Design Notes: How do rust monsters affect warforged? Check out the Eberron Campaign Setting, p. 23:
"The iron in the body of a warforged makes him vulnerable to rusting grasp. The creature takes 2d6 points of damage from the spell (Reflex half; save DC 14 + caster's ability modifier). A warforged takes the same damage from a rust monster's touch (Reflex DC 17 half)." That's the same save DC listed in the Monster Manual entry; Con-based and with a +4 racial bonus.
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