Secure ECMAScript

Usage no npm install needed!

<script type="module">
  import ses from '';



SES is hardened JavaScript. SES stands for fearless cooperation. This package is a SES [shim][define shim] for JavaScript features [proposed][SES proposal] to ECMA TC-39. Hardened JavaScript is highly compatible with ordinary JavaScript. Most existing JavaScript libraries can run on hardened JavaScript.

  • Compartments Compartments are separate execution contexts: each one has its own global object and global lexical scope.
  • Frozen realm Compartments share their intrinsics to avoid identity discontinuity. By freezing the intrinsics, SES protects programs from each other. By sharing the intrinsics, programs from separate compartments can recognize each other's arrays, data objects, and so on.
  • Strict mode SES enforces JavaScript strict mode that enhances security, for example by changing some silent errors into throw errors.
  • POLA (Principle of Least Authority) By default, Compartments receive no ambient authority. They are created without host-provided APIs, (for example no fetch). Compartments can be selectively endowed with powerful arguments, globals, or modules.

SES safely executes third-party JavaScript 'strict' mode programs in compartments that have no excess authority in their global scope. SES runs atop an ES6-compliant platform, enabling safe interaction of mutually-suspicious code, using object-capability -style programming.

See to see how SES fits into the various flavors of confined JavaScript execution. And visit for a demo.

Derived from the Caja project,

Still under development: do not use for production systems yet, there are known security holes that need to be closed.


npm install ses


The SES shim runs in most engines, either as an ESM module ses or as a <script> tag. For a script tag, the content encoding charset must be UTF-8, either by virtue of <head><meta charset="utf-8"></head> (a general best practice for all HTML files) or specifically <script src="node_modules/ses/dist/ses.umd.min.js" charset="utf-8">.

SES can be bundled by Webpack, Browseriy, Rollup, and Parcel, but any of these tools could be coopted with a supply-chain attack to invalidate the security properties of SES. We generally recommend installing SES as a separate script tag.


SES introduces the lockdown() function. Calling lockdown() alters the surrounding execution environment, or realm, such that no two programs running in the same realm can observe or interfere with each other until they have been introduced.

To do this, lockdown() tamper-proofs all of the JavaScript intrinsics, to prevent prototype pollution. After that, no program can subvert the methods of these objects (preventing some man in the middle attacks). Also, no program can use these mutable objects to pass notes to parties that haven't been expressly introduced (preventing some covert communication channels).

Lockdown freezes all objects accessible to any program in the realm. The set of accessible objects includes but is not limited to: globalThis, [].__proto__, {}.__proto__, (() => {}).__proto__ (async () => {}).__proto__, and the properties of any accessible object.

The lockdown() function also tames some objects including regular expressions, locale methods, and errors. A tamed RegExp does not have the deprecated compile method. A tamed error does not have a V8 stack, but the console can still see the stack. Lockdown replaces locale methods like String.prototype.localeCompare with lexical versions that do not reveal the user locale.

import 'ses';
import 'my-vetted-shim';


// true

Lockdown does not erase any powerful objects from the initial global scope. Instead, Compartments give complete control over what powerful objects exist for client code.

See lockdown options for configuration options to lockdown. However, all of these have sensible defaults that should work for most projects out of the box.


SES introduces the harden function. After calling lockdown, the harden function ensures that every object in the transitive closure over property and prototype access starting with that object has been frozen by Object.freeze. This means that the object can be passed among programs and none of those programs will be able to tamper with the surface of that object graph. They can only read the surface data and call the surface functions.

import 'ses';


let counter = 0;
const capability = harden({
  inc() {

// true
// true

Note that although the surface of the capability is frozen, the capability still closes over the mutable counter. Hardening an object graph makes the surface immutable, but does not make methods pure.


SES introduces the Compartment constructor. A compartment is an evaluation and execution environment with its own globalThis and wholly independent system of modules, but otherwise shares the same batch of intrinsics like Array with the surrounding compartment. The concept of a compartment implies an initial compartment, the initial execution environment of a realm.

In the following example, we create a compartment endowed with a print() function on globalThis.

import 'ses';

const c = new Compartment({
  print: harden(console.log),

  print('Hello! Hello?');

The new compartment has a different global object than the start compartment. The global object is initially mutable. Locking down the realm hardened the objects in global scope. After lockdown, no compartment can tamper with these intrinsics and undeniable objects. Many of these are identical in the new compartment.

const c = new Compartment();
c.globalThis === globalThis; // false
c.globalThis.JSON === JSON; // true

Other pairs of compartments also share many identical intrinsics and undeniable objects of the realm. Each has a unique, initially mutable, global object.

const c1 = new Compartment();
const c2 = new Compartment();
c1.globalThis === c2.globalThis; // false
c1.globalThis.JSON === c2.globalThis.JSON; // true

The global scope of every compartment includes a shallow, specialized copy of the JavaScript intrinsics, omitting and Math.random. Comaprtments leave these out since they can be used as covert communication channels between programs. However, a compartment may be expressly given access to these objects through:

  • the first argument to the compartment constructor or
  • by assigning them to the compartment's globalThis after construction.
const powerfulCompartment = new Compartment({ Math });
powerfulCompartment.globalThis.Date = Date;

Compartment + Lockdown

Together, Compartment and lockdown isolate client code in an environment with limited powers and communication channels. A compartment has only the capabilities it is expressly given and cannot modify any of the shared intrinsics. Every compartment gets its own globals, including such objects as the Function constructor. Yet, compartment and lockdown do not break instanceof for any of these intrinsics types!

