With the continuous development of functionalities and the expansion of applications, smartphones face many security threats. The objectives of Supplement 19 to ITU-T X.1120 series of Recommendations are to protect the personal privacy of users and to improve information security of smartphones. In order to satisfy these security objectives, this Supplement specifies a hierarchical security framework and relevant security considerations for smartphones. This Supplement identifies smartphone threats, which are categorized into vulnerabilities and attacks. With regard to the security framework, this Supplement provides necessary security solutions through system improvements and security tools.