Cloud　computing　is　an　innovative　technique　that　offers　shared　resources　for　stock　cache　and　server　management.　Cloud　computing　saves　time　and　monitoring　costs　for　any　organization　and　turns　technological　solutions　for　large-scale　systems　into　server-to-service　frameworks.　However,　just　like　any　other　technology,　cloud　computing　opens　up　many　forms　of　security　threats　and　problems.　In　this　work,　we　focus　on　discussing　different　cloud　models　and　cloud　services,　respectively.　Next,　we　discuss　the　security　trends　in　the　cloud　models.　Taking　these　security　trends　into　account,　we　move　to　security　problems,　including　data　breaches,　data　confidentiality,　data　access　controllability,　authentication,　inadequate　diligence,　phishing,　key　exposure,　auditing,　privacy　preservability,　and　cloud-assisted　IoT　applications.　We　then　propose　security　attacks　and　countermeasures　specifically　for　the　different　cloud　models　based　on　the　security　trends　and　problems.　In　the　end,　we　pinpoint　some　of　the　futuristic　directions　and　implications　relevant　to　the　security　of　cloud　models.　The　future　directions　will　help　researchers　in　academia　and　industry　work　toward　cloud　computing　security.