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fix(commons): skip __proto__ / constructor / prototype keys in _.merge #3687

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ridingsa wants to merge 1 commit into from
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fix(commons): skip __proto__ / constructor / prototype keys in _.merge #3687

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13 changes: 13 additions & 0 deletions packages/commons/src/index.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -75,6 +75,19 @@ export const _ = {
merge(target: any, source: any) {
if (_.isObject(target) && _.isObject(source)) {
Object.keys(source).forEach((key) => {
// Skip prototype-chain-mutating keys. `Object.keys` returns
// `__proto__` as an own enumerable when the source object came
// from `JSON.parse('{"__proto__":...}')`; without this filter
// the recursive `_.merge(target[key], source[key])` below
// resolves `target['__proto__']` to `Object.prototype` and
// writes attacker-controlled keys onto it.
if (
key === '__proto__' ||
key === 'constructor' ||
key === 'prototype'
) {
return
}
if (_.isObject(source[key])) {
if (!target[key]) {
Object.assign(target, { [key]: {} })
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20 changes: 20 additions & 0 deletions packages/commons/test/utils.test.ts
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Expand Up @@ -212,5 +212,25 @@ describe('@feathersjs/commons utils', () => {

assert.equal(_.merge('hello', {}), 'hello')
})

it('merge does not mutate Object.prototype via __proto__ keys', () => {
// `Object.keys` returns `__proto__` as an own enumerable key when the
// source object came from `JSON.parse('{"__proto__":...}')`. Without
// the filter in the merge implementation, the recursive call resolves
// `target['__proto__']` to `Object.prototype` and writes attacker-
// controlled keys onto it (affecting every plain object in the
// process).
const target = {} as any
_.merge(target, JSON.parse('{"__proto__":{"polluted":"X"}}'))
assert.strictEqual(({} as any).polluted, undefined)
assert.strictEqual((Object.prototype as any).polluted, undefined)
assert.strictEqual(target.polluted, undefined)

// Same protection for `constructor` and `prototype` keys (the rest
// of the standard prototype-mutating triad).
_.merge(target, JSON.parse('{"constructor":{"prototype":{"polluted2":"Y"}}}'))
assert.strictEqual(({} as any).polluted2, undefined)
assert.strictEqual((Object.prototype as any).polluted2, undefined)
})
})
})