An issue was discovered in Oniguruma 6.x before 6.9.4_rc2. In the function fetch_interval_quantifier (formerly known as fetch_range_quantifier) in regparse.c, PFETCH is called without checking PEND. This leads to a heap-based buffer over-read.
https://usn.ubuntu.com/4460-1/
https://lists.debian.org/debian-lts-announce/2019/12/msg00002.html