State laws differ but in general both the car owner and the driver can be held responsible (liable) if an uninsured vehicle is at-fault in an auto accident.

Typically the owner can be held responsible because they own the vehicle and in most states as the registered owner they are required to have at least state minimum Liability limits on the vehicle. If the car was driven without being insured than the owner was negligent on their duties as the registered owner. An owner also has vicarious liability for anyone they allow to operate their vehicle, whether the car is insured or not.

The driver can be held liable for the accident if they were at-fault and thus caused the accident. The driver should also have made certain that the vehicle they were operating was properly insured so that an accident would be covered and not leave any damages for the driver and owner to pay for personally.

So if you loaned out an uninsured vehicle to a friend and they were in an accident than you as the owner and the driver who caused the accident could both normally be found responsible for the damages the operator of the car caused. This means if person harmed by the accident went through their own insurance than their insurer could subrogate with both the owner and driver. Or if the other party instead took you to court they could usually sue both the driver and owner for their damages.

If you are in a situation where you are the driver or owner of an uninsured vehicle or were harmed by a driver (that was not the owner) of an uninsured vehicle than you should contact a lawyer in your state to find out the liability of both the driver and car owner of the uninsured vehicle are according to state laws.

Just to keep in mind penalties for driving without insurance are normally harsh though they vary by state. Penalties can be just a fine or be severe like jail time, driver's license suspension, registration suspension, or a SR-22 required to be carried. If the car owner and/or driver are held liable and do not pay the victim or their insurer back for the cost of the damages than in many states their driver's licenses can be suspended indefinitely.