Security Overview

The expression evaluator is designed from the ground up to safely evaluate untrusted user input. It uses a defense-in-depth approach with multiple layers of protection.

Sandboxed Execution Model

Expressions run in a fully sandboxed interpreter. Unlike approaches that compile to JavaScript and use eval(), this library:

• Parses expressions into an AST (Abstract Syntax Tree)
• Interprets the AST directly with controlled semantics
• Never generates or executes arbitrary JavaScript code

This means expressions can only do what the interpreter explicitly allows.

What Expressions Cannot Do

• No side effects -- expressions cannot modify the context, environment, or any external state
• No loops -- there are no for, while, or do constructs (array functions like map/filter are the only iteration mechanism and are bounded)
• No dynamic code execution -- no eval(), Function(), or import()
• No prototype access -- __proto__, constructor, and prototype are blocked by default
• No arbitrary function calls -- only pre-registered built-in and custom functions can be called

Defense Layers

Layer	Protection	Default
Property Access Control	Allowlist/denylist for property names	Prototype blocked
Timeout	Maximum execution time	1000ms
Complexity Limit	Maximum AST nodes	100
Call Stack Depth	Maximum function nesting	50
Loop Iterations	Maximum iterations in array functions	10000
Feature Toggles	Disable arrays, template strings, arrow functions	Arrays on; template strings and arrow functions off

Recommended Configuration

For evaluating fully untrusted input, use restrictive settings:

const options = {
  timeout: 500,
  maxComplexity: 50,
  maxCallStackDepth: 20,
  maxLoopIterations: 1000,
  enableTemplateStrings: false,
  enableArrowFunctions: false,
  deniedProperties: ['password', 'secret', 'token', 'key'],
};

For trusted internal use where you control the expressions, you can use more permissive settings:

const options = {
  enableTemplateStrings: true,
  enableArrowFunctions: true,
  timeout: 5000,
  maxComplexity: 500,
};