# CVE-2026-100721: vm2 before 3.12.2 Authorization Bypass via Custom Resolver

> Attackers can escape the vm2 sandbox and execute arbitrary code in the host process if a custom resolver is configured without proper path boundaries.

- **Severity:** Critical 9.5 (`CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 Sep 2026
- **Updated:** 28 Sep 2026
- **Also known as:** CLEANSTART-2026-CN31177, CLSA-2026-1791887363, CLSA-2026-1791888008, ECHO-923f-7c77-29c1, GHSA-5h3f-q97h-ccvc, MINI-52gv-5c3m-r2x9, ROOT-APP-NPM-CVE-2026-100721

## What is CVE-2026-100721?

CVE-2026-100721 is a critical authentication bypass vulnerability in patriksimek vm2, published on 27 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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.

Weakness: CWE-863 (Incorrect Authorization)

## Which versions are affected by CVE-2026-100721?

2 open source packages ship an affected version, including vm2.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `vm2` | npm | All versions | 3.12.2 |
| `vm2` | npm | All versions | 3.12.2 |

## How do I fix CVE-2026-100721?

Upgrade vm2 to 3.12.2 or later.

## Is CVE-2026-100721 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-100721 exists.

Public exploit code:

- CVE-2026-100721 PoC: vm2 <3.12.2 NodeVM external allowlist bypass → sandbox escape / host RCE. Local Node lab (evil-left-pad), remote sandbox API mass exploit, colorful CLI. PoCbit — https://pocbit.org/pocs/cve-2026-100721 (github_poc): https://github.com/murrez/CVE-2026-100721

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-100721 and fix it if it is affected.

What it is: CVE-2026-100721 is a critical authentication bypass vulnerability in patriksimek vm2, published on 27 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-100721 exists.

Affected packages and fixed versions:
- vm2 (npm): affected all versions; fixed in 3.12.2
- vm2 (npm): affected all versions; fixed in 3.12.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-100721/ (Markdown: https://insights.sbomify.com/CVE-2026-100721/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-100721
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-100721
- OSV: https://osv.dev/vulnerability/CVE-2026-100721

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-100721/
Summaries are generated and may contain mistakes; the official description above is the record.