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CRITICAL severity

CVE-2021-23594 — realms-shim

CRITICAL

CVE-2021-23594 is a critical-severity (CVSS 9.8) CWE-1321 vulnerability in realms-shim. 1 public exploit reference exists, so weaponization risk is real. No vendor fix is recorded yet; mitigation options are listed below.

Prototype Pollution in realms-shim

Published
Updated
Affected
1 pkg
Patched
See advisory
Exploits
1 known
Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
1.8%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs77th percentile — riskier than 77% of all scored CVEsHighest risk
0.06%0.79%1.53%2.26%0.6%1.8%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-23594 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.

realms-shimnpm
707downloads / week

Description

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

Affected Packages

1 total
EcosystemPackageVulnerable rangeFix
📦npmrealms-shimall versionsNo fix

Affected Products

1 product · 1 configurations
Application
realms-shimagoric
all
Exploits & PoCs
1

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

All versions of package realms-shim are vulnerable to Sandbox Bypass via…

snyk.ioNVDJan 2022

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for realms-shim, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Remediation status

    No patched version of realms-shim has shipped for CVE-2021-23594 yet. Where your build allows, override or pin the dependency away from the vulnerable range, and apply any maintainer-recommended mitigation.

  3. Mitigate without a patch

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

All versions of package realms-shim are vulnerable to Sandbox Bypass via a Prototype Pollution attack vector.
O3 Security · Impact-Aware SCA

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CVE-2021-23594: realms-shim: Prototype Pollution