How worried should I be about a bathroom fan venting into the attic?

It’s worth fixing, but finding it doesn’t automatically mean there’s already major damage. A fan sends warm, damp air into the attic, and that can contribute to condensation, wet insulation, mold, or wood damage. How much risk there is depends on things like how often the bathroom is used, attic conditions, and whether moisture is actually lingering there.

Have a look for damp or matted insulation, staining, musty smells, or visible growth around the fan and nearby roof framing. If you find those, get the area assessed. Even if everything looks dry, I’d still plan to duct the fan to an exterior termination rather than leave it as-is.

The fix is to run a properly sized duct from the fan to outdoors, with the connection secured and the duct routed so it doesn’t sag and collect water. The exterior outlet needs to be placed where bathroom exhaust won’t just get drawn back into the attic through an intake vent. A suitable wall or gable exit may be an option, depending on the layout. A roof exit can also be done, but with a newer metal roof I’d have someone familiar with that roof system handle the penetration.

Local requirements can depend on the home and jurisdiction, so ask the installer to confirm the applicable rules and show you where the duct will end. I wouldn’t accept “it’s common” as a substitute for a clear plan to vent it outside.

Short version: don’t panic, but don’t treat it as fine either. Some older homes have fans that were never ducted, and a long time without obvious damage is useful context, not proof that the setup is correct or risk-free. Venting it outside is the sensible repair.

If the original contractor won’t clearly agree to the correction, getting an independent quote may save a lot of back-and-forth. Ask the contractor to inspect the existing fan, attic, and possible exit routes, and to explain how the exterior opening will be sealed against weather.