CVE-2026-33940 Handlebars.js has JavaScript Injection via AST Type Confusion when passing an object as dynamic partial

High 8.1 Published 27 Mar 2026 Updated 4 Sep 2026

Attackers can execute arbitrary code on the server by injecting a crafted object into the template context during dynamic partial resolution.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-33940?

CVE-2026-33940 is a high-severity code injection vulnerability in handlebars-lang handlebars.js, published on 27 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, a crafted object placed in the template context can bypass all conditional guards in `resolvePartial()` and cause `invokePartial()` to return `undefined`. The Handlebars runtime then treats the unresolved partial as a source that needs to be compiled, passing the crafted object to `env.compile()`. Because the object is a valid Handlebars AST containing injected code, the generated JavaScript executes arbitrary commands on the server. The attack requires the adversary to control a value that can be returned by a dynamic partial lookup. Version 4.7.9 fixes the issue. Some workarounds are available. First, use the runtime-only build (`require('handlebars/runtime')`). Without `compile()`, the fallback compilation path in `invokePartial` is unreachable. Second, sanitize context data before rendering: Ensure no value in the context is a non-primitive object that could be passed to a dynamic partial. Third, avoid dynamic partial lookups (`{{> (lookup ...)}}`) when context data is user-controlled.

Weakness: CWE-94, CWE-843

Which versions are affected by CVE-2026-33940?

2 open source packages ship an affected version, including handlebars. It affects 1 product.

Package	Affected	Fixed in
`handlebars` npm	From `4.0.0`	`4.7.9`
`handlebars` npm	From `4.0.0`	`4.7.9`

Products include Handlebarsjs Handlebars.

How do I fix CVE-2026-33940?

Upgrade handlebars to 4.7.9 or later.

Upgrade, and never evaluate templates, expressions or code built from user input. More about Code injection.

Is CVE-2026-33940 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33940 exists.

Sources

Also known as CLEANSTART-2026-OR11868, CLEANSTART-2026-XH19639, DEBIAN-CVE-2026-33940, GHSA-xhpv-hc6g-r9c6, MINI-26q8-5v4j-3gj6, MINI-2p9g-24rf-j696 and 14 more.