How severe is CVE-2026-100721?

No CVSS score has been assigned to CVE-2026-100721 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-100721?

Affected package information for CVE-2026-100721 is not yet fully mapped. Check the official advisory links for details.

How do I fix CVE-2026-100721?

Upgrade the affected component to the fixed release for CVE-2026-100721, or apply your distribution's backported patch — distro builds are often patched at an older version number, so check your vendor's advisory rather than the upstream version alone.

How do I detect CVE-2026-100721 in my code and dependencies?

Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).

How do I mitigate CVE-2026-100721 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2026-100721 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-100721 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-100721?

CVE-2026-100721 has an EPSS (Exploit Prediction Scoring System) score of 0.4%, placing it in the 32th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-100721?

CVE-2026-100721 is classified as CWE-863 (CWE-863). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-100721 published, and has it been updated?

CVE-2026-100721 was published on September 27, 2026 and was last updated on September 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

Not in CISA KEV

CVE-2026-100721 — vm2

Fix: patriksimek/vm2@6ac3916

CVE-2026-100721 is a CWE-863 vulnerability. A fix is available — see the affected versions and patch details below.

vm2 before 3.12.2 Authorization Bypass via Custom Resolver

Also known asGHSA-5h3f-q97h-ccvc

Published

Sep 27, 2026

Updated

Sep 28, 2026

Affected

37 versions

Patched

See advisory

Exploits

None indexed

Exploitation data as of Sep 27, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs32th percentile — riskier than 32% of all scored CVEsHighest risk

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

Description

vm2 before 3.12.2 contains an authorization bypass in the NodeVM external-module resolver. When an embedder configures require.external with a custom resolver (and context: 'host'), LegacyResolver.customResolve in lib/resolver-compat.js records the resolved module directory in this.externals as new RegExp('^' + escapeRegExp(resolvedPath)), without requiring a path separator or end-of-string boundary. Untrusted guest code can therefore require the allowlisted module (e.g. foo) and then require the absolute path of a non-allowlisted sibling whose path merely shares the resolved prefix (e.g. .../node_modules/foo2/index.js); the sibling passes isPathAllowedForModule and is loaded through hostRequire, so its top-level code runs in the host process before the exports are wrapped with vm.readonly, resulting in a sandbox escape and arbitrary code execution in the host context.

Detection & mitigation playbook

Vulnerability

Detect
Identify every host running the affected component and compare the installed build against the fixed version below — for source-built or distro-packaged software the version string, not a lockfile, is the source of truth (`dpkg -l`, `rpm -q`, or the binary's own `--version`).
Fix
Upgrade the affected component to the fixed release for CVE-2026-100721, or apply your distribution's backported patch — distro builds are often patched at an older version number, so check your vendor's advisory rather than the upstream version alone.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

vm2 before 3.12.2 contains an authorization bypass in the NodeVM external-module resolver. When an embedder configures `require.external` with a custom resolver (and `context: 'host'`), `LegacyResolver.customResolve` in lib/resolver-compat.js records the resolved module directory in `this.externals` as `new RegExp('^' + escapeRegExp(resolvedPath))`, without requiring a path separator or end-of-string boundary. Untrusted guest code can therefore require the allowlisted module (e.g. `foo`) and then require the absolute path of a non-allowlisted sibling whose path merely shares the resolved prefi

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