CVE-2026-55091

Description

### Summary `convert()` builds the nested tree by using each flat record's `id` and `parent` field values directly as object keys, with no guard against `__proto__` / `constructor` / `prototype`. A record whose `parent` is the string `"__proto__"` makes `temp[parent]` resolve to `Object.prototype`, and the following `initPush(...)` writes attacker-controlled data onto the global prototype. Any application that passes attacker-influenced records to `convert()` is affected, and the base prototype methods stay intact so the pollution is stealthy. ### Details In `index.js`, `convert()` (`FlatToNested.prototype.convert`): - `temp = {}` (line 45) and `pendingChildOf = {}` (line 46) are plain objects, so they inherit from `Object.prototype`. - For each record, `parent = flatEl[this.config.parent]` (line 51) is taken verbatim from input. - Line 57: `if (temp[parent] !== undefined)` — when `parent === "__proto__"`, `temp["__proto__"]` resolves via the prototype chain to `Object.prototype`, which is `!== undefined`, so the branch is taken. - Line 59: `initPush(this.config.children, temp[parent], flatEl)` → effectively `initPush("children", Object.prototype, flatEl)`. - `initPush` (lines 4-9): `Object.prototype["children"] = []` then `Object.prototype["children"].push(flatEl)` — **attacker-controlled data is written onto the global `Object.prototype`.** There is no sanitization of `id` / `parent` anywhere; they flow straight into `temp[id]`, `temp[parent]`, and `pendingChildOf[parent]` as dynamic keys. ### PoC ```js const FlatToNested = require('flat-to-nested'); new FlatToNested().convert([ { id: 1, parent: '__proto__', polluted: 'PWNED' } ]); console.log(({}).children); // => [ { id: 1, polluted: 'PWNED' } ] A freshly-created, unrelated object {} now carries an attacker-controlled children property. ({}).toString === Object.prototype.toString remains true, so existing methods are untouched (stealthy). If the consumer configures a custom children key, that arbitrary prototype property is polluted instead. ``` ### Impact Prototype pollution (CWE-1321). Any service that builds a tree from attacker-influenced flat records (the package's core purpose — e.g. records derived from a DB/REST/user input) can have Object.prototype polluted. Consequences range from application-logic corruption and denial of service to serving as a gadget toward privilege escalation or RCE depending on downstream sinks. No special privileges or user interaction required; the malicious value is ordinary input data. ### Suggested fix Use prototype-less lookup tables so inherited keys like __proto__ cannot be reached: var temp = Object.create(null); var pendingChildOf = Object.create(null); (Optionally also reject id/parent values equal to __proto__, constructor, or prototype.) Verified: with Object.create(null) for both temp and pendingChildOf, the PoC no longer pollutes Object.prototype and normal nesting output is unchanged. A patch with a regression test is ready.