An issue was discovered in Oniguruma 6.x before 6.9.4rc2. In the function gb18030mbcenclen in file gb18030.c, a UChar pointer is dereferenced without checking if it passed the end of the matched string. This leads to a heap-based buffer over-read.

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.

A NULL Pointer Dereference in matchat() in regexec.c in Oniguruma 6.9.2 allows attackers to potentially cause denial of service by providing a crafted regular expression. Oniguruma issues often affect Ruby, as well as common optional libraries for PHP and Rust.