During the upgrade to 6.4.2 the certificates were renewed for splunkd, but not for splunkweb
"/usr/splunk/etc/auth/ca.pem": certificate renewed
"/usr/splunk/etc/auth/cacert.pem": certificate renewed
"/usr/splunk/etc/auth/server.pem": certificate renewed
Process for self-signed certs: http://wiki.splunk.com/Community:SplunkWeb_SSL_SelfSignedCert_NewRootCA
Process for 3rd party CA certs: http://wiki.splunk.com/Community:SplunkWeb_SSL_3rdPartyCA
Those certs, for splunkweb, doesn't come by default with splunk installation and hence not updated during upgrade. You would've either used a self signed certs OR third party certs, so follow the same process to get new certs and update the same in your web.conf.