Trust Unearned? Evaluating CA Trustworthiness Across 5 Billion Certificates

Security relies on trust, especially when it comes to Certificate Authorities. Browsers ship with many root CAs built in, but are they all equally trustworthy? I examined over 5 billion recent TLS certificates and now I know! In this session I’ll reveal the most and least trustworthy CAs, factors influencing their trust ratings, and how to use this data to protect your organization.

[As presented at RSAC 2023, 27 April 2023]

Transcript

1. SESSION ID: #RSAC David J. Bianco Trust Unearned? Evaluating CA Trustworthiness Across 5 Billion Certificates Staff Security Strategist SURGe by Splunk @DavidJBianco / @DavidJBianco@infosec.exchange HT-R03
2. #RSAC Disclaimer Presentations are intended for educational purposes only and do not replace independent professional judgment. Statements of fact and opinions expressed are those of the presenters individually and, unless expressly stated to the contrary, are not the opinion or position of RSA Conference™ or any other co-sponsors. RSA Conference does not endorse or approve, and assumes no responsibility for, the content, accuracy or completeness of the information presented. Attendees should note that sessions may be audio- or video-recorded and may be published in various media, including print, audio and video formats without further notice. The presentation template and any media capture are subject to copyright protection. © 2023 RSA Conference LLC or its affiliates. The RSA Conference logo and other trademarks are proprietary. All rights reserved. 2
3. #RSAC The Secret Origins of the CA Trust Project As a new SURGeon, your first project is an important choice! I wanted something research-y, but practical. Ideally, something that could benefit from our unlimited Splunk license. I’d worked with Certificate Transparency before and thought it was neat. What could I do with ALL THE TLS CERTS ON THE INTERNET? 3 Source: DALL-E
4. #RSAC CA Trust: Is it Truly Earned? Internet security is built on TLS, which anchors its trust with Root CAs. OS and browser manufacturers decide for us which roots we trust. • Chrome: 138 trusted roots • Firefox: 54 trusted roots • Safari / MacOS: 154 trusted roots How do we know they are all worthy of our trust? Let’s measure! 4
5. #RSAC Splunkvengers, Assemble! 5 Mikael Bjerkeland Apps/Sys Engineer Kelcie Bourne Security Strategist, SURGe Philipp Drieger Principal Architect
6. #RSAC Methodology 6 In theory, this is simple: 1. Download all TLS certificates on the Internet and load them into Splunk 2. Mark all certificates observed doing Bad Things 3. Calculate the ”worst” and “best” CAs Source: DALL-E
7. #RSAC Certificate Review 7 Subject (“Whose cert is this?”) Issuer (“Who created this cert?”)
8. #RSAC Certificate Review 8 SANs (“What other names is this good for?”) Chain of Trust (“Can I trust this cert?”) Root CA (“Who says I can trust it?”)
9. #RSAC Superpower: Certificate Validation 9 Browsers must validate a certificate’s chain of trust, but that’s not all! What if the site owner loses control of their secret key? • Certificate Revocation Lists (CRLs) or Online Certificate Status Protocol (OCSP) But what if the CA loses control of its signing key? Source: DALL-E
10. #RSAC Certificate Transparency Explained Simply 10 Certificate Transparency (CT) provides public, verifiable records of WebPKI certificate issuance. Chrome has required CT since April 2018. CTLs are open to all via a standard API. I used this to download certificates from (mostly) 2021 and 2022. Source: https://certificate.transparency.dev/howctworks/
11. #RSAC Data: Certificates 11 5 Billion Unique Certificates 78k Issuing CAs 497 Root CAs 15 Certificate Transparency Logs
12. #RSAC Root CAs and Issuers 12
13. #RSAC Typical Root/Issuer CA Relationships 13
14. #RSAC Highly-Connected Relationships 14
15. #RSAC Data: Intel 15 185 Million Malicious Observations 7 Providers
16. #RSAC Data: Intel - Big THANK YOUs to the Providers! 16
17. #RSAC Analysis: Challenges and Limitations • This was some big “tres commas” data! – Needed custom download & parsing tool – Ingest was tricky • Matching intel to certificate subjects isn’t straightforward • Extreme variation in CA population sizes makes fair comparisons difficult 17
18. #RSAC Riskiest Root CAs (Single Pool) 18
19. #RSAC Riskiest Issuing CAs (Single Pool) 19
20. #RSAC Total Certificates by Root CA 20
21. #RSAC Total Certificates by Issuing CA 21
22. #RSAC Creating Root CA Tiers is Much Fairer! 22
23. #RSAC All Roots – Tier 1 23
24. #RSAC All Roots – Tier 2 24
25. #RSAC All Roots – Tier 3 25
26. #RSAC Riskiest Roots – Tier 4 26
27. #RSAC Trustiest Roots – Tier 4 27
28. #RSAC Issuing CA Tiers 28
29. #RSAC Riskiest Issuers – Tier 1 29
30. #RSAC Trustiest Issuers – Tier 1 30
31. #RSAC Riskiest Issuers – Tier 2 31
32. #RSAC Trustiest Issuers – Tier 2 32
33. #RSAC Riskiest Issuers – Tier 3 33
34. #RSAC Trustiest Issuers – Tier 3 34
35. #RSAC Riskiest Issuers – Tier 4 35
36. #RSAC Trustiest Issuers – Tier 4 36
37. #RSAC Conclusions • We identified 4 risky outlier root CAs and 10 issuers – Tier 4 roots were drowned in noise, so we ignored those • Blocking specific CAs is probably not warranted for most orgs • Logging certificates enables CAs to be used as observations with Risk- Based Alerting (RBA) or as data enrichment for threat hunting 37
38. #RSAC Room for Future Work • Longer certificate history + longer intel history would allow us to track trustworthiness changes over time, which could be interesting • More precise subject matching • Why don’t more intel providers track malicious certificates? • Someone (else) should compute & publish CA risk rankings on a regular basis 38
39. #RSAC Apply Days • Download the CA Trust risk rankings from https://splk.it/CATrust • Start logging TLS certificates w/Root and Issuing CA details Weeks • Do some HTTPS-focused threat hunting, using certificates and our risk data for enrichment 39 Months • Beg your CTI vendors to make CA risk rankings a regular part of their service offering!
40. SESSION ID: #RSAC David J. Bianco Trust Unearned? Evaluating CA Trustworthiness Across 5 Billion Certificates Staff Security Strategist SURGe by Splunk @DavidJBianco / @DavidJBianco@infosec.exchange HT-R03