Bad heat exchanger? Very likely.
The soot on the side of this furnace tell us that sometimes the fire wanders aimlessly around the attic. Flames are licking the outside of this heater. That can't be good.
Aside from the obvious, a defective heat exchanger can make you or others in your family very sick. If exhaust gasses are crossing over into your warm air as you are heating your home, carbon monoxide and carbon dioxide may be present without you knowing it. Known as the silent killers, CO and CO2 can make you very ill at low levels and can be fatal at higher levels. Most defective heat exchangers are not this obvious so have your gas furnace checked every heating season. I prefer other methods of heating but I like gas heaters until they begin to age. Once a gas heating unit becomes 11 or 12 years old, it should be checked at the beginning of each heating season. By year 15, the system is aging and becomes more of a liability each year after. [This unreliability grows exponentially, not incrementally.]