TC39 Module Keys (Stage 1 Proposal)

Lets project teams trust code they know with more than code they don't.

This proposal adds per-module APIs visible within a ModuleBody that enable secure communication channels between modules to enable a large application to grant different degrees of trust to different modules.

Quick link to code.

Slides for May TC39 meeting

Implementation and polyfilling babel plugin on NPM

It is based on a proof of concept in Node.js require()d modules. It is largely a rewrite of the Node frenemies design into an ES6 modules context.


Large EcmaScript projects are collaborations between different authors:

  • First-party authors who understand the project's goals and security needs.
  • Direct dependency authors -- those that have been explicitly imported by first-party authors and are familiar with the security needs of the kinds of projects that need them.
  • Deep dependencies authored by people trying to solve a general problem with little insight into any particular project's specific security needs. A dependency is "deep" when you have to go through multiple layers of imports to find out why it is loaded.


Allow a project's authors to grant more authority to modules they are familiar with while still granting some authority to deep dependencies.

We want to make sure the path of least resistance for a deep dependency is to degrade gracefully when granted less authority instead of working around security measures to provide a higher quality of service but at a higher level of risk.


It is not a goal to allow one to safely load malicious third-party code. Even code written in good faith can put a project at risk if parts are written carelessly or if its authors define "secure" differently from the first-party authors.

We assume that all module authors are acting in good faith but that first-party and third-party authors still end up working at cross purposes and that first-party authors are better placed to represent end-users' interests.

API Sketch

We use the name frenemies below. This is a terrible name. If this advances to stage 3, we expect to bikeshed a better name.

Per ModuleBody, we make available a frozen API:

  • frenemies.publicKey(): a function that returns true when there is a private key on the call stack and the shallowest private key on the call stack corresponds to this key, or false otherwise.
  • frenemies.privateKey(f): a function that calls f and returns its result. See call stack relationship for .publicKey.
  •, mayOpen): returns a unique Box instance.
  • frenemies.unbox(box, ifFrom, fallback): awaits box. Let value and mayOpen be the arguments provided when box was created and boxer be the Frenemies instance used to create the box. The call to unbox returns value if mayOpen(frenemies.publicKey) is true when called in the context of frenemies.privateKey, and ifFrom(boxer.publicKey) is true when called in the context of boxer.privateKey. Otherwise, returns fallback.

Before execution of the module body starts, we export frenemies.publicKey as publicKey. Like default exports publicKey does not participate in re-exporting; it is included in * for the purposes of export * from ....

Per realm, we make available:

  • a makeFrenemies function that produces a unique frenemies instance. This would help source code rewriters preserve module semantics. See below.
  • a class Box { toString() { return '[Box]' } }.
  • an isPublicKey(x) function that returns true only for public keys produced by makeFrenemies. This makes it easier to write solid mayOpen and ifFrom predicates.

Use Case Summary

These use cases are discussed in more detail later when we relate them to the propsed solution.

Most of the use cases revolve around treating modules known to align with the project's security goals differently from the mass of deep dependencies whose behavior we want to bound.

  • Server-side, use JS engine hooks to restrict use of new Function to intentional uses in carefully reviewed modules.
  • Prevent or warn on powerful APIs (those that send messages, invoke shells) by unwhitelisted modules.
  • Restrict use of unsafe or error-prone APIs that require specific expertise and care to use correctly. Error messages could steer users at wrapping APIs that are suitable for general use.

This can also enable safe-by-construction guarantees:

  • Allow some modules to mint values that pass a verifier so that any module can verify they are safe-by-construction regardless of what other modules they pass through. This is especially useful for contract values that encapsulate a security property. E.g. trusted types benefit from being able to limit the ability to create values that pass a verifier to those that have been reviewed by domain experts.

Opaque values make it easier to keep sensitive data private:

  • Allow a layer that extracts sensitive inputs to ferry them to their destination without having to worry about intermediate modules. For example, code for a reset password or credit card processing form might want to ensure that neither shows up in log. Being able to wrap a value lets us guarantee that the sensitive input is not going to show up in logs or error traces.
  • Similarly for PII like physical locations from client-side Geo APIs.

Solution: Node module loader acts as trusted intermediary to enable mutual suspicion

We can realize these use cases if we have a way for one module to open a reliable channel to another.

Reliable Channel Example

Two modules alice.js and bob.js wish to collaborate in a tightly coupled manner (be less suspicious of requests from one another) while checking inputs from other modules more carefully. They may wish to use carol.js to carry messages, but do not wish to grant read access to carol.js.

One way to allow this would be for Alice to create a box containing the value she wishes to communicate that only Bob can open (confidentiality), and provide Bob a way to know that the box came from Alice (authenticity).

