Cause of this?

Any idea on the cause of this? Is it heat, leakage, both or other?

Look’s like both Further evaluation by a qualified person .
Looks like an old tired furnace

sorry, attached photo now

It needs the gas pressure checked at the control valve as there appears to be serious flame roll out. If the heat exchanger is in tact based on age I would consider the unit near end of life

Definitely looks like flame roll-out. Isn’t the plastic box from a disconnected control of some sort located in the upper right hand corner of the picture melted?

Yes it was melted. I didn’t see any flame roll out upon the boiler firing up. It may have already been corrected.

That ol girl looks like she needs a pension ! What’s the age of it ?

I would also check the flue on that puppy.