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 write in onigencunicodegetcasefoldcodesbystr() occurs during regular expression compilation. Code point 0xFFFFFFFF is not properly handled in unicodeunfoldkey(). A malformed regular expression could result in 4 bytes being written off the end of a stack buffer of expandcasefoldstring() during the call to onigencunicodegetcasefoldcodesbystr(), a typical stack buffer overflow. Upstream bug:

https://github.com/kkos/oniguruma/issues/56

Upstream patch:

https://github.com/kkos/oniguruma/commit/166a6c3999bf06b4de0ab4ce6b088a468cc4029f