Yes, Very much so. I have a large enclosed glass room in "Hell" (My mp netherworld) that stands over a great cavern with a lava lake beneath. If there is a single hole in the room a ghast will fireball it and blast all the glass around you sending you spiraling into the lava below.

Can blazes break glass?

EDIT: Nope! Just tested! They cannot see through glass panes or glass.

What is resistant to ghast fireballs?

Ghasts cannot destroy any blocks with a blast resistance of 26 or higher (eg iron bars, Nether Brick Blocks, or cobblestone).