flat-to-nested: Prototype pollution in flat-to-nested convert() via __proto__ parent/id key

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No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

flat-to-nested converts a hierarchy from a flat representation to a nested representation. Prior to 1.1.2, FlatToNested.prototype.convert in index.js uses attacker-influenced id and parent record fields directly as keys in the plain temp and pendingChildOf objects. When parent or id is __proto__, temp[parent] can resolve to Object.prototype, and initPush() can write attacker-controlled data to the global children prototype property while existing prototype methods remain intact. Any application that passes attacker-influenced flat records to convert() can therefore expose unrelated objects to polluted inherited state, causing application-logic corruption or denial of service and potentially enabling greater impact when a downstream prototype-pollution gadget is present. The constructor and prototype strings are also unsafe inherited-key values in the same lookup design. This issue is fixed in version 1.1.2.

Source published

14 Sep 2026 17:26:38 UTC

Source modified

14 Sep 2026 20:05:59 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

flat-to-nested converts a hierarchy from a flat representation to a nested representation. Prior to 1.1.2, FlatToNested.prototype.convert in index.js uses attacker-influenced id and parent record fields directly as keys in the plain temp and pendingChildOf objects. When parent or id is __proto__, temp[parent] can resolve to Object.prototype, and initPush() can write attacker-controlled data to the global children prototype property while existing prototype methods remain intact. Any application that passes attacker-influenced flat records to convert() can therefore expose unrelated objects to polluted inherited state, causing application-logic corruption or denial of service and potentially enabling greater impact when a downstream prototype-pollution gadget is present. The constructor and prototype strings are also unsafe inherited-key values in the same lookup design. This issue is fixed in version 1.1.2.

Source published

Sep 14, 2026, 5:26:38 PM (timezone not stated)

Source modified

Sep 14, 2026, 8:05:59 PM (timezone not stated)

Observed here

11 Nov 2026 16:11:16 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

flat-to-nested converts a hierarchy from a flat representation to a nested representation. Prior to 1.1.2, FlatToNested.prototype.convert in index.js uses attacker-influenced id and parent record fields directly as keys in the plain temp and pendingChildOf objects. When parent or id is __proto__, temp[parent] can resolve to Object.prototype, and initPush() can write attacker-controlled data to the global children prototype property while existing prototype methods remain intact. Any application that passes attacker-influenced flat records to convert() can therefore expose unrelated objects to polluted inherited state, causing application-logic corruption or denial of service and potentially enabling greater impact when a downstream prototype-pollution gadget is present. The constructor and prototype strings are also unsafe inherited-key values in the same lookup design. This issue is fixed in version 1.1.2.

Source published

14 Sep 2026 18:17:54 UTC

Source modified

30 Sep 2026 17:51:56 UTC

Observed here

25 Oct 2026 13:17:34 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

### 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.

Source published

19 Jun 2026 20:47:52 UTC

Source modified

19 Jun 2026 21:11:28 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

Publisher rating · HIGH