libxml2 before 2.12.10 and 2.13.x before 2.13.6 has a use-after-free in xmlSchemaIDCFillNodeTables and xmlSchemaBubbleIDCNodeTables in xmlschemas.c. To exploit this, a crafted XML document must be validated against an XML schema with certain identity constraints, or a crafted XML schema must be used.
In libxml2 2.11 before 2.11.9, 2.12 before 2.12.9, and 2.13 before 2.13.3, the SAX parser can produce events for external entities even if custom SAX handlers try to override entity content (by setting "checked"). This makes classic XXE attacks possible.