Embed this noticeAndrew Ayer (agwa@follow.agwa.name)'s status on Wednesday, 21-Oct-2026 02:14:13 JST
Andrew AyerThe lack of intermediate certificate revocation on Linux has long been a top concern of mine. eMudhra is likely not the first instance of an intermediate private key being mishandled. There have been many examples of CAs "accidentally" issuing unconstrained intermediates without any audits whatsoever (not that audits are a panacea). Those intermediates were revoked and added to OneCRL/CRLSets/etc. but Linux systems continued to trust them. Hopefully eMudhra will provide impetus for change. upki is looking promising! https://github.com/rustls/upki
The people who should be really scared right now are those using WebPKI CAs for mTLS / TLS client authentication. Attackers can just mint a certificate and authenticate to the mTLS endpoint, which probably does not know about the intermediate revocation. No need to pull off a BGP hijack or other network attack.
(Yes, Chrome is banning clientAuth EKU in the WebPKI, but they extended the deadline and I know of 6 examples of services changing their mTLS verifiers to accept serverAuth EKU in lieu of clientAuth, so mTLS in the WebPKI is still alive and kicking despite it being a bad idea.)
The first rogue 1.1.1.1 certificate was issued by Fina and logged to Certificate Transparency over a year ago. AFAICT, the first person to notice any of this was Hacker News user JXzVB0iA, two days ago: https://news.ycombinator.com/item?id=45089708 This morning, it was reported to the certificate-transparency mailing list, with attribution to JXzVB0iA. A few hours later, it was reported to the mozilla-dev-security-policy mailing list, without attribution. Then Dan Goodin wrote his article, citing the mozilla-dev-security-policy post. Very surprising that Cloudflare did not notice given they operate a CT monitor.
Over two years ago the library's developers were warned about the need to monitor the ct-policy mailing list if they were going to consume the Chrome CT log lists.
The Appmatus developers ignored this, so when the change was published today, Android apps using Appmatus stopped working. Affected app developers are now flooding CT mailing lists demanding the log list change be reverted. The right fix is to stop using Appmatus - it's clearly not fit for purpose.
Folks at Google have taken time on a Saturday to roll out a creative hack to unbreak the apps using this library. The folks at Apple, Google, and Mozilla who work on WebPKI and CT policy are truly some of the Internet's unsung heroes, for this and many other reasons.
This can used to intercept traffic to Google from Edge and other Windows applications (except Chrome and Firefox). Hug-ops to Google folks.
Microsoft are well aware of the extensive history of problems with this CA - I emailed them my concerns in 2021, and further issues were raised during a public CCADB discussion in 2022 - but they clearly don't care. I hope this incident prompts some change; Windows users deserve better!
As I expected, many clients will accept Entrust certificates issued after Nov 30. But to my surprise, several providers of PEM root bundles, notably Certifi/mkcert and curl, will begin automatically omitting Entrust roots after Nov 30 - meaning previously-issued Entrust certs will stop working! This is a complete misinterpretation of Mozilla's Distrust After attribute, and will cause unexpected breakage. I have opened bugs, but I expect other consumers have also misinterpreted Distrust After, so it would be prudent to replace existing Entrust certs.
Cert Spotter lets you know if any of your certificates will be revoked in the upcoming DigiCert mass revocation. They appear in your Cert Spotter dashboard with the original expiration replaced by the revocation date.
Handy, because if you log into your DigiCert account, they just give you a list of serial numbers and expect you to figure out what certs they correspond to.
@agwa@agwa.nameBootstrapped founder of SSLMate (https://sslmate.com) and DNS Helper (https://www.dnshelper.com). Making SSL certificates and DNS records easier. #WebPKI and #CertificateTransparency research on the side.