CVE-2026-33938 Handlebars.js has JavaScript Injection via AST Type Confusion by tampering @partial-block

High 8.1 Published 27 Mar 2026 Updated 4 Sep 2026

Attackers can execute arbitrary JavaScript on the server by tampering with the @partial-block variable in Handlebars.js templates.

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

What is CVE-2026-33938?

CVE-2026-33938 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, the `@partial-block` special variable is stored in the template data context and is reachable and mutable from within a template via helpers that accept arbitrary objects. When a helper overwrites `@partial-block` with a crafted Handlebars AST, a subsequent invocation of `{{> @partial-block}}` compiles and executes that AST, enabling arbitrary JavaScript execution on the server. Version 4.7.9 fixes the issue. Some workarounds are available. First, use the runtime-only build (`require('handlebars/runtime')`). The `compile()` method is absent, eliminating the vulnerable fallback path. Second, audit registered helpers for any that write arbitrary values to context objects. Helpers should treat context data as read-only. Third, avoid registering helpers from third-party packages (such as `handlebars-helpers`) in contexts where templates or context data can be influenced by untrusted input.

Weakness: CWE-917, CWE-94, CWE-843

Which versions are affected by CVE-2026-33938?

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-2026-33938?

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-2026-33938 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-33938 exists.

Sources

Also known as CLEANSTART-2026-HJ03986, CLEANSTART-2026-SJ54112, DEBIAN-CVE-2026-33938, GHSA-3mfm-83xf-c92r, MINI-36gh-g93m-h9c9, MINI-46j2-m5vq-x4cv and 14 more.