We could do this via asymmetric crypto if all we wanted to pass were strings and Alice and Bob had a way to exchange keys, but JavaScript objects that close over local variables do not serialize well.

The random oracle model explains cryptographic primitives in terms of a stable mapping from inputs to unpredictable strings. JavaScript does not allow forging object references (e.g. casting ints to pointers) or extracting object references from a closure's local scope. These two properties, the uniqueness and privacy of new Objects, let us get the benefits of crypto-style operators for JavaScript objects without serializing by replacing that model with a distinct object reference oracle model.

In frenemies.js we implement a pure-JavaScript analogue of secure channels that can convey arbitrary values providing Confidentiality and Authenticity and building on top of a JavaScript analogue of private/public key pairs.

It does not provide Integrity or Non-Repudiation. If the boxed value is available via other means, it might be modified between being boxed and being unboxed. For this reason, although a boxer can't repudiate the identity of the boxed value, they could repudiate the value (or absence) of any property reachable via properties that were not frozen/sealed when the object was boxed. To get non-repudiation, we would need to provide an unbox operator that additionally checks that the object was deeply-frozen and sealed before boxing completed.

Availability doesn't have a clear meaning in this context. There is no guarantee that Carol, in the example above, will deliver Alice's box to Bob; an out of memory error, stack overflow, or OS interrupt could prevent any step in the process.

A private key is a per-module function that takes a function. It calls its argument in such a way that a call to the corresponding public key will return its first argument instead of its second.

A public key is a per-module function that returns its first argument (defaults to true) when called in the context of the corresponding private key, or its second argument (default to false) otherwise.

Example Code

// alice.js
'use strict';
// Alice sends a message to Bob.

import * as bob from './bob.js';
import * as carol from './carol.js';

const mayOpen = (opener) => opener === bob.publicKey && opener();

export function send () {
  const messageForBob =
    'Have a nice day, Bob! Sincerely, Alice',
    mayOpen);'Alice is sending');
  carol.convey(bob, messageForBob);
// bob.js
'use strict';
// Bob gets a message from Alice and verifies that it comes from her.
import * as alice from './alice.js';

function ifFrom(sender) {
  return sender === alice.publicKey && sender();

// Carol puts messages in mailboxes.
export function mailbox(box) {
  const value = frenemies.unbox(
    box, ifFrom, 'a message of questionable provenance!');
  console.log(`Bob read: ${value}`);
// carol.js
'use strict';
// Carol passes messages between Alice and Bob.
// Maybe she's a message bus.  Who knows?!

// Maybe Carol is evil!  Maybe not!  Again, who knows?!
const evil = Math.random() >= 0.5;

export function convey(recipient, message) {
  if (evil) {
    console.log('Carol got ' + message);  // OPAQUE.  No leak.
    // No leak.  Denied by since alice.mayOpen gets called
    // in the context of Alice's private key, not Bob's.
    console.log('Carol unboxed ' + frenemies.unbox(message, (x) => true, 'Fallback value'));
  // Carol delivers Bob's mail.  She may be evil, but she's not a monster!
  if (evil) {
      // Bob will not open it since his ifFrom predicate expects
      // Alice's public key, not Carol's.'Have an evil day! Sincerely, Alice', (x) => true));

Use Case Solution Sketches

Here we sketch out how we address the use cases above.

Contract Values

Project members may trust some modules to produce HTML:

  • widely-used-html-sanitizer.js which filters HTML tags and attributes
  • autoescaping-template-engine.js which plugs untrusted values into a trusted HTML template

based on their extensive experience with these modules, but not others:

  • mysterious-markdown-converter.js which shows up in the project's dependencies for reasons no one has investigated.

Luckily all these modules produce TrustedHtml values which takes a box containing the string of HTML.

When it's time to flush a chunk of HTML to an HTTP response buffer, the request handler unboxes the TrustedHTML content if it comes from one of the first two. If it comes from another source it treats it with more suspicion, perhaps passing it back to the sanitizer which unboxes it without regards to its source and whitelists tags and attributes.

The template engine is also a consumer -- when rendering it gets the project policy to decide when to inline a chunk of trusted HTML without escaping.

Other project teams might trust widely-used-html-sanitizer.js with their own policy, but don't trust third-party code to craft their own policies, so only whitelist a wrapped version of widely-used-html-sanitizer.js.

This proposal provides publicKeys per module which provides a basis for defining whitelists, and a mechanism for consumers of values to check whitelists.

Boxes are tamper-proof in transit, so don't require reorganizing the way data flows through a system as long as the intermediate layers don't insist on coercing values to strings.

