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.