An issue was discovered in Oniguruma 6.2.0, as used in Oniguruma-mod in Ruby through 2.4.1 and mbstring in PHP through 7.1.5. A heap out-of-bounds write occurs in bitset_set_range() during regular expression compilation due to an uninitialized variable from an incorrect state transition. An incorrect state transition in parse_char_class() could create an execution path that leaves a critical local variable uninitialized until it's used as an index, resulting in an out-of-bounds write memory corruption.