OpenRedirect: treat assignments to Url.Path as a barrier

ql/src/semmle/go/security/OpenUrlRedirectCustomizations.qll

@@ -50,6 +50,41 @@ module OpenUrlRedirect {
     }
   }
 
+  /**
+   * Holds if `a` and `b` read the same variable, field, or method.
+   */
+  predicate readsSameEntity(Read a, Read b) {
+    exists(DataFlow::Node aBase, DataFlow::Node bBase | readsSameEntity(aBase, bBase) |
+      exists(Field f | a.readsField(aBase, f) | b.readsField(bBase, f))
+      or
+      exists(Method m | a.readsMethod(aBase, m) | b.readsMethod(bBase, m))
+    )
+  }
+
+  /**
+   * An access to a variable that is preceded by an assignment to its `Path` field.
+   *
+   * This is overapproximate; this will currently remove flow through all `Url.Path` assignments
+   * which contain a substring that could sanitize data.
+   */
+  class PathAssignmentBarrier extends Barrier, Read {
+    PathAssignmentBarrier() {
+      exists(Write w, Field f, Read writeBase, ValueEntity v |
+        f.getName() = "Path" and
+        hasHostnameSanitizingSubstring(w.getRhs()) and
+        readsSameEntity(this, writeBase)
+      |
+        w.writesField(writeBase, f, _) and
+        w.getBasicBlock().(ReachableBasicBlock).dominates(this.asInstruction().getBasicBlock()) and
+        (
+          not w.getBasicBlock() = this.asInstruction().getBasicBlock()
+          or
+          w.getASuccessor+() = this.asInstruction()
+        )
+      )
+    }
+  }
+
   /**
    * A call to a function called `isLocalUrl` or similar, which is
    * considered a barrier for purposes of URL redirection.

ql/src/semmle/go/security/UrlConcatenation.qll

@@ -55,7 +55,7 @@ predicate sanitizingPrefixEdge(DataFlow::Node source, DataFlow::Node sink) {
  * In the latter two cases, the additional check is necessary to avoid a `/` that could be interpreted as
  * the `//` separating the (optional) scheme from the hostname.
  */
-private predicate hasHostnameSanitizingSubstring(DataFlow::Node nd) {
+predicate hasHostnameSanitizingSubstring(DataFlow::Node nd) {
   nd.getStringValue().regexpMatch(".*([?#]|[^?#:/\\\\][/\\\\]).*|[/\\\\][^/\\\\].*")
   or
   hasHostnameSanitizingSubstring(StringConcatenation::getAnOperand(nd))