A cryptographic error compromised eight eMudhra private keys, leading to weak TLS and code signing certificates. Microsoft blocked the affected authorities and issued patches.
Cryptographic error affects eMudhra keys
A serious cryptographic error compromised eight private keys belonging to the eMudhra certificate authority. The error occurred during a cryptographic key generation operation on April 30, 2025, resulting in eight intermediate certificate authorities inheriting abnormally weak private keys.Weak keys pose security risks
Six of these keys were intended for issuing TLS certificates, used to authenticate websites and secure web communications, while the other two were for code signing, which allows verification of the origin of software. The flaw was in the very value of these keys, which, although relying on P-384, an elliptic curve cryptography algorithm, had values below 256.Error traced to a script flaw
The company admitted to an error in a script during a key manipulation operation, which produced a second set of vulnerable keys, subsequently associated with public certificates. The anomaly had gone undetected for nearly eighteen months, despite two independent audit cycles.Flaw detected and browsers respond
The flaw was detected on October 17, leading major browser publishers to block the compromised authorities. Microsoft took initial measures on October 20 by listing the eight compromised intermediate authorities on the prohibited certificates list of Windows.Microsoft expands security patches
On October 26, Microsoft expanded its response by excluding six eMudhra G3 root certificates and one cross-signed AAA Certificate Services certificate from the trust list of Windows. The same measure was applied to Edge the following day.Updates target specific Windows versions
For Windows 11, this decision was accompanied by three exceptional cumulative updates: KB5134133 for Windows 11 24H2, 25H2 and 26H2, KB5134134 for 26H1 and KB5134130 for 23H2. Windows 10 and Windows Server also received patches.eMudhra revokes compromised authorities
eMudhra revoked the eight authorities on October 18 and claims that none had issued a final certificate. However, an attacker in possession of their private keys could have issued fake certificates for domains they did not control.Compromised certificates could enable impersonation
On a device still recognizing the compromised authorities, these certificates could facilitate the impersonation of legitimate websites and the interception of encrypted communications. The code signing keys could also be used to present malicious software as coming from a trusted publisher.Microsoft extends measures to trust hierarchy
Microsoft has extended these measures to part of the Indian group's certification hierarchy, risking disruption to services that still rely on it. Companies must also inventory the websites, applications and services whose certificate chain passes through one of the seven removed certificates.Replacement of certificates advised
Their replacement, or the use of another recognized trust chain, will allow to avoid validation errors. Microsoft advises against restoring this trust locally without having previously assessed the risks.