All of the evaluators in one compartment are captured by that compartment's global scope, including Function, indirect eval, dynamic import, and its own Compartment constructor for child compartments. For example, the Function constructor in one compartment creates functions that evaluate in the global scope of that compartment.

const f = new Function("return this");
f() === globalThis
// true

Lockdown prepares for compartments with separate globals by freezing their shared prototypes and replacing their prototype constructors with powerless dummies. So, Function is different in two compartments, Function.prototype is the same, and Function is not the same as Function.prototype.constructor. The Function.prototype.constructor can only throw exceptions. So, a function passed between compartments does not carry access to its compartment's globals along with it. Yet, f instanceof Function works, even when f and Function are from different compartments.

The globalThis in each compartment is mutable. This can and should be frozen before running any dynamic code in that compartment, yet is not strictly necessary if the compartment only runs code from a single party.


Any code executed within a compartment shares a set of module instances. For modules to work within a compartment, the creator must provide a resolveHook and an importHook. The resolveHook determines how the compartment will infer the full module specifier for another module from a referrer module and the import specifier. The importHook accepts a full specifier and asynchronously returns a StaticModuleRecord for that module.

import 'ses';
import { StaticModuleRecord } from '@endo/static-module-record`;

const c1 = new Compartment({}, {}, {
  name: "first compartment",
  resolveHook: (moduleSpecifier, moduleReferrer) => {
    return resolve(moduleSpecifier, moduleReferrer);
  importHook: async moduleSpecifier => {
    const moduleLocation = locate(moduleSpecifier);
    const moduleText = await retrieve(moduleLocation);
    return new StaticModuleRecord(moduleText, moduleLocation);

The SES language specifies a global StaticModuleRecord, but this is not provided by the shim because it entrains a full JavaScript parser that is an unnecessary performance penalty for the SES runtime. Instead, the SES shim accepts a compiled static module record duck-type that is tightly coupled to the shim implementation. Third party modules can provide suitable implementations and even move the compile step to build time instead of runtime.

A compartment can also link a module in another compartment. Each compartment has a module function that accepts a module specifier and returns the module exports namespace for that module. The module exports namespace is not useful for inspecting the exports of the module until that module has been imported, but it can be passed into the module map of another Compartment, creating a link.

const c2 = new Compartment({}, {
  'c1': c1.module('./main.js'),
}, {
  name: "second compartment",

importHook aliases

If a compartment imports a module specified as "./utility" but actually implemented by an alias like "./utility/index.js", the importHook may follow redirects, symbolic links, or search for candidates using its own logic and return a module that has a different "response specifier" than the original "request specifier". The importHook may return an "alias" object with record, compartment, and module properties.

  • record must be a static module record, either a third-party module record or a compiled static module record.
  • compartment is optional, to be specified if the alias transits to a different compartment, and
  • specifier is the full module specifier of the module in its compartment. This defaults to the request specifier, which is only useful if the compartment is different.

In the following example, the importHook searches for a file and returns an alias.

const importHook = async specifier => {
  const candidates = [specifier, `${specifier}.js`, `${specifier}/index.js`];
  for (const candidate of candidates) {
    const record = await wrappedImportHook(candidate).catch(_ => undefined);
    if (record !== undefined) {
      return { record, specifier };
  throw new Error(`Cannot find module ${specifier}`);

const compartment = new Compartment({}, {}, {


The module map above allows modules to be introduced to a compartment up-front. Some modules cannot be known that early. For example, in Node.js, a package might have a dependency that brings in an entire subtree of modules. Also, a pair of compartments with cyclic dependencies between modules they each contain cannot use compartment.module to link the second compartment constructed to the first. For these cases, the Compartment constructor accepts a moduleMapHook option that is like the dynamic version of the static moduleMap argument. This is a function that accepts a module specifier and returns the module namespace for that module specifier, or undefined. If the moduleMapHook returns undefined, the compartment proceeds to the importHook to attempt to asynchronously obtain the module's source.

const moduleMapHook = moduleSpecifier => {
  if (moduleSpecifier === 'even') {
    return even.module('./index.js');
  } else if (moduleSpecifier === 'odd') {
    return odd.module('./index.js');

const even = new Compartment({}, {}, {
  resolveHook: nodeResolveHook,
  importHook: makeImportHook(''),

const odd = new Compartment({}, {}, {
  resolveHook: nodeResolveHook,
  importHook: makeImportHook(''),

Third-party modules

To incorporate modules not implemented as JavaScript modules, third-parties may implement a StaticModuleRecord interface. The record must have an imports array and an execute method. The compartment will call execute with:

  1. the proxied exports namespace object,
  2. a resolvedImports object that maps import names (from imports) to their corresponding resolved specifiers (through the compartment's resolveHook), and
  3. the compartment, such that importNow can obtain any of the module's specified imports.

