When you enter your barbarian rage, your body becomes infused with cold energy.
Prerequisite: Ability to rage.
Benefit: As long as you are under the effects of a barbarian rage, you gain the cold subtype. You gain immunity to cold, but have vulnerability to fire, which means you take half again as much (+50%) damage as normal from fire, regardless of whether a saving throw is allowed, or if the save is a success or failure.
Source: Frostburn, page 48.