Microsoft has released out-of-band (OOB) Windows updates to remove trust in certificates tied to compromised eMudhra certificate authority keys, protecting systems that do not receive automatic certificate trust updates.
The KB5134133 update for Windows 11 versions 24H2, 25H2, and 26H2 and the KB5134125 update for Windows 10 versions 21H2 and 22H2 were released on October 26.
The updates are for systems without Internet access, devices that cannot access Microsoft's certificate-update service, and those where automatic Certificate Trust List (CTL) updates have been disabled.
The updates were released after the discovery of eight eMudhra intermediate certificate authority private keys that were so weak they could be recovered almost instantly.
All eight keys were generated in April 2025 and had values smaller than 256, allowing anyone to recover them by testing just 256 possible numbers.
The compromised keys could have allowed attackers to forge certificates for legitimate websites and intercept encrypted communications.
After the disclosure, eMudhra revoked the eight compromised intermediate certificates, and Mozilla, Google, Apple, and other software vendors also began revoking them.
In response, Microsoft previously updated the Windows Disallowed Certificate Trust List to block the eight affected intermediate certificate authorities, including six used for TLS and two for code signing.
The company also released a CRLSet update for Microsoft Edge and WebView2 to block affected TLS certificates, which enabled the block without requiring a browser update.
Microsoft is now taking further steps by removing seven certificates associated with the affected certificate hierarchies from its Windows trust list.
"On October 26, Microsoft released an out-of-band Windows update that removes seven certificates associated with the affected hierarchies: six eMudhra G3 roots and an AAA Certificate Services cross-sign, from the Windows trust list," Microsoft explained.
The new update goes further by also removing trust in the eMudhra root certificates used to verify other certificates issued by the CA.
Microsoft says the same seven certificates will also be removed from the Edge trust list on October 27.
"Removing a root from a trust list is not the same as revoking the known impacted intermediate certificate authorities," explains Microsoft.
"This is a separate trust decision, broader than blocking the identified compromised intermediate keys. Organizations with a specific compatibility need may choose to trust a certificate locally, subject to their own security assessment and policies."
The company advises admins to install the update on disconnected Windows systems and identify any certificates that depend on the removed roots, as they may need to be replaced.
Microsoft is also investigating the remaining eMudhra roots and may make additional trust changes as needed.
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