--- 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;

Summary

flatted's parse() function uses a recursive revive() phase to resolve circular references in deserialized JSON. When given a crafted payload with deeply nested or self-referential $ indices, the recursion depth is unbounded, causing a stack overflow that crashes the Node.js process.

Impact

Denial of Service (DoS). Any application that passes untrusted input to flatted.parse() can be crashed by an unauthenticated attacker with a single request.

flatted has ~87M weekly npm downloads and is used as the circular-JSON serialization layer in many caching and logging libraries.

Proof of Concept

javascript const flatted = require('flatted');

// Build deeply nested circular reference chain const depth = 20000; const arr = new Array(depth + 1); arr[0] = '{"a":"1"}'; for (let i = 1; i <= depth; i++) { arr[i] = {"a":"${i + 1}"}; } arr[depth] = '{"a":"leaf"}';

const payload = JSON.stringify(arr); flatted.parse(payload); // RangeError: Maximum call stack size exceeded

Fix

The maintainer has already merged an iterative (non-recursive) implementation in PR #88, converting the recursive revive() to a stack-based loop.

Affected Versions

All versions prior to the PR #88 fix.