CVE-2026-55091 flat-to-nested: Prototype pollution in flat-to-nested convert() via __proto__ parent/id key
Prototype pollution in flat-to-nested allows attackers to corrupt application state or cause denial of service via crafted input.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-55091?
CVE-2026-55091 is a high-severity vulnerability in joaonuno flat-to-nested-js, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
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.
Which versions are affected by CVE-2026-55091?
2 open source packages ship an affected version, including flat-to-nested.
| Package | Affected | Fixed in |
|---|---|---|
flat-to-nested npm | All versions | 1.1.2 |
flat-to-nested npm | All versions | 1.1.2 |
How do I fix CVE-2026-55091?
Upgrade flat-to-nested to 1.1.2 or later.
Is CVE-2026-55091 being exploited?
There is no known exploitation of CVE-2026-55091 in the wild and no public exploit code.
Sources
Also known as GHSA-hp36-v28f-w3r4.