A verification server provides certificate verification services to users of third-party application sites. In some embodiments, a verifier component of a user's client device provides the verification server with a certificate of a third-party application site, and the verification server indicates whether the certificate is successfully verified. In response to successful verification, the verifier component of the user's client device takes an action such as permitting the user's credentials to be provided to the third-party application site. In some embodiments, verifier components of numerous client devices provide certificates to the verification server, based on which the verification server learns which certificates are valid for a given third-party application site.