The word closest to that definition is probably opportunist, although historically it has been used in a rather specialised sense. I feel that the word most closely associated with taking advantage of a situation is exploit which also carries a slightly negative connotation.
2. Advantage, benefit, profit all mean something that is of use or value. Advantage is anything that places one in an improved position, especially in coping with
Comprehensive list of synonyms for to have or to take advantage of an opportunity, phrase. to get an advantage from a situation, especially a bad situation
Synonyms for take advantage of at Thesaurus with free online thesaurus, early 14c., avantage, "position of being in advance of another," from Old French