the possibility to see the hostname. Issued at 2019-10-30 15:09 UTC, CT log here: https://crt.sh/?id=2053730087. It was active on my blog until 2020-01-15 13:55 UTC.

As both certificates were issued by Let’s Encrypt, their validity expired after 90 days.

Since I am controlling the authoritative DNS servers for *.ib.weberdns.de (Infoblox VMs with query logging enabled) as well as the IPv6 space 2001:470:765b::/64 (a HE tunnel broker connection through my Palo Alto Networks firewall) I was able to catch every single DNS query and IP connection attempt. ;) However, all connections were blocked completely. No service was listening on those IPv6 addresses at all.

Many DNS Queries and IP Connections

After the two certificates were issued, I put the second one (with the SAN) on my blog. For both hostnames, I saw a couple of DNS requests on the DNS servers immediately. As well as incoming connections to the IPv6 addresses:

Here is a screenshot from the Palo Alto, showing the two blocking rules and their hits:

Note that I did *only* analyze incoming IPv6 connections. Yes, this is not optimal, but it is 100 % likely that all those attempts came from explicit stations querying my hostnames rather than normal port scans. (Port scans to random IPv6 addresses are unlikely as the host ID space in a /64 is 2^64. Refer to my post about the Internet’s Noise.)

My Analysis

I grepped through the log files from the authoritative DNS servers as well as from the firewall. As expected, there are hundreds of connection attempts to both hostnames:

Cert 1
only CT
Cert 2
CT + Blog SAN
DNS queries for A
(unique sources)
DNS queries for AAAA
(unique sources)
DNS queries for CAA
(unique sources)
All queried RRs642 A
22 MX
6 DS
5 NS
357 A
117 AAAA
29 MX
24 CAA
8 NS
4 DS
IPv6 connections
(unique sources)
[unique /32]
Destination Ports73x 443154x 80
122x 443
Sourcing ASes14 DigitalOcean, LLC
1 Google LLC
14 DigitalOcean, LLC
8 Quintex Alliance Consulting
6 Emerald Onion
4 Google LLC
4 F3 Netze e.V.
2 Zwiebelfreunde e.V.
2 Nexeon Technologies, Inc.
1 OVH Ltd
1 Keyweb AG
1 Joey Julian Koenig
1 Hydra Communications Ltd
1 Hurricane Electric LLC
1 Alec Larsen


While the first FQDN got more DNS queries, the second one received much more connection attempts.

I have not looked into every single IPv6 source address, but there seem to be some interesting ones. Of course, it’s Google. And quite often DigitalOcean which probably only hosts some kind of stuff? (Does anyone know where Let’s Encrypt itself is hosted?) Furthermore, at least one Tor exit node is present according to the whois search. And Technische Universitaet Muenchen querying the CAA records of the hostname just a few seconds after it appeared in the CT log. Probably some research going on here? Same for Cloudflare, which only queried the CAA record.

Further Analysis

As always, you could do so much more with this data. At first, drawing some more nice graphs to have an easier understanding of the data than the raw values. ;) But also some more analysis such as:

  • timeline of queries <- this would probably reveal much more queries in the first few hours compared to the rest of the test period
  • correlation of the DNS clients (that sent the DNS query) and the IPv6 addresses that sourced actual connections to the servers <- probably no big correlation here since the first show the recursive DNS resolvers while the second the Google crawlers
  • a deeper look at the source IPv6 addresses <- more details about search engine crawlers, university projects, TOR exit nodes, and random private clicks as someone saw the weird hostname on my blog’s certificate.


Don’t expect that you can receive valid X.509 certificates on the Internet for private use cases. Every single FQDN is immediately publicized in the CT logs and will be scanned! Keep that in mind.

My analysis used only IPv6 addresses behind those hostnames. Since the query rate for A records was much higher, it is likely that there are a couple of hundred connection attempts for every hostname that ever appeared in the public CT logs or that is just a subject alternative name on a publicly used certificate.


If you want to have a look at the raw logs, here we go:

I used the following shell commands for my analysis (refer to Logfile Parsing):

God’s blessing!

Photo by AbsolutVision on Unsplash.

5 thoughts on “Certificate Transparency & Alternative Name Disclosure

  1. Some of the queries came from the following Cloudflare IP ranges (these are listed on https://cloudflare.com/ips):

    They appear to be recursive queries sent by the resolver on behalf of its users. I expect Google to be in a similar situation, with queries going to

  2. I was planning to take advantage of this CT log, to monitor for new hosts in my company which skip our Security approval. Im planning to write a small tool to keep checking for unauthenticated endpoints for a subscribed domain in a CT log.

  3. This is the reason, why i only use wildcard certificates for my private stuff. No CT privacy issues anymore. Cheers to the DNS-01 acme challenge and Let’s Encrypt.

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