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Critical, CVSS 9.2amended

Handlebars: JavaScript Injection via Own Property Check Bypass

CVE-2026-106445GHSA-p8wg-vrv2-v86f

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (handlebars 4.7.10), 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

  • npm handlebars

    Affected
    >= 3.0.0, <= 4.7.9
    Fixed in
    4.7.10

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS 4.0
9.2 · CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N · score source
EPSS
  • CVE-2026-106445: 0.59% estimated exploitation activity in the next 30 days; percentile 48.20 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.
Source description

As published by the source; long descriptions are shortened.

Summary Handlebars can expose the Function constructor despite its prototype-access deny list. When a template reaches Function.prototype, its own constructor property is returned before the deny list is checked. An attacker who can render a controlled template with allowProtoMethodsByDefault: true can inject and execute arbitrary JavaScript, leading to Remote Code Execution on the server. Description lookupProperty trusts own properties: Normally, prototype-derived values reach resultIsAllowed, which blocks dangerous method names such as constructor. However, constructor is itself an own property of prototype objects: As a result, once a template reaches Function.prototype, looking up constructor returns Function without consulting the deny list. Proof of Concept An attacker needs to reach any prototype object where constructor is an own property, then access constructor to obtain Function. The shortest path requires only an accessible function in the template context: 1. Access a…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 4 · amended ·

    Amended from ghsa record GHSA-p8wg-vrv2-v86f

  2. Revision 3 · amended ·

    Reviewed OSV alias/range/CVSS4 correction; package source scopes and original publication preserved. No customer finding resolved.

  3. Revision 2 · amended ·

    Amended from osv record GHSA-p8wg-vrv2-v86f

  4. Revision 1 · created ·

    Published from ghsa record GHSA-p8wg-vrv2-v86f

Agent brief

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# Patch Next advisory brief: CVE-2026-106445

Advisory: Handlebars: JavaScript Injection via Own Property Check Bypass
IDs: CVE-2026-106445, GHSA-p8wg-vrv2-v86f
Record: https://patchnext.com/advisories/cve-2026-106445 (revision 4, source checked 26 Oct 2026, 13:07 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: handlebars 4.7.10. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm handlebars: affected >= 3.0.0, <= 4.7.9; fixed in 4.7.10

## Suggested inspection (read-only)
- `npm ls handlebars --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 handlebars.
- 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://github.com/advisories/GHSA-p8wg-vrv2-v86f (GitHub-reviewed advisory GHSA-p8wg-vrv2-v86f lists affected ranges: npm handlebars >=3.0.0 <=4.7.9)
- Verified: https://osv.dev/vulnerability/GHSA-p8wg-vrv2-v86f (OSV record GHSA-p8wg-vrv2-v86f lists affected ranges: npm handlebars >=3.0.0 <4.7.10 (fixed))

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