liquidjs/docs/source/tutorials/security-model.md

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Security Model

LiquidJS provides DoS-oriented limits (parseLimit, renderLimit, memoryLimit) to reduce risk. This page summarizes those limits, ownPropertyOnly, custom Drop usage, and the security boundary to assume in production.

Security boundary

The built-in limits are cooperative safeguards, not strict runtime isolation.

• They do not equal process RSS/heap usage.
• They do not sandbox JavaScript execution.
• They should be combined with process/container limits and request timeouts for defense in depth.

Limits at a glance

• parseLimit: limit total template size per parse() call.
• renderLimit: limit total render time per render() call.
• memoryLimit: cooperatively limit memory-sensitive allocations counted by LiquidJS.

Limit details

parseLimit

parseLimit restricts the size (character length) of templates parsed in each .parse() call, including referenced partials and layouts. Since LiquidJS parses template strings in near O(n) time, limiting total template length is usually sufficient.

A typical PC handles 1e8 (100M) characters without issues.

renderLimit

Restricting template size alone is insufficient because dynamic loops with large counts can occur in render time. renderLimit mitigates this by limiting the time consumed by each render() call.

{%- for i in (1..10000000) -%}
    order: {{i}}
{%- endfor -%}

Render time is checked on a per-template basis (before rendering each template). In the above example, there are 2 templates in the loop: order: and {{i}}, render time will be checked 10000000x2 times.

renderLimit is not a hard CPU limiter. It is checked between template renders, so compute-intensive filters/tags/user-defined functions or deeply nested template execution between checks can still cause DoS.

memoryLimit

memoryLimit only limits operations that LiquidJS explicitly counts.

• Counted: memory-sensitive LiquidJS operations that call internal memory accounting.
• Not guaranteed counted: arbitrary user object behavior such as custom toValue()/toString() chains, or other host-side code that allocates outside LiquidJS accounting points.

In other words, memoryLimit limits what LiquidJS counts, not every byte your process may allocate.

Even with small number of templates and iterations, memory usage can grow exponentially. In the following example, memory doubles with each iteration:

{% assign array = "1,2,3" | split: "," %}
{% for i in (1..32) %}
    {% assign array = array | concat: array %}
{% endfor %}

As JavaScript uses GC to manage memory, memoryLimit may not reflect the actual memory footprint.

ownPropertyOnly and scope data

With ownPropertyOnly true, plain scope objects only expose own properties (no inherited / Object.prototype keys). Default false follows normal JS property access. Use true for untrusted or polluted objects; add strictVariables if missing paths should error. Override per render via RenderOptions. This is a read policy for scope data—not a sandbox for filters, tags, or your code.

Custom Drop classes

Drop values are not restricted the same way: LiquidJS still reads the prototype chain and may call liquidMethodMissing. You control what a drop exposes; narrow APIs and never feed unsafe data into drops unless the class is built for template access. ownPropertyOnly alone does not harden custom drops—audit them like any privileged code.

Online service guidance

If you run an online service, avoid rendering fully user-defined templates whenever possible.

• Prefer curated templates or a restricted template subset.
• If user-defined templates are required, isolate rendering (worker/process/container), enforce OS/container memory and CPU limits, and apply request rate limits.
• Treat parseLimit/renderLimit/memoryLimit as one layer in a broader DoS defense strategy.

For heavy single-template operations, process-level isolation is still recommended (for example with paralleljs).