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 stack out-of-bounds
read occurs in match_at() during regular expression searching. A logical
error involving order of validation and access in match_at() could result
in an out-of-bounds read from a stack buffer.

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.