Handlebars Improper Control of Generation of Code (Code Injection) Vulnerability - CVE-2026-33940

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(39handlebars/runtime39)). 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 (gt (lookup ...)) when context data is user-controlled.