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fix: prevent proto-access bypass via Symbol.toStringTag and escape bypass via toHTML #2148

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fix: prevent proto-access bypass via Symbol.toStringTag and escape bypass via toHTML #2148

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fix: prevent proto-access bypass via Symbol.toStringTag and escape by…
eddieran Apr 12, 2026
0e4b212
fix: scope PR to Map/Set spoofing fix only
eddieran Apr 16, 2026
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17 changes: 6 additions & 11 deletions lib/handlebars/utils.js
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Original file line number Diff line number Diff line change
Expand Up @@ -35,18 +35,13 @@ export function isFunction(value) {
return typeof value === 'function';
}

function testTag(name) {
const tag = '[object ' + name + ']';
return function (value) {
return value && typeof value === 'object'
? toString.call(value) === tag
: false;
};
}

export const isArray = Array.isArray;
export const isMap = testTag('Map');
export const isSet = testTag('Set');
export function isMap(value) {
return value instanceof Map;
}
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Copilot AI Apr 12, 2026

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Given isMap now gates proto-access control behavior in runtime.lookupProperty, it would be good to add a security regression test that a plain object spoofing [Symbol.toStringTag] = 'Map' is not treated as a Map and cannot be used to read blocked proto properties (e.g. constructor). No existing spec appears to cover the spoofing case.

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export function isSet(value) {
return value instanceof Set;
}

// Older IE versions do not directly support indexOf so we must implement our own, sadly.
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@kibertoad kibertoad Apr 21, 2026

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this needs a regression test, something like

it('does not treat Symbol.toStringTag-spoofed objects as Map', function () {
    expectTemplate('{{value.constructor}}')
      .withInput({ value: { [Symbol.toStringTag]: 'Map', get: () => 'pwned' } })
      .toCompileTo(''); // or whatever the proto-controlled path produces
  });

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@kibertoad kibertoad Apr 21, 2026

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Add one each for isMap / isSet at the unit level (spec/utils.js already has the happy-path tests) plus an end-to-end runtime.js lookup test.

export function indexOf(array, value) {
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