A NULL pointer dereference vulnerability exists in the xpath.c:xmlXPathCompOpEval() function of libxml2 through 2.9.8 when parsing an invalid XPath expression in the XPATH_OP_AND or XPATH_OP_OR case. Applications processing untrusted XSL format inputs with the use of the libxml2 library may be vulnerable to a denial of service attack due to a crash of the application.
https://access.redhat.com/errata/RHSA-2019:1543
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901817
https://bugzilla.redhat.com/show_bug.cgi?id=1595985
https://gitlab.gnome.org/GNOME/libxml2/issues/10
https://lists.debian.org/debian-lts-announce/2018/09/msg00035.html
https://lists.debian.org/debian-lts-announce/2020/09/msg00009.html
https://security.netapp.com/advisory/ntap-20190719-0002/