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.