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guard mergeObjects against prototype-polluting keys #40516

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guard mergeObjects against prototype-polluting keys #40516

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10 changes: 8 additions & 2 deletions extensions/amp-analytics/0.1/config.js
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Original file line number Diff line number Diff line change
Expand Up @@ -499,8 +499,14 @@ export function mergeObjects(from, to, opt_predefinedVendorConfig) {
opt_predefinedVendorConfig || property != 'iframePing',
'iframePing config is only available to vendor config.'
);
// Only deal with own properties.
if (hasOwn(from, property)) {
// Only deal with own properties, and never let a key reach the prototype
// chain (remote/inline config is attacker reachable).
if (
hasOwn(from, property) &&
property != '__proto__' &&
property != 'constructor' &&
property != 'prototype'
) {
if (isArray(from[property])) {
if (!isArray(to[property])) {
to[property] = [];
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14 changes: 14 additions & 0 deletions extensions/amp-analytics/0.1/test/test-config.js
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Original file line number Diff line number Diff line change
Expand Up @@ -338,6 +338,20 @@ describes.realWin(
'foobar': {'foobar': ['abc', 'def']},
});
});

it('does not pollute the prototype via __proto__', () => {
const from = JSON.parse('{"__proto__": {"polluted": "yes"}}');
mergeObjects(from, {});
expect({}.polluted).to.be.undefined;
});

it('does not pollute the prototype via constructor', () => {
const from = JSON.parse(
'{"constructor": {"prototype": {"polluted": "yes"}}}'
);
mergeObjects(from, {});
expect({}.polluted).to.be.undefined;
});
});

describe('vendor only configs', () => {
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