There are lots of API’s is used with WordPress in which some are;
Dashboard Widgets API: The Dashboard Widgets API makes it very simple to add new widgets to the Administration Dashboard Panel.
Database API: This page lists all holistic pages of a given Database related API. Each covers the functions involved in and use of a given set of functionality.
HTTP API: Within PHP, there are different ways to send an HTTP request. For simplicity, that different ways are called 'transports' and will be used from now on.
File Header API: The File Header API consists of all functions and hooks that are part of the WordPress API regarding the use of File Headers.
Setting API: The Settings API, added in WordPress 2.7, allows admin pages containing settings forms to be managed semi-automatically.
Theme Modification API: Theme Modification API consists of all functions and hooks that are part of the WordPress API regarding the use of Theme modification values.
Security Implementation in WordPress:
To implement security in WordPress; the following component plays an important role.
1. Security Theme
2. Vulnerability on your computer
3. Vulnerability on WordPress
4. Web server vulnerability
5. Network Vulnerability
These are the five factors which are used to implement security with WordPress.