Web application frameworks often use a templating engine which allows them to easily create web pages of html from a combination of html and data. A template file (sample.html.twig) using the PHP template system Twig might look like
The PHP code to render this as an html page would then be:
echo $twig->render('sample.html.twig', ['name' => 'John Doe',
'occupation' => 'Penetration Tester']);
A vulnerability would occur with code that works as follows:
echo $twig->render('sample.html.twig', ['name' => $_GET[‘name’],
'occupation' => $_GET[‘occupation’]]);
An attacker can detect the injection by passing the string “” as the name argument and seeing 49 displayed in the rendered page. For the Python template engine Jinja2, the string ‘” produces a different result of “7777777” which enables us to differentiate the underlying code environment.