CVE-2026-33940

01/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.

02/Affected products

handlebarsjs: handlebars

03/References8

• github.com/handlebars-lang/handlebars.js/security/advisories/GH… ExploitVendor Advisory
• github.com/handlebars-lang/handlebars.js/commit/68d8df5a88e0a26… Patch
• github.com/handlebars-lang/handlebars.js/releases/tag/v4.7.9 Release Notes
• access.redhat.com/errata/RHSA-2026:10175
• access.redhat.com/errata/RHSA-2026:34342
• access.redhat.com/security/cve/CVE-2026-33940
• bugzilla.redhat.com/show_bug.cgi?id=2452521
• security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-33940…