In Artifex MuPDF 1.13.0, the fz_append_byte function in fitz/buffer.c allows remote attackers to cause a denial of service (segmentation fault) via a crafted pdf file. This is caused by a pdf/pdf-device.c pdf_dev_alpha array-index underflow.
https://lists.debian.org/debian-lts-announce/2020/07/msg00019.html