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Critical, CVSS 9.8amended

Prototype Pollution in realms-shim

CVE-2021-23543GHSA-q4j7-v27r-fgcx

What to do

Vendor response not established. Follow the primary source's mitigation guidance and check back for a fix.

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

  • npm realms-shim

    Affected
    <= 1.2.2
    Fixed in
    no fixed version recorded

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
9.8 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS
  • CVE-2021-23543: 1.82% estimated exploitation activity in the next 30 days; percentile 78.77 (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.

All versions of package realms-shim are vulnerable to Sandbox Bypass via a Prototype Pollution attack vector.

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

    Amended from ghsa record GHSA-q4j7-v27r-fgcx

  2. Revision 1 · created ·

    Published from osv record GHSA-q4j7-v27r-fgcx

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: CVE-2021-23543

Advisory: Prototype Pollution in realms-shim
IDs: CVE-2021-23543, GHSA-q4j7-v27r-fgcx
Record: https://patchnext.com/advisories/cve-2021-23543 (revision 2, source checked 26 Oct 2026, 12:21 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.8)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm realms-shim: affected <= 1.2.2

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

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on realms-shim.
- 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-q4j7-v27r-fgcx (OSV record GHSA-q4j7-v27r-fgcx lists affected ranges: npm realms-shim >=0 <=1.2.2)
- Verified: https://github.com/advisories/GHSA-q4j7-v27r-fgcx (GitHub-reviewed advisory GHSA-q4j7-v27r-fgcx lists affected ranges: npm realms-shim >=0 <=1.2.2)

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