:warning: A future breaking version may allow the importNow and the execute method of third-party static module records to return promises, to support top-level await.

Compiled modules

Instead of the StaticModuleRecord constructor specified for the SES language, the SES shim uses compiled static module records as a stand-in. These can be created with a StaticModuleRecord constructor from a package like @endo/static-module-record. We omitted StaticModuleRecord from the SES shim because it entrains a heavy dependency on a JavaScript parser. The shim depends upon a StaticModuleRecord constructor to analyze and transform the source of a JavaScript module (known as an ESM or a .mjs file) into a JavaScript program suitable for evaluation with compartment.evaluate using a particular calling convention to initialize a module instance.

A compiled static module record has the following shape:

  • imports is a record that maps partial module specifiers to a list of names imported from the corresponding module.
  • exports is an array of all the names that the module will export.
  • reexports is an array of partial module specifier for which this module exports all imported names. This field is optional.
  • __syncModuleProgram__ is a string that evaluates to a function that accepts an initialization record and initializes the module. This property distinguishes this type of module record. The name implies a future record type that supports top-level await.
    • An initialization record has the properties imports, liveVar, and onceVar.
      • imports is a function that accepts a map from partial import module specifiers to maps from names that the corresponding module exports to notifier functions. A notifier function accepts an update function and registers to receive updates for the value exported by the other module.
      • liveVar is a record that maps names exported by this module to a function that may be called to initialize or update the corresponding value in another module.
      • onceVar is a record that maps constants exported by this module to a function that may be called to initialize the corresponding value in another module.
  • __liveExportsMap__ is a record that maps import names or names in the lexical scope of the module to export names, for variables that may change after initialization. Any reexported name is assumed to possibly change. The exported name is wrapped in a duple array like ["exportedName", true]. The second value, a boolean, indicates that the variable has a temporal dead-zone (a time between creation and initialization) when access to that name should throw a ReferenceError.
  • __fixedExportsMap__ is a record that maps import names to export names for constants exported by this module. The fixed exports map is an aesthetic subtype of the live exports map, so the value is wrapped in a simple array like ["exportedName"]


The Compartment constructor accepts a transforms option. This is an array of JavaScript source to source translation functions, in the order they should be applied. Passing the source to the first function's input, then from each function's output to the next's input, the final function's output must be a valid JavaScript "Program" grammar construction, code that is valid in a <script>, not a module.

const transforms = [addCodeCoverageInstrumentation];
const globalLexicals = { coverage };
const c = new Compartment({ console }, null, { transforms, globalLexicals });

The evaluate method of a compartment also accepts a transforms option. These apply before and in addition to the compartment-scoped transforms.

const transform = source => source.replace(/Farewell/g, 'Hello');
const transforms = [transform];
c.evaluate('console.log("Farewell, World!")', { transforms });
// Hello, World!

These transforms do not apply to modules. To transform the source of a JavaScript module, the importHook must intercept the source and transform it before passing it to the StaticModuleRecord constructor. These are distinct because programs and modules have distinct grammar productions.

An internal implementation detail of the SES-shim is that it converts modules to programs and evaluates them as programs. So, only for this implementation of Compartment, it is possible for a program transform to be equally applicable for modules, but that transform will have a window into the internal translation, will be sensitive to changes to that translation between any pair of releases, even those that do not disclose any breaking changes, and will only work on SES-shim, not any other implementation of Compartment like the one provided by XS.

The SES-shim Compartment constructor accepts a __shimTransforms__ option for this purpose. For the Compartment to use the same transforms for both evaluated strings and modules converted to programs, pass them as __shimTransforms__ instead of transforms.

const __shimTransforms__ = [addMetering];
const globalLexicals = { meter };
const c = new Compartment({ console }, null, {

The __shimTransforms__ feature is designed to uphold the security properties of compartments, since an attacker may use all available features, whether they are standard or not.

Logging Errors

lockdown() adds new global assert and tames the global console. The error taming hides error stacks, accumulating them in side tables. The assert system generated other diagnostic information hidden in side tables. The tamed console uses these side tables to output more informative diagnostics. Logging Errors explains the design.

Bug Disclosure

Please help us practice coordinated security bug disclosure, by using the instructions in [][] to report security-sensitive bugs privately.

For non-security bugs, please use the [regular Issues page][SES Issues].

[define shim]: [SES proposal]: []: [SES Issues]: