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we have a CA certificate template to verify, which contains an extendedkeyusage extension with the following values

  • "clientAuthentication"
  • "emailProtection" and
  • "id-kp-OCSPSigning"

Is this a mistake or is there really a usecase for extendedkeyusage in a CA certificate?

2 Answers 2

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CA certificate itself is never used during authentication, instead it is used for chain validation (I'm not talking about signing). In most cases, applications check particular EKU presence in end-entity certificate only. Thus, EKU presence in CA certificate has no effect.

There are known vendor-specific implementations when EKU in CA certificate has effect. This is called "constrained EKU", when application verifies whether particular EKU is valid for entire certificate chain and reject the certificate if particular EKU is presented in end entity certificate, but not specified on CA level in non-empty EKU extension.

For more details, please read my blog post about constrained EKU implementation in Microsoft certificate chaining engine: https://www.sysadmins.lv/blog-en/constraining-extended-key-usages-in-microsoft-windows.aspx

very brief explanation, in this thread: Root CA with Extended Key Usage fields

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The EKUs on CAs are used to limit which EKUs can be effective for entity certs. Even if a CA goes rogue and issues server auth EKU, in your case, verifiers won't allow it. As part of chain validation, a client will see the lack of server EKU on the CA and kill the handshake.

This is what makes a policy CA a policy CA. It limits blast radius if compromised.

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  • This answer is incorrect. Not all clients verify if particular EKU is valid for entire chain. In many cases they simply verify if requested EKU is shown in leaf certificate only. There is no requirement for EKU to be valid for entire chain.
    – Crypt32
    Commented Feb 10, 2023 at 15:15

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