Allow MaD sanitizers for java/xml/xpath-injection

Author: owen-mc

java/ql/lib/semmle/code/java/security/XPath.qll

@@ -27,3 +27,10 @@ private class DefaultXPathInjectionSink extends XPathInjectionSink {
     )
   }
 }
+
+/** A sanitizer for XPath injection. */
+abstract class XPathInjectionSanitizer extends DataFlow::Node { }
+
+private class ExternalXPathInjectionSanitizer extends XPathInjectionSanitizer {
+  ExternalXPathInjectionSanitizer() { barrierNode(this, "xpath-injection") }
+}

java/ql/lib/semmle/code/java/security/XPathInjectionQuery.qll

@@ -13,6 +13,8 @@ module XPathInjectionConfig implements DataFlow::ConfigSig {
 
   predicate isSink(DataFlow::Node sink) { sink instanceof XPathInjectionSink }
 
+  predicate isBarrier(DataFlow::Node node) { node instanceof XPathInjectionSanitizer }
+
   predicate observeDiffInformedIncrementalMode() { any() }
 }