Not a bug report. Moved to Platform Pagoda.
No, the launcher does not steal your Mojang account credentials. The entire login process takes place between you and Mojang. 3rd parties are never involved in this process.
The launcher is open source, you can view the login code here: https://github.com/TechnicPack/LauncherV3/blob/master/src/main/java/net/technicpack/minecraftcore/mojang/auth/AuthenticationService.java#L97:L111