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refactor: replace eval with ShadowRealm for safe expression evaluation #103

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mattapperson merged 2 commits into from
Dec 3, 2025
Merged

refactor: replace eval with ShadowRealm for safe expression evaluation #103

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2987e7f
refactor: replace eval with ShadowRealm for safe expression evaluation
devin-ai-integration[bot] Dec 3, 2025
099e569
fix: address PR feedback - make ShadowRealm usage correct
devin-ai-integration[bot] Dec 3, 2025
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33 changes: 30 additions & 3 deletions examples/tools-example.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -163,6 +163,34 @@ async function generatorToolExample() {
const _message = await response.getMessage();
}

/**
* Safely evaluate a math expression using ShadowRealm
*
* ShadowRealm provides a secure, isolated JavaScript execution environment
* that prevents access to the host realm's globals (no access to DOM, fetch,
* filesystem, etc.). This makes it safe for evaluating untrusted expressions.
*
* Browser support: Chrome 124+, Edge 124+, Firefox 128+, Safari 18+
* Node.js support: Available behind --experimental-shadow-realm flag (v19+)
* See: https://tc39.es/proposal-shadowrealm/
*/
async function safeEvaluateMath(expression: string): Promise<number> {
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The function signature declares async and returns Promise<number>, but since ShadowRealm.prototype.evaluate() is synchronous (not a Promise), this function should not be async. Change to:

function safeEvaluateMath(expression: string): number {

This also requires updating the call site at line 236 to remove await.

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// ShadowRealm is a TC39 Stage 3 proposal for isolated JavaScript execution
// It creates a separate realm with its own global object, preventing access
// to the host environment's APIs (no DOM, no Node.js APIs, no fetch, etc.)
const realm = new ShadowRealm();
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TypeScript may not have built-in type definitions for ShadowRealm since it's still a Stage 3 proposal. Consider adding a type declaration at the top of the file to avoid potential TypeScript errors:

// Type declaration for ShadowRealm (TC39 Stage 3 proposal)
declare class ShadowRealm {
  constructor();
  evaluate(sourceText: string): any;
  importValue(specifier: string, bindingName: string): Promise<any>;
}

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// The expression is evaluated in complete isolation - even if malicious code
// is injected, it cannot access anything outside the shadow realm
const result = await realm.evaluate(`(${expression})`);
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The ShadowRealm.prototype.evaluate() method returns synchronously, not a Promise. According to the TC39 spec (https://tc39.es/proposal-shadowrealm/#sec-shadowrealm.prototype.evaluate), this method should not be awaited. Remove the await keyword here.

const result = realm.evaluate(`(${expression})`);
Suggested change
const result = await realm.evaluate(`(${expression})`);
const result = realm.evaluate(`(${expression})`);

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Wrapping the expression in parentheses allows arbitrary JavaScript expressions, not just math operations. For example, "1, console.log('hi'), 2" would be valid. Consider validating the expression contains only allowed characters (digits, operators, parentheses, whitespace) before evaluation:

// Validate expression contains only safe math characters
if (!/^[\d\s+\-*/.()]+$/.test(expression)) {
  throw new Error('Expression contains invalid characters');
}
const result = realm.evaluate(expression);

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if (typeof result !== 'number' || !Number.isFinite(result)) {
throw new Error('Expression did not evaluate to a finite number');
}

return result;
}

/**
* Example 3: Manual Tool Execution
* Define a tool without execute function for manual handling
Expand Down Expand Up @@ -203,10 +231,9 @@ async function manualToolExample() {
}
).expression;

// In a real app, you would safely evaluate this
// For demo purposes only - don't use eval in production!
// Safely evaluate the math expression using ShadowRealm
try {
const _result = eval(expression);
const _result = await safeEvaluateMath(expression);
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Since safeEvaluateMath() should be synchronous (not async), remove the await keyword here:

const _result = safeEvaluateMath(expression);
Suggested change
const _result = await safeEvaluateMath(expression);
const _result = safeEvaluateMath(expression);

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} catch (_error) {}
}
}
Expand Down