Security researcher Scott Helme revealed that eight intermediate certificate authority (CA) private keys from eMudhra were effectively just 8-bit numbers, making them trivially easy to brute-force. The keys, used for TLS and code-signing certificates, were created in April 2025 and cross-signed by Sectigo. On October 17, a researcher reported the issue to major browser vendors and eMudhra. Chrome and Mozilla blocked the certificates within hours, and by October 20, Mozilla publicly announced the compromise and removed the eMudhra root from its trust store.
The flaw means that anyone could have recovered these private keys in milliseconds using a standard laptop. This compromises the entire security model of certificates issued by these intermediates. While browsers acted quickly, many other software applications that rely on TLS remain vulnerable because they do not check certificate revocation or Certificate Transparency logs by default.
How the eMudhra CA keys were compromised
The private keys are elliptic-curve keys on the P-384 curve. In such cryptography, the private key is a random number d, and the public key is d times G, where G is a fixed point. Security relies on d being enormous—a 384-bit random number. However, eMudhra's keys were not random; they were small integers like 4, 56, and 115. This means an attacker could simply compute multiples of G until finding the match, requiring at most 255 steps. Scott Helme demonstrated this by brute-forcing all 16 certificates in under a second on his laptop.
The certificates themselves appeared valid: the public keys were legitimate points on the curve, and all structural checks passed. The only way to detect the weakness was to attempt the brute-force. Unlike a typical key theft, where a key is stolen, here the key was effectively public from the moment the certificate was issued. Anyone who saw the certificate could have derived the private key instantly.
- The private keys were 384-bit numbers that should have been random, but instead were tiny values like 4, 56, and 115.
