EPA thinks otherwise. Then when you think about it, it would be hard to boast about how clean things will be when all trucks use DEF if it had a bunch of hazmat stickers on it.
Not particularly corrosive, not hazardous material, and actually a fairly benign material. Don't go by what you hear over the CB, urine is not a concentrated enough source, and there are other things in it.
Certified diesel exhaust fluid is a mixture of 67% deionized water and 33% urea, if I remember the percentages right, I know they are close, and is a concentration that will not salt out the urea if allowed to freeze.