# Handlebars.js prototype method access control gap via missing __lookupSetter__

Source: https://junglewise.ai/threats/handlebars-js-prototype-method-access-control-gap-via-missing-f97c2407  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-03-29. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

Severity: low · CVSS 3.1 · Published 2026-03-29

Handlebars.js is a popular template engine used to render dynamic content in web applications. When a non-default security setting (allowProtoMethodsByDefault: true) is enabled, a missing blocklist entry allows attackers to access the __lookupSetter__ prototype method in templates, potentially leading to unauthorized data access or modification.

Technologies: [handlebars (npm)](https://junglewise.ai/threats/technologies/npm-handlebars). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is This vulnerability exploited in the wild?** No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

**How severe is This vulnerability?** Low severity, with a CVSS score of 3.1, published 29 March 2026.

**Which products and versions are affected?** handlebars (npm) SEMVER:introduced=4.6.0,fixed=4.7.9.

**Is there a fix for This vulnerability?** Yes. handlebars (npm): upgrade to 4.7.9 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is This vulnerability attacked?** It is attacked over the network.

**Am I affected?** Check whether you run handlebars (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- handlebars (npm): upgrade to 4.7.9 or later

  ```sh
  npm install handlebars@4.7.9
  ```


## Technical details

The vulnerability is a prototype method access control gap in lib/handlebars/internal/proto-access.js. The blocklist blocks constructor, __defineGetter__, __defineSetter__, and __lookupGetter__ but omits __lookupSetter__, creating an asymmetric security boundary. When the non-default runtime option allowProtoMethodsByDefault: true is set, any prototype method not in the blocklist is permitted by default. An attacker can reference __lookupSetter__ through untrusted template input to bypass access controls. This only affects applications that explicitly enable allowProtoMethodsByDefault: true; the default configuration is not affected. The vulnerability was fixed in version 4.7.9.

## Affected products

- Handlebars Handlebars.js 4.6.0 through 4.7.8

## Timeline

- 2026-03-29: disclosed: GHSA-7rx3-28cr-v5wh published
- 2026-03-26: patched: Fixed in version 4.7.9

## References

- [https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-7rx3-28cr-v5wh](https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-7rx3-28cr-v5wh)

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