GHSA-HP36-V28F-W3R4

Vulnerability from github – Published: 2026-06-19 20:47 – Updated: 2026-06-19 20:47
VLAI
Summary
flat-to-nested: Prototype pollution in flat-to-nested convert() via __proto__ parent/id key
Details

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.

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
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{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.1.1"
      },
      "package": {
        "ecosystem": "npm",
        "name": "flat-to-nested"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.1.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-55091"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1321",
      "CWE-915"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-06-19T20:47:52Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "### Summary\n  `convert()` builds the nested tree by using each flat record\u0027s `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.\n\n  ### Details\n  In `index.js`, `convert()` (`FlatToNested.prototype.convert`):\n\n  - `temp = {}` (line 45) and `pendingChildOf = {}` (line 46) are plain objects, so they inherit from `Object.prototype`.\n  - For each record, `parent = flatEl[this.config.parent]` (line 51) is taken verbatim from input.\n  - Line 57: `if (temp[parent] !== undefined)` \u2014 when `parent === \"__proto__\"`, `temp[\"__proto__\"]` resolves via the prototype chain to `Object.prototype`, which is `!== undefined`, so the\n  branch is taken.\n  - Line 59: `initPush(this.config.children, temp[parent], flatEl)` \u2192 effectively `initPush(\"children\", Object.prototype, flatEl)`.\n  - `initPush` (lines 4-9): `Object.prototype[\"children\"] = []` then `Object.prototype[\"children\"].push(flatEl)` \u2014 **attacker-controlled data is written onto the global `Object.prototype`.**\n\n  There is no sanitization of `id` / `parent` anywhere; they flow straight into `temp[id]`, `temp[parent]`, and `pendingChildOf[parent]` as dynamic keys.\n\n  ### PoC\n  ```js\n  const FlatToNested = require(\u0027flat-to-nested\u0027);\n\n  new FlatToNested().convert([\n    { id: 1, parent: \u0027__proto__\u0027, polluted: \u0027PWNED\u0027 }\n  ]);\n\n  console.log(({}).children); // =\u003e [ { id: 1, polluted: \u0027PWNED\u0027 } ]\n  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.\n ```\n \n  ### Impact\n\n  Prototype pollution (CWE-1321). Any service that builds a tree from attacker-influenced flat records (the package\u0027s core purpose \u2014 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.\n\n  ### Suggested fix\n\n  Use prototype-less lookup tables so inherited keys like __proto__ cannot be reached:\n  var temp = Object.create(null);\n  var pendingChildOf = Object.create(null);\n  (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.",
  "id": "GHSA-hp36-v28f-w3r4",
  "modified": "2026-06-19T20:47:52Z",
  "published": "2026-06-19T20:47:52Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/joaonuno/flat-to-nested-js/security/advisories/GHSA-hp36-v28f-w3r4"
    },
    {
      "type": "WEB",
      "url": "https://github.com/joaonuno/flat-to-nested-js/commit/680a5ebe1194edda16fa93baaa56ff14fe0e3d7f"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/joaonuno/flat-to-nested-js"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "flat-to-nested: Prototype pollution in flat-to-nested convert() via __proto__ parent/id key"
}



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