How can I securely execute some user supplied JS code using Java8 Nashorn?
The script extends some computations for some servlet based reports. The app has many different (untrusted) users. The scripts should only be able to access a Java Object and those returned by the defined members. By default the scripts could instantiate any class using Class.forName() (using .getClass() of my supplied object). Is there any way to prohibit access to any java class not explicitly specified by me?
I asked this question on the Nashorn mailing list a while back:
Are there any recommendations for the best way to restrict the classes that Nashorn scripts can create to a whitelist? Or is the approach the same as any JSR223 engine (custom classloader on the ScriptEngineManager constructor)?
And got this answer from one of the Nashorn devs:
Hi,
Nashorn already filters classes - only public classes of non-sensitive packages (packages listed in package.access security property aka 'sensitive'). Package access check is done from a no-permissions context. i.e., whatever package that can be accessed from a no-permissions class are only allowed.
Nashorn filters Java reflective and jsr292 access - unless script has RuntimePermission("nashorn.JavaReflection"), the script wont be able to do reflection.
The above two require running with SecurityManager enabled. Under no security manager, the above filtering won't apply.
You could remove global Java.type function and Packages object (+ com,edu,java,javafx,javax,org,JavaImporter) in global scope and/or replace those with whatever filtering functions that you implement. Because, these are the only entry points to Java access from script, customizing these functions => filtering Java access from scripts.
There is an undocumented option (right now used only to run test262 tests) "--no-java" of nashorn shell that does the above for you. i.e., Nashorn won't initialize Java hooks in global scope.
JSR223 does not provide any standards based hook to pass a custom class loader. This may have to be addressed in a (possible) future update of jsr223.
Hope this helps,
-Sundar
Added in 1.8u40, you can use the ClassFilter to restrict what classes the engine can use.
Here is an example from the Oracle documentation:
import javax.script.ScriptEngine; import jdk.nashorn.api.scripting.ClassFilter; import jdk.nashorn.api.scripting.NashornScriptEngineFactory; public class MyClassFilterTest { class MyCF implements ClassFilter { @Override public boolean exposeToScripts(String s) { if (s.compareTo("java.io.File") == 0) return false; return true; } } public void testClassFilter() { final String script = "print(java.lang.System.getProperty(\"java.home\"));" + "print(\"Create file variable\");" + "var File = Java.type(\"java.io.File\");"; NashornScriptEngineFactory factory = new NashornScriptEngineFactory(); ScriptEngine engine = factory.getScriptEngine( new MyClassFilterTest.MyCF()); try { engine.eval(script); } catch (Exception e) { System.out.println("Exception caught: " + e.toString()); } } public static void main(String[] args) { MyClassFilterTest myApp = new MyClassFilterTest(); myApp.testClassFilter(); } }This example prints the following:
C:\Java\jre8 Create file variable Exception caught: java.lang.RuntimeException: java.lang.ClassNotFoundException: java.io.File
I've researched ways of allowing users to write a simple script in a sandbox that is allowed access to some basic objects provided by my application (in the same way Google Apps Script works). My conclusion was that this is easier/better documented with Rhino than with Nashorn. You can:
Define a class-shutter to avoid access to other classes: http://codeutopia.net/blog/2009/01/02/sandboxing-rhino-in-java/
Limit the number of instructions to avoid endess-loops with observeInstructionCount: http://www-archive.mozilla.org/rhino/apidocs/org/mozilla/javascript/ContextFactory.html
However be warned that with untrusted users this is not enough, because they can still (by accident or on purpose) allocate a hugh amount of memory, causing your JVM to throw an OutOfMemoryError. I have not found a safe solution to this last point yet.
You can quite easily create a ClassFilter which allows fine-grained control of which Java classes are available in JavaScript.
Following the example from the Oracle Nashorn Docs:
class MyCF implements ClassFilter {
@Override
public boolean exposeToScripts(String s) {
if (s.compareTo("java.io.File") == 0) return false;
return true;
}
}
I have wrapped this an a few other measures in a small library today: Nashorn Sandbox (on GitHub). Enjoy!
So far as I can tell, you can't sandbox Nashorn. An untrusted user can execute the "Additional Nashorn Built-In Functions" listed here:
https://docs.oracle.com/javase/8/docs/technotes/guides/scripting/nashorn/shell.html
which include "quit()". I tested it; it exits the JVM entirely.
(As an aside, in my setup the global objects, $ENV, $ARG, did not work, which is good.)
If I'm wrong about this, someone please leave a comment.
