Go: Add html/template as XSS queries sanitizer

go/ql/lib/semmle/go/security/Xss.qll

@@ -127,4 +127,20 @@ module SharedXss {
       )
     }
   }
+
+  /**
+   * A `Template` from `html/template` will HTML-escape data automatically
+   * and therefore acts as a sanitizer for XSS vulnerabilities.
+   */
+  class HtmlTemplateSanitizer extends Sanitizer, DataFlow::Node {
+    HtmlTemplateSanitizer() {
+      exists(Method m, DataFlow::CallNode call | m = call.getCall().getTarget() |
+        m.hasQualifiedName("html/template", "Template", "ExecuteTemplate") and
+        call.getArgument(2) = this
+        or
+        m.hasQualifiedName("html/template", "Template", "Execute") and
+        call.getArgument(1) = this
+      )
+    }
+  }
 }

go/ql/test/query-tests/Security/CWE-079/ReflectedXssGood.go

@@ -3,16 +3,22 @@ package main
 import (
 	"fmt"
 	"html"
+	"html/template"
 	"net/http"
 )
 
 func serve1() {
+	var template template.Template
+
 	http.HandleFunc("/user", func(w http.ResponseWriter, r *http.Request) {
 		r.ParseForm()
 		username := r.Form.Get("username")
 		if !isValidUsername(username) {
 			// GOOD: a request parameter is escaped before being put into the response
 			fmt.Fprintf(w, "%q is an unknown user", html.EscapeString(username))
+			// GOOD: using html/template escapes values for us
+			template.Execute(w, username)
+			template.ExecuteTemplate(w, "test", username)
 		} else {
 			// TODO: do something exciting
 		}

go/ql/test/query-tests/Security/CWE-079/StoredXssGood.go

@@ -2,15 +2,18 @@ package main
 
 import (
 	"html"
+	"html/template"
 	"io"
 	"io/ioutil"
 	"net/http"
 )
 
 func ListFiles1(w http.ResponseWriter, r *http.Request) {
+	var template template.Template
 	files, _ := ioutil.ReadDir(".")
 
 	for _, file := range files {
 		io.WriteString(w, html.EscapeString(file.Name())+"\n")
+		template.Execute(w, file.Name())
 	}
 }