Library code can define common key predicates:

 * A public key predicate that checks whether a key
 * is allowed.
export function makeWhitelist(...allowed) {
  const idSet = new Set(allowed)
  return (publicKey) => (
      frenemies.isPublicKey(publicKey) &&
      publicKey() &&
      // TODO: avoid depending on Set.prototype

and a configuration could create whitelists:

import {publicKey as fooKey} from 'foo';
import {publicKey as barKey} from 'bar';
import {makeWhitelist} from '/whitelists.js';

const myWhitelist = Object.freeze(makeWhitelist([fooKey, barKey]);

Opaque Values

We may trust our framework code to route inputs to the right place when everything is ok, but there's too much code that looks like

function execute(...args) {
  try {
    // ...
  } catch (exc) {
    log('Execution failed given %o: %o', args, exc);
    // ...

dataBundle may contain sensitive information:

  • Real names
  • Unsalted passwords
  • Geo locations
  • Credit cards

We need to encourage developers to build systems that they can debug in the field, but we still need to keep sensitive data out of logs that might not be as resistant to attack as our key stores.

Reliably opaque values can help us balance these two needs.

As soon as request handler extracts sensitive data, it can box it and specify which modules can open it using the same kind of module whitelist as above.

Opaque values don't require mutual suspicions, so reasonably reliable opaque values could be had by other means.

Reifying Permissions

What about eval explains that quirks of JavaScript make it easier to unintentionally convert a string to code than in other languages, and Dynamically bounding eval proposes a way to allow some uses of eval without the blanket ban implied by --disallow_code_generation_from_strings.

We can represent permissions as boxes, and the permission checker can unbox it with an ifFrom parameter that only accepts the permission granter.

To request permissions, the requestor would create a box and give it to the grantor which could use the whitelist scheme above.

These permissions would be delegatable with the usual caveats.

Permissions can also be revocable.

// Granter
const whitelist = (see ^ above);
function mayI(request) {
  let permission () => false
  let revoke = () => {}
  if (frenemies.unbox(request, whitelist, false)) {
    let youMay = true;
    permission = () => youMay
    revoke = () => { youMay = false }
  return {
    permission:, () => true),

// Requester
import * as granter from '/granter.js';
const { permission, revoke } = granter.mayI(, () => true));
// Do something that invokes checker with permission

// Checker
import * as granter from '/granter.js';
function check(permission) {
  if (frenemies.unbox(
          permission, (k) => k === granter.publicKey && k(), () => false)()) {
    // Grant
  } else {
    // Deny

Access Restrictions

Some modules are inherently sensitive. For example, in a Node.js context:

  • fs -- file system access
  • child_process -- shell access
  • net -- network sends
  • user-libraries that wrap the same

Client-side, future sensitive APIs might be exposed as builtin modules.

These are incredibly powerful, and most large projects use one or all of these to great effect.

They or their counterparts in other languages are also involved in many large-scale exploits.

Most third-party modules do not require all of these.

It would be nice for a project team to be able to bound which code should use those, review that code, and then enforce that going forward. Then if a new dependency needs access, it should fail loudly so that they can incrementally review the new use.

It would also be nice if tricky use of reflective operators, like that which exploits the Function constructor for which we advocated permissions, failed safely.

We have shown above that this proposal provides a basis for whitelists of modules which lets us define the boundary of what has been reviewed.

We could enforce this with extra syntax if we could rely on sensitive modules to opt-in export if (importerPublicKey => ...);.

[Resolve hooks][resolve hooks] would allow vetoing arbitrary imports based on a check that takes into account the "parent" and "child" modules as identified by their keys.

Alternate approaches

Here are alternate approaches that have been suggested to granting different authority to different code.

We're all adults here.

Most JavaScript projects takes a "we're all adults here" attitude to development. None of is going to do something stupid like undefined = 1337, so we can all get along without worrying about corner cases that only occur when undefined === 1337.

This argument is raised in two different ways:

  • We're all adults so we can provide third-party library authors with all the authority they could need and trust them to manage risk on their clients' behalf.
  • We're all adults so, if we do need to implement a security policy in code, we can use half-measures since adults don't try to work around security policies.

This attitude is great in moderation, but we're not all adults -- we're a large collection of adults who can't possibly all talk to each other because that's an O(n²) problem.

Large groups don't have the common context that "we're all adults here" implies.

There are several kinds of context that affect the security of an end product.

  • Deep dependency authors don't understand the specific security needs of that end product.
  • Deep dependency authors are often domain experts in the problem their library solves, but often are not experts in how an attacker can turn powerful features they may use can be turned against the end product.
  • End product developers do not understand how the deep dependency author solves the problem.

If a third-party developer has a choice between using a powerful feature to definitely solve a problem for one client who is requesting it and maybe keeping risk low for other clients who are not present requesting they don't, they will likely do so. Ones who consistently don't will not gain users.

It is unreasonable to expect third-party developers to approximate POLA for a specific end product.

We need to enable a small group of security specialists to guarantee security properties while third-party developers focus on bringing their strengths to bear.