The best way to secure a JS execution in Nashorn is to enable the SecurityManager and let Nashorn deny the critical operations. In addition you can create a monitoring class that check the script execution time and memory in order to avoid infinite loops and outOfMemory. In case you run it in a restricted environment without possibility to setup the SecurityManager, you can think to use the Nashorn ClassFilter to deny all/partial access to the Java classes. In addition to that you must overwrite all the critical JS functions (like quit() etc.). Have a look at this function that manage all this aspects (except memory management):
public static Object javascriptSafeEval(HashMap<String, Object> parameters, String algorithm, boolean enableSecurityManager, boolean disableCriticalJSFunctions, boolean disableLoadJSFunctions, boolean defaultDenyJavaClasses, List<String> javaClassesExceptionList, int maxAllowedExecTimeInSeconds) throws Exception {
System.setProperty("java.net.useSystemProxies", "true");
Policy originalPolicy = null;
if(enableSecurityManager) {
ProtectionDomain currentProtectionDomain = this.getClass().getProtectionDomain();
originalPolicy = Policy.getPolicy();
final Policy orinalPolicyFinal = originalPolicy;
Policy.setPolicy(new Policy() {
@Override
public boolean implies(ProtectionDomain domain, Permission permission) {
if(domain.equals(currentProtectionDomain))
return true;
return orinalPolicyFinal.implies(domain, permission);
}
});
}
try {
SecurityManager originalSecurityManager = null;
if(enableSecurityManager) {
originalSecurityManager = System.getSecurityManager();
System.setSecurityManager(new SecurityManager() {
//allow only the opening of a socket connection (required by the JS function load())
@Override
public void checkConnect(String host, int port, Object context) {}
@Override
public void checkConnect(String host, int port) {}
});
}
try {
ScriptEngine engineReflex = null;
try{
Class<?> nashornScriptEngineFactoryClass = Class.forName("jdk.nashorn.api.scripting.NashornScriptEngineFactory");
Class<?> classFilterClass = Class.forName("jdk.nashorn.api.scripting.ClassFilter");
engineReflex = (ScriptEngine)nashornScriptEngineFactoryClass.getDeclaredMethod("getScriptEngine", new Class[]{Class.forName("jdk.nashorn.api.scripting.ClassFilter")}).invoke(nashornScriptEngineFactoryClass.newInstance(), Proxy.newProxyInstance(classFilterClass.getClassLoader(), new Class[]{classFilterClass}, new InvocationHandler() {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
if(method.getName().equals("exposeToScripts")) {
if(javaClassesExceptionList != null && javaClassesExceptionList.contains(args[0]))
return defaultDenyJavaClasses;
return !defaultDenyJavaClasses;
}
throw new RuntimeException("no method found");
}
}));
/*
engine = new jdk.nashorn.api.scripting.NashornScriptEngineFactory().getScriptEngine(new jdk.nashorn.api.scripting.ClassFilter() {
@Override
public boolean exposeToScripts(String arg0) {
...
}
});
*/
}catch(Exception ex) {
throw new Exception("Impossible to initialize the Nashorn Engine: " + ex.getMessage());
}
final ScriptEngine engine = engineReflex;
if(parameters != null)
for(Entry<String, Object> entry : parameters.entrySet())
engine.put(entry.getKey(), entry.getValue());
if(disableCriticalJSFunctions)
engine.eval("quit=function(){throw 'quit() not allowed';};exit=function(){throw 'exit() not allowed';};print=function(){throw 'print() not allowed';};echo=function(){throw 'echo() not allowed';};readFully=function(){throw 'readFully() not allowed';};readLine=function(){throw 'readLine() not allowed';};$ARG=null;$ENV=null;$EXEC=null;$OPTIONS=null;$OUT=null;$ERR=null;$EXIT=null;");
if(disableLoadJSFunctions)
engine.eval("load=function(){throw 'load() not allowed';};loadWithNewGlobal=function(){throw 'loadWithNewGlobal() not allowed';};");
//nashorn-polyfill.js
engine.eval("var global=this;var window=this;var process={env:{}};var console={};console.debug=print;console.log=print;console.warn=print;console.error=print;");
class ScriptMonitor{
public Object scriptResult = null;
private boolean stop = false;
Object lock = new Object();
@SuppressWarnings("deprecation")
public void startAndWait(Thread threadToMonitor, int secondsToWait) {
threadToMonitor.start();
synchronized (lock) {
if(!stop) {
try {
if(secondsToWait<1)
lock.wait();
else
lock.wait(1000*secondsToWait);
} catch (InterruptedException e) {
throw new RuntimeException(e);
}
}
}
if(!stop) {
threadToMonitor.interrupt();
threadToMonitor.stop();
throw new RuntimeException("Javascript forced to termination: Execution time bigger then " + secondsToWait + " seconds");
}
}
public void stop() {
synchronized (lock) {
stop = true;
lock.notifyAll();
}
}
}
final ScriptMonitor scriptMonitor = new ScriptMonitor();
scriptMonitor.startAndWait(new Thread(new Runnable() {
@Override
public void run() {
try {
scriptMonitor.scriptResult = engine.eval(algorithm);
} catch (ScriptException e) {
throw new RuntimeException(e);
} finally {
scriptMonitor.stop();
}
}
}), maxAllowedExecTimeInSeconds);
Object ret = scriptMonitor.scriptResult;
return ret;
} finally {
if(enableSecurityManager)
System.setSecurityManager(originalSecurityManager);
}
} finally {
if(enableSecurityManager)
Policy.setPolicy(originalPolicy);
}
}
The function currently use the deprecated Thread stop(). An improvement can be execute the JS not in a Thread but in a separate Process.
PS: here Nashorn is loaded through reflexion but the equivalent Java code is also provided in the comments
I'd say overriding the supplied class's classloader is easiest way to control access to classes.
(Disclaimer: I'm not really familiar with newer Java, so this answer may be old-school/obsolete)
An external sandbox library can be used if you don't want to implement your own ClassLoader & SecurityManager (that's the only way of sandboxing for now).
I've tried "The Java Sandbox" (http://blog.datenwerke.net/p/the-java-sandbox.html) although it's a bit rough around the edges, but it works.
Without the use of Security Manager it is not possible to securely execute JavaScript on Nashorn.
In all releases of Oracle Hotspot that included Nashorn one can write JavaScript that will execute any Java/JavaScript code on this JVM. As of January 2019, Oracle Security Team insist that use of Security Manager is mandatory.
One of the problems is already discussed in https://github.com/javadelight/delight-nashorn-sandbox/issues/73
