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Medium, CVSS 4.7amended

Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection

CVE-2026-33916GHSA-2qvq-rjwj-gvw9

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (handlebars 4.7.9), 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
    >= 4.0.0, < 4.7.9
    Fixed in
    4.7.9

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
4.7 · CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
EPSS
  • CVE-2026-33916: 0.28% estimated exploitation activity in the next 30 days; percentile 20.57 (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 resolvePartial() in the Handlebars runtime resolves partial names via a plain property lookup on options.partials without guarding against prototype-chain traversal. When Object.prototype has been polluted with a string value whose key matches a partial reference in a template, the polluted string is used as the partial body and rendered without HTML escaping, resulting in reflected or stored XSS. Description The root cause is in lib/handlebars/runtime.js inside resolvePartial() and invokePartial(): hasOwnProperty is never checked, so if Object.prototype has been seeded with a key whose name matches a partial reference in the template (e.g. widget), the lookup succeeds and the polluted string is returned. The runtime emits a prototype-access warning, but the partial is still resolved and its content is inserted into the rendered output unescaped. This contradicts the documented security model and is distinct from CVE-2021-23369 and CVE-2021-23383, which addressed data…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 2 · amended ·

    Amended from ghsa record GHSA-2qvq-rjwj-gvw9

  2. Revision 1 · created ·

    Published from osv record GHSA-2qvq-rjwj-gvw9

Agent brief

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# Patch Next advisory brief: CVE-2026-33916

Advisory: Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection
IDs: CVE-2026-33916, GHSA-2qvq-rjwj-gvw9
Record: https://patchnext.com/advisories/cve-2026-33916 (revision 2, source checked 26 Oct 2026, 12:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.7)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: handlebars 4.7.9. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm handlebars: affected >= 4.0.0, < 4.7.9; fixed in 4.7.9

## 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://osv.dev/vulnerability/GHSA-2qvq-rjwj-gvw9 (OSV record GHSA-2qvq-rjwj-gvw9 lists affected ranges: npm handlebars >=4.0.0 <4.7.9 (fixed))
- Verified: https://github.com/advisories/GHSA-2qvq-rjwj-gvw9 (GitHub-reviewed advisory GHSA-2qvq-rjwj-gvw9 lists affected ranges: npm handlebars >=4.0.0 <4.7.9)

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