Standard shader: transparent detail and mask interactions

I want to "apply" a transparent texture as the detail over an opaque base texture using the standard shader. But if I don't add a mask, the color of the last non-transparent pixel of the the detail gets repeated until the end.

W/o mask:


With detail texture itself as mask: (this is what I want)


I'm completely new to shaders so I'd really appreciate if someone could explain my why this happens. Is it a problem with the texture? or just the way the shader works?

The transparent pixels in the detail texture are 100% transparent.