Add documentation for URL opening sinks

Author: tamasvajk

java/ql/src/Security/CWE/CWE-319/HttpsUrls.ql

@@ -36,7 +36,7 @@ class HTTPStringToURLOpenMethodFlowConfig extends TaintTracking::Configuration {
 
   override predicate isSource(DataFlow::Node src) { src.asExpr() instanceof HTTPString }
 
-  override predicate isSink(DataFlow::Node sink) { sinkNode(sink, "open-url") }
+  override predicate isSink(DataFlow::Node sink) { sink instanceof URLOpenSink }
 
   override predicate isAdditionalTaintStep(DataFlow::Node node1, DataFlow::Node node2) {
     exists(UrlConstructorCall u |
@@ -50,6 +50,13 @@ class HTTPStringToURLOpenMethodFlowConfig extends TaintTracking::Configuration {
   }
 }
 
+/**
+ * A sink that represents a URL opening method call, such as a call to `java.net.URL.openConnection()`.
+ */
+private class URLOpenSink extends DataFlow::Node {
+  URLOpenSink() { sinkNode(this, "open-url") }
+}
+
 from DataFlow::PathNode source, DataFlow::PathNode sink, MethodAccess m, HTTPString s
 where
   source.getNode().asExpr() = s and