Access is encouraged to see that Yahoo! is now supporting HTTPS globally for its mail and messaging services, an important and overdue step for the security and privacy of its users. Pending technical analysis of its implementation, we believe this decision by Yahoo! responds to some of the concerns raised by civil society and security experts, and signals a continuing strengthening of their services’ privacy protections.
HTTPS is now enabled by default on Yahoo!'s mobile apps for Apple iOS, Windows 8, and Android. For users accessing Yahoo! Mail through their computer's browser, HTTPS can be switched on via the mail settings in the top right of the screen (go here to find out how). Once it's activated, all Yahoo! Messenger conversations in the browser window are also protected. According to the company, HTTPS is coming later this year to the desktop version of Messenger.
Source : Direct Link