CVE-2026-33228 — Prototype Pollution via parse() in flatted — CVE...

--- **Summary** The parse() function in flatted can use attacker-controlled string values from the parsed JSON as direct array index keys, without validating that they are numeric. Since the internal input buffer is a JavaScript Array, accessing it with the key "\_\_proto\_\_" returns Array.prototype via the inherited getter. This object is then treated as a legitimate parsed value and assigned as a property of the output object, effectively leaking a live reference to Array.prototype to the consumer. Any code that subsequently writes to that property will pollute the global prototype. --- **Root Cause** File: esm/index.js:29 (identical in cjs/index.js) ``` const resolver = (input, lazy, parsed, $) => output => { for (let ke = keys(output), {length} = ke, y = 0; y < length; y++) { const k = ke[y]; const value = output[k]; if (value instanceof Primitive) { const tmp = input[value]; // Bug is here ``` No validation that value is a safe numeric index input is built as a plain Array. JavaScript's property lookup on arrays traverses the prototype chain for non-numeric keys. The key "\_\_proto\_\_" resolves to Array.prototype, which: - has type "object" → passes the typeof tmp === object guard at line 30 - is not in the parsed Set yet → passes the !parsed.has(tmp) guard. - The reference to Array.prototype is then enqueued in lazy and later unconditionally assigned to the output object. --- **Replication Steps** ``` const Flatted = require('flatted'); const parsed = Flatted.parse('[{"x":"__proto__"}]'); parsed.x.polluted = 'pwned'; console.log([].polluted); // Returns true ``` --- **Impact** An attacker can supply a crafted flatted string to parse() that causes the returned object to hold a live reference to Array.prototype, enabling any downstream code that writes to that property to pollute the global prototype chain, potentially causing denial of service or code execution. **Recommended solution** Validate that the index string represents an integer within the bounds of input before accessing it: // Before (vulnerable) const tmp = input[value]; // After (safe) const idx = +value; // coerce boxed String → number const tmp = (Number.isInteger(idx) && idx >= 0 && idx < input.length) ? input[idx] : undefined;