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

Severity: critical (CVSS 9.2 v4.0)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.6% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-10-06 · updated 2026-10-12
Weakness: CWE-184

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.

Affected products

Npm Handlebars: from 3.0.0 before 4.7.10 — fixed in 4.7.10
handlebars-lang handlebars.js: from 3.0.0, < 4.7.10

How to fix

Upgrade to a fixed version: Npm Handlebars 4.7.10. Check the vendor advisory for mitigations.

References

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