Vulnerability · Published Sep 14, 2026 · Assigned by GitHub_M
CVE-2026-55091flat-to-nested: Prototype pollution in flat-to-nested convert() via __proto__ parent/id key
High 7.5
In plain English
A high-severity prototype pollution in flat-to-nested-js from joaonuno. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.5% (higher than 43% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| npm | flat-to-nested | 1.1.2 | GHSA-hp36-v28f-w3r4 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| joaonuno | flat-to-nested-js | < 1.1.2 |
Weakness: CWE-915, CWE-1321 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N