Handlebars Improper Control of Generation of Code (Code Injection) Vulnerability - CVE-2026-33938

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 gt 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(39handlebars/runtime39)). 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.