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Critical, CVSS 9.8amended

Sandbox Breakout in realms-shim

GHSA-7cg8-pq9v-x98q

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (realms-shim 1.2.1), 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
9.8 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
EPSS
No dated EPSS signal available.
Source description

As published by the source; long descriptions are shortened.

Versions of realms-shim prior to 1.2.1 are vulnerable to a Sandbox Breakout. The Realms evaluation function has an option to apply Babel-like transformations to the source code before it reaches the evaluator. One portion of this transform pipeline exposed a primal-Realm object to the rewriting function. Confined code which used the evaluator itself could provide a malicious rewriter function that captured this object, and use it to breach the sandbox. Recommendation Upgrade to version 1.2.1 or later.

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

    Amended from ghsa record GHSA-7cg8-pq9v-x98q

  2. Revision 1 · created ·

    Published from osv record GHSA-7cg8-pq9v-x98q

Agent brief

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

# Patch Next advisory brief: GHSA-7cg8-pq9v-x98q

Advisory: Sandbox Breakout in realms-shim
IDs: GHSA-7cg8-pq9v-x98q
Record: https://patchnext.com/advisories/ghsa-7cg8-pq9v-x98q (revision 2, source checked 26 Oct 2026, 12:07 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 fix available; fixed versions by package: realms-shim 1.2.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm realms-shim: affected < 1.2.1; fixed in 1.2.1

## 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
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 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-7cg8-pq9v-x98q (OSV record GHSA-7cg8-pq9v-x98q lists affected ranges: npm realms-shim >=0 <1.2.1 (fixed))
- Verified: https://github.com/advisories/GHSA-7cg8-pq9v-x98q (GitHub-reviewed advisory GHSA-7cg8-pq9v-x98q lists affected ranges: npm realms-shim >=0 <1.2.1)

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