Mozilla, Google and Apple block weak eMudhra certificates

Our opinion

Eight eMudhra intermediate certificate authorities had their private keys compromised after a key-generation error produced values small enough to recover almost instantly. Mozilla, Google and Apple have now blocked the affected certificates in their browser and operating-system trust systems.¹² The problem was discovered on 17 October by a security researcher scanning publicly available certificate data. Mozilla's incident report says the affected P-384 keys were generated with values smaller than 256, leaving only 256 possible values to test. A published demonstration key had a value of just 4.²³ That matters because intermediate certificate authorities can issue trusted TLS certificates for websites. If an attacker recovered one of the weak private keys, they could potentially create a certificate for a legitimate domain and use it to impersonate the site or intercept encrypted traffic. BleepingComputer independently matched one published key to an affected eMudhra certificate.¹ Mozilla blocked the certificates through OneCRL on 17 October, so most standard Firefox installations received protection without a browser update or restart. Mozilla also released Firefox 158.0.1 and updates for its ESR branches for users whose settings prevented the earlier revocation update from arriving automatically.² Google Chrome blocked the six affected intermediates authorised to issue TLS certificates, while Apple added protections on 19 October. eMudhra said the certificates were selected because a script mistakenly chose a second set of weak keys whose labels resembled the correctly generated keys. The company said most were never used in production and that it had found no evidence they issued certificates to end users.¹² Software with its own certificate store, or software that does not properly check revocation information, may still need an update. Administrators can use the incident tracker and vendor advisories to check their exposure rather than assuming that a browser update alone covers every application.¹²⁴ This incident is a sharp reminder that certificate trust is only as strong as the key-generation process behind it. The rapid browser response is reassuring, but the long tail is in specialised software and command-line tools that maintain their own trust decisions. Organisations should treat the vendor advisories as an inventory exercise: identify non-browser TLS clients, confirm their trust-store behaviour and apply the relevant updates instead of relying on Chrome or Firefox protection as a universal fix.