WordNet sense 1 (lacking protection or a guard):

unguarded

WordNet sense 2 (lacking protection or support):

defenceless

WordNet sense 3 (not defended or capable of being defended):

assailable

WordNet sense 4 ((often followed by `of' or `to') yielding readily to or capable of):

susceptible

WordNet sense 5 (having little physical or spiritual strength):

weak