WoSign has been in the media for offering free server and client certificates. Since one of my (free) S/MIME certificate from another CA is about to expire soon after one year and WoSign offers certificates, which are valid for thee years, I applied for their certificate, because I hoped to be able to skip the re-creation of new certificates in the next years.
During the process I wondered, why there was no indication from my browser, that it was creating a key pair, as I am used to from my last applications at different CAs. When the process was finished, a PFX file was offered for download. No need to export the certificate from the browser certificate storage.
According to their policy (in chapter 3.2.1), they offer to generate a private key for their subscribers. I understand, that this should be avoided, because nobody except the owner should ever be in possession of a private key. While they state, that subscribers may also choose to create own private keys and CSRs for server certificates, they don't make any statement about creating the private key for client certificate in another way.
Is my assumption correct, that the private keys used for their client certificates are really created on the server side, and is it correct, that (while they claim they don't) they could keep it and use it to decrypt/sign messages, exactly as the subscribers could do with their certificate? This should be totally unacceptable, because one would have to trust them, that they delete the private key, that they should not even had in the first place. But since I was unable to find any criticism about such an issue with their client certificate issuing process, I might have gotten something wrong.