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.

It was discovered that Oniguruma incorrectly handled certain regular
expressions. An attacker could possibly use this issue to obtain
sensitive information, cause a denial of service or execute arbitrary
code.