This write-up by CitizenLab [1] describes the Trident iOS exploit chain. Excerpt:
CVE-2016-4657 [2]: Visiting a maliciously crafted website may lead to arbitrary code execution
CVE-2016-4655 [3]: An application may be able to disclose kernel memory
CVE-2016-4656 [4]: An application may be able to execute arbitrary code with kernel privileges
[1] https://citizenlab.org/2016/08/million-dollar-dissident-ipho...
[2] http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=2016-4657
[3] http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=2016-4655
[4] http://www.cve.mitre.org/cgi-bin/cvename.cgi?name=2016-4656