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 SIGSEGV occurs in leftadjustcharhead() during regular expression compilation. Invalid handling of reg->dmax in forwardsearch_range() could result in an invalid pointer dereference, normally as an immediate denial-of-service condition.