eMudhra CAs Under Scrutiny After Private Key Leak

Indian certificate authority eMudhra issued eight intermediate certificates with private keys that were only one byte long, a configuration that allows attackers to impersonate any domain and sign malware as legitimate software. The exposure was disclosed this week and requires immediate updates across devices and applications.

In standard certificate validation, a website's certificate is signed by an intermediate certification authority, whose own certificate is signed by a trusted root certificate. The private keys for root certificates are stored offline. In this case, eMudhra issued six TLS and two code-signing intermediate certificates with private keys that were one byte long, meaning they consisted of a single byte, a number from 0 to 255. Such keys can be brute-forced in a split second. An attacker positioned anywhere in a network path, including on a corporate or government network, public Wi-Fi, an ISP, or a government entity, could intercept a connection and pose as any website, and code-signing certificates could make a downloaded application appear to come from a legitimate source such as Microsoft or Google. The cause of the one-byte keys is under investigation.

Software checks a certificate by verifying its chain of trust, and operating systems and applications maintain lists of trusted root certificates and certification authorities. Browsers maintain a list of certificates to reject, known as a revocation list. These lists are updated as problems arise, and all affected certificates now require revocation, removal of the affected root certificate, or a hard-coded rejection of the eight affected keys.

Recent versions of Chrome, Brave, Firefox, and Safari enforce a feature called Certificate Transparency that consults a public log of certificate issuances and blocks any that are not listed. Google reports that no certificates signed by the six affected intermediate CAs were logged, which suggests that routine web browsing may be unaffected. Software updates are still advised.

The scope of systems requiring updates is extensive: PCs, servers, operating systems, phones, tablets, routers, web browsers, web browsers wrapped in apps, virtual servers, virtual machines, Docker and other containers, programming languages such as Python, Go, Node.js, and Rust, and standalone download utilities such as curl. Other categories may also be affected. Older routers, smart and IoT appliances, and old computers could present problems later.

No timeline has been provided for when all affected systems will be patched, and the investigation into how the one-byte keys were issued remains open.