All launcher text replaced by black boxes (caused by outdated Java)

All text in the launcher, both static text and text input field, shows as black boxes. The only exception in the password box, which correctly showed dots.

I tested this with a blank .minecraft directory and a new copy of the launcher jar from the website, and the issue persisted.

Playing around with the username box seems to indicate that the letters are the correct widths, and that the glyphs have just been replaced by solid black boxes of the appropriate widths.