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High, CVSS 7.5amended

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

CVE-2026-55091GHSA-hp36-v28f-w3r4

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (flat-to-nested 1.1.2), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
EPSS
  • CVE-2026-55091: 0.50% estimated exploitation activity in the next 30 days; percentile 42.69 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

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:…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-hp36-v28f-w3r4

  2. Revision 1 · created ·

    Published from osv record GHSA-hp36-v28f-w3r4

Agent brief

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# Patch Next advisory brief: CVE-2026-55091

Advisory: flat-to-nested: Prototype pollution in flat-to-nested convert() via __proto__ parent/id key
IDs: CVE-2026-55091, GHSA-hp36-v28f-w3r4
Record: https://patchnext.com/advisories/cve-2026-55091 (revision 2, source checked 26 Oct 2026, 12:56 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: flat-to-nested 1.1.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm flat-to-nested: affected <= 1.1.1; fixed in 1.1.2

## Suggested inspection (read-only)
- `npm ls flat-to-nested --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on flat-to-nested.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-hp36-v28f-w3r4 (OSV record GHSA-hp36-v28f-w3r4 lists affected ranges: npm flat-to-nested >=0 <1.1.2 (fixed))
- Verified: https://github.com/advisories/GHSA-hp36-v28f-w3r4 (GitHub-reviewed advisory GHSA-hp36-v28f-w3r4 lists affected ranges: npm flat-to-nested >=0 <=1.1.1)

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