CVE-2021-23594
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedAll versions of package realms-shim are vulnerable to Sandbox Bypass via a Prototype Pollution attack vector.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidenceThe realms-shim package has a prototype pollution vulnerability that allows attackers to modify JavaScript object prototypes, enabling a sandbox bypass. This critical flaw affects all versions of the package, potentially allowing malicious code execution outside the intended security boundary.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE dataall versionsCVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Changed
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify if realms-shim is installedRun 'npm list realms-shim' or check package.json dependencies for the realms-shim packageAffected if The package appears in installed dependencies or package.json
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Check the package versionRun 'npm list realms-shim --depth=0' to see the installed version, or inspect node_modules/realms-shim/package.jsonAffected if Any version of realms-shim is present, as all versions are affected
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Determine if untrusted input reaches realms-shimReview code that imports or uses realms-shim and trace whether any user-supplied data is passed to its API methods (e.g., Realm.write, compartment evaluation)Affected if Untrusted or unsanitized data is passed to realms-shim functions that can affect object prototypes
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Check for prototype pollution sink usageSearch codebase for direct manipulation of __proto__, constructor, or prototype properties via realms-shim evaluationsAffected if Code allows setting __proto__ or constructor properties on objects created within realms-shim contexts
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Verify sandbox boundariesInspect whether realms-shim is used to evaluate code from external or user-controlled sourcesAffected if Realms-shim evaluates code from untrusted sources without additional sanitization layers
If realms-shim is installed and processes any untrusted or user-supplied data, the environment is affected by this prototype pollution vulnerability.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpgrade to a patched version of realms-shim if available; otherwise, implement strict input validation on all user-supplied data and consider using an alternative sandboxed execution library with active security maintenance.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $6,176.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-23594 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-23594 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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