NULL Pointer Dereference
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.
CWE-476 - NULL Pointer Dereference
A NULL pointer dereference occurs when the application dereferences a pointer that it expects to be valid, but is NULL, typically causing a crash or exit.