Affected: npm: handlebars

## Summary

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.

## Description

Handlebars stores `@partial-block` in the `data` frame that is accessible to templates. In nested contexts, a parent frame's `@partial-block` is reachable as `@_parent.partial-block`. Because the data frame is a mutable object, any registered helper that accepts an object reference and assigns properties to it can overwrite `@partial-block` with an attacker-controlled value.

When `{{> @partial-block}}` is subsequently evaluated, `invokePartial` receives the crafted object. The runtime, finding an object that is not a compiled function, falls back to **dynamically compiling** the value via `env.compile()`. If that value is a well-formed Handlebars AST containing injected code, the injected JavaScript runs in the server process.

The `handlebars-helpers` npm package (commonly used with Handlebars) includes several helpers such as `merge` that can be used as the mutation primitive.

## Proof of Concept

Tested with Handlebars 4.7.8 and `handlebars-helpers`:

```javascript const Handlebars = require('handlebars'); const merge = require('handlebars-helpers').object().merge; H…