Weak Keys Found in Eight eMudhra Certificates

On October 17, 2026, a researcher notified Apple, Google, Microsoft, Mozilla, and eMudhra that the private keys of eight of the company’s intermediate certificates had been compromised. The certificates were created in April 2025.

Six were TLS certificate authorities and two were code-signing authorities. All the keys used the P-384 elliptic curve, but instead of a random 384-bit number, they contained zero bytes and one non-zero byte. Finding the value of that byte required trying no more than 255 possibilities.

Helme recovered the keys from the public certificates in less than a second. Each of the eight intermediate certificates had also been cross-signed by a Sectigo root certificate. The researcher therefore checked 16 certificates: two for each key.

That same evening, Chrome blocked the intermediate certificates with a CRLSet update, and Mozilla added them to OneCRL. On October 20, Mozilla announced the incident and released an NSS update that removes the eMudhra root certificate. Chrome confirmed that the root certificate will be included in Chrome Root Store version 43.

According to Helme’s article, Chrome and Firefox directly block these intermediate certificates and require Certificate Transparency checks. Many other TLS clients do not check certificate revocation or Certificate Transparency by default, so revoking the intermediate certificates does not by itself affect them. The author recommends updating the root certificate sets in use; devices that cannot be updated will continue to trust these certificates until they expire in July 2039.

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Translation editors: Соловьев В., Пеков А.