CVE-2026-55091: Joaonuno Flat-To-Nested-Js

High severity, CVSS 7.5. EPSS: 0.5% chance of exploitation in the next 30 days.

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 products

• Joaonuno Flat-To-Nested-Js: before 1.1.2 (fixed in 1.1.2)

Published 2026-09-14. Last modified 2026-09-30.