Ajax: Mitigate possible XSS vulnerability

Proposed by @jaubourg

Cherry-picked from b078a62013
Fixes gh-2432
Closes gh-2588

src/ajax/script.js

@@ -4,6 +4,13 @@ define( [
 	"../ajax"
 ], function( jQuery, document ) {
 
+// Prevent auto-execution of scripts when no explicit dataType was provided (See gh-2432)
+jQuery.ajaxPrefilter( function( s ) {
+    if ( s.crossDomain ) {
+        s.contents.script = false;
+    }
+} );
+
 // Install script dataType
 jQuery.ajaxSetup( {
 	accepts: {

test/unit/ajax.js

@@ -85,6 +85,54 @@ QUnit.module( "ajax", {
 		};
 	} );
 
+	ajaxTest( "jQuery.ajax() - do not execute js (crossOrigin)", 2, function( assert ) {
+		return {
+			create: function( options ) {
+				options.crossDomain = true;
+				return jQuery.ajax( url( "data/script.php?header=ecma" ), options );
+			},
+			success: function() {
+				assert.ok( true, "success" );
+			},
+			complete: function() {
+				assert.ok( true, "complete" );
+			}
+		};
+	} );
+
+	ajaxTest( "jQuery.ajax() - execute js for crossOrigin when dataType option is provided", 3,
+		function( assert ) {
+			return {
+				create: function( options ) {
+					options.crossDomain = true;
+					options.dataType = "script";
+					return jQuery.ajax( url( "data/script.php?header=ecma" ), options );
+				},
+				success: function() {
+					assert.ok( true, "success" );
+				},
+				complete: function() {
+					assert.ok( true, "complete" );
+				}
+			};
+		}
+	);
+
+	ajaxTest( "jQuery.ajax() - do not execute js (crossOrigin)", 2, function( assert ) {
+		return {
+			create: function( options ) {
+				options.crossDomain = true;
+				return jQuery.ajax( url( "data/script.php" ), options );
+			},
+			success: function() {
+				assert.ok( true, "success" );
+			},
+			complete: function() {
+				assert.ok( true, "complete" );
+			}
+		};
+	} );
+
 	ajaxTest( "jQuery.ajax() - success callbacks (late binding)", 8, function( assert ) {
 		return {
 			setup: addGlobalEvents( "ajaxStart ajaxStop ajaxSend ajaxComplete ajaxSuccess", assert ),