Re whether we can trust adults not to work around half-measures, in goals above we wanted to

make sure the path of least resistance for a deep dependency is to degrade gracefully

A library author wants to provide a high level of service. If they can by peeking inside a box and don't clearly see how this increases the risk to an end product, then they are likely to peek. If they can't peek inside, the path of least resistance is to degrade gracefully.

We're all adults here; sometimes adults with deadlines and not enough coffee. Strong fences make good neighbours, let security specialists manage product's security, and guide third-party developers towards graceful degradation and away from hacks that work around policies.

Unit testing

Why not write unit test that your code doesn't do things with untrusted inputs that it shouldn't?

Unit test suites can give confidence that a system does what it should, but do a poor job checking that it doesn't do what it shouldn't. Mechanisms that limit or contain the damage that code can do when exposed to malicious inputs can help large development teams maintain velocity while giving confidence in the latter.

Turn off unneeded functionality

This is mostly raised in the context of Node.js but has also been raised in terms of Feature-Policy.

We talk about powerful modules like fs and sh and powerful operators like eval and note that few modules need one of these but that most projects do. If this were not the case, then shell injection and leaks of sensitive files would not be as frequent as they are.

Project teams benefit from having them when they need them if we can limit the downside.

Examine all third-party dependencies

Some argue that developers should not depend on any code that they haven't read, or that they wouldn't be happy writing.

Developers do use code that they haven't read.

We don't advocate reading all your dependencies because that sounds super boring and would probably become your full-time job.

Write your own instead of using third-party code

This may be a fine approach for something project-critical where you have domain expertise on the project team, but doesn't scale.

If a large project tried this, they would have to become large enough that internally and would have enough pressure to provide reusable components, they would recreate the first-party/third-party disconnect.

Load modules in separate contexts

Some have proposed loading each module in its own realm, with its own version of globals.

It may be a fine idea, but probably breaks modules that assume Object.prototype and other builtins are identical across modules.

It also does not, by itself, address these use cases, though might, if there were a way to prevent some modules from require-ing specific others.

It could be complementary with this proposal.

Implications for code rewriters.

Code rewriters that merge or bundle modules, will have to change to accommodate this proposal by manufacturing a public/private key pair for modules that mention frenemies.

Even if frenemies is not mentioned, rewriters also adds an implicit export of a public key. If a module never mentions frenemies, then it never uses its private key, so attaching a well-known public key that whose corresponding private key has been discarded should suffice. This well-known public key would be equivalent to (a, b = false) => b but would pass the isPublicKey predicate. Code rewriters that eliminate dead code should be able to eliminate this vestige in many cases.

Module loader hooks

We assume module loader hooks are trusted so do not defend against module hooks that collect private keys. Any "security considerations" section in published guidelines for loader hook authors should mention that loader hook authors are responsible for not leaking private keys.

Failure modes

Implicit in "grant ... to module" is the idea that a module is a principal. This brings along the usual risks when authorizing based on principals:

Impersonation - one module masquerading as another. For example, the vm module uses the filename option to load code in a way that makes it obvious in stack traces where code originated. Modules that use stack introspection (e.g. node-sec-patterns) to authenticate a caller may mistakenly authorize code run in a context created with { filename: './high-authority-module.js' }.

Another way a module might try to prove its identity to a sensitive function that it calls is to pass in a value that only it knows. This is susceptible to replay attacks -- once passed, the receiver knows the value, and can save the value so that it can later use the privileges of the caller.

The attached code should not be susceptible to impersonation as long as a module does not leak its privateKey, box, or unbox functions.

If a module identity were based on a string name, then loading the same module in a separate realm (e.g. via <iframe>) might allow bypasses. All mutable state in this proposal is per realm, so this proposal should not suffer this vector.

Attacking the policy - For server-side code, if we store grant decisions in a configuration file like package.json, a lower privileged module could (temporarily or not) edit that file to grants itself more privileges.

Attack of the clones - For server-side code, a lower privileged module could prepend script it wishes to run to the main file of a more highly privileged module.

We do not attempt to solve these last two problems. Existing techniques like denying the node runtime write access to source and configuration files and doing resource integrity checks on load should suffice.

Attacking objects - For server-side code, Node.js does not run JS. C++ addons may be able to violate the assumptions that underlie our random oracle replacement. Programmatic access to out-of-band debuggers (1 2), and deserialize APIs have also allowed object forgery in similar systems.

We do not attempt to solve these problems either. "If you can't trust native code who can you trust" is not an ideal security posture, but project teams should already be careful about which C++ addons they load in production, and a feature like this might allow bounding access to out-of-band APIs (like debug hooks and deserialization APIs) which would be a better security situation than having no such bounds. (Debug APIs should probably be turned off in production.)