flat-to-nested CVE-2026-55091

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

  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.