What method does Shopify use to prevent XSS attacks in Liquid templates?

Other Questions Shopify
1

I’ve realized that Shopify includes automatic safeguards against XSS when utilizing Liquid templates, yet I haven’t seen these measures applied in the regular liquid gem.

Here’s an illustration of what I’m experiencing:

Template code: <span title="{{ customer_input }}">{{ customer_input }}</span>

Provided input: '" onmouseover="alert('Attack')'"

Output from Shopify:

<span title="&quot; onmouseover=&quot;alert('Attack')&quot;">" onmouseover="alert('Attack')"</span>

Output from the standard Liquid gem:

<span title="" onmouseover="alert('Attack')">" onmouseover="alert('Attack')"</span>

My Ruby testing script:

template_string = '<span title="{{ customer_input }}">{{ customer_input }}</span>'
parsed_template = Liquid::Template.parse(template_string)

result = parsed_template.render!('customer_input' => '" onmouseover="alert(\'Attack\')')

What’s the trick that Shopify uses? I am aware of the escape filter and the option to sanitize on the backend, but Shopify’s method appears more efficient as it reduces the risk of neglecting to sanitize inputs. Using {{ data }} seems tidier than repeatedly applying {{ data | escape }}.

Does anyone know how they achieve this automatic escaping?

2

there’s actually a monkey patch that does this - you override the to_liquid_value method and check if the output context needs escaping. I saw it on GitHub but can’t remember which repo. it intercepts variable rendering before it reaches the template output.

3

I’ve dug into Shopify’s developer docs and done some reverse engineering - here’s how they pull it off. They override the default variable rendering in their custom Liquid setup. Basically, they wrap every {{ }} output with an HTML escape function that kicks in during rendering, not when parsing the template. The magic happens in the Variable class - they modified the render_to_output_buffer method to automatically HTML-encode any string output. Want raw output? You’ve got to explicitly use the raw filter to skip the escaping. This works for them because they control their entire Liquid environment. They can make breaking changes without worrying about backwards compatibility like the public gem has to. If you want the same thing, you’ll need to monkey-patch the Liquid::Variable class or build a custom Liquid environment with modified rendering defaults.

4

Shopify baked context-aware escaping straight into their Liquid engine by tweaking the core rendering pipeline. They hijacked the default variable output to automatically spot HTML contexts and escape stuff properly. It’s not just basic HTML entity encoding - the engine actually figures out where your variable sits in the template and uses different escaping rules for attributes vs regular content. The engine tracks whether you’re outputting inside HTML attributes, JavaScript, or normal content, then picks the right escaping method. This needs deep integration with the template parser instead of just using filters, which is why you won’t find it in the standard Liquid gem. You’d have to fork Liquid and build similar context detection into the rendering engine to get the same automatic protection.

5

shopify uses some clever stuff to auto-escape outputs unless you go raw or use risky filters. they altered the rendering but its kinda vague in docs. u can try extending the variable class and change the render method to escape by default.