Node.js SSTI reaches code execution through require('child_process').execSync (or process / global), since a template that can run JavaScript can load any core module. The engines differ in how much JavaScript they let the template express.

Pug compiles templates to JavaScript and allows inline JS in several constructs, so injection is close to direct evaluation. Handlebars is logic-light by design and exposes no direct JS, so its exploitation is a constructor/prototype walk that reassembles a function at runtime. Detection differs too: Pug responds to #{7*7}, while Handlebars uses {{ }} but will not evaluate {{7*7}} as arithmetic, so it is fingerprinted by its error messages and helper syntax.

Engines#

• Handlebars: the constructor.constructor / #with prototype walk to a function.
• Pug: inline JavaScript in interpolation and buffered code to require('child_process').

References#

• Handlebars and Pug documentation
• PortSwigger Web Security Academy: Server-side template injection