LiquidJS has a renderLimit DoS guard bypass via empty `{% for %}` body

Summary

The renderLimit option — documented in docs/source/tutorials/dos.md as the mechanism that "mitigates this by limiting the time consumed by each render() call" — can be fully bypassed by a {% for %} (or {% tablerow %}) tag whose body is empty. The per-iteration time check is reached only when the body contains at least one template node, so a template like {%- for i in (1..N) -%}{%- endfor -%} iterates the full collection without ever consulting renderLimit. With a configured renderLimit of 50 ms, a single parseAndRenderSync call has been observed to consume 2.26 seconds (~45× over the limit) and scales linearly with N up to memoryLimit, allowing a low-privileged template author to wedge an event-loop thread for an attacker-chosen duration.

Details

Render.renderTemplates is the single point at which renderLimit is consulted:

// src/render/render.ts
14:  public * renderTemplates (templates: Template[], ctx: Context, emitter?: Emitter): IterableIterator<any> {
15:    if (!emitter) {
16:      emitter = ctx.opts.keepOutputType ? new KeepingTypeEmitter() : new SimpleEmitter()
17:    }
18:    const errors = []
19:    for (const tpl of templates) {
20:      ctx.renderLimit.check(getPerformance().now())
21:      try {
22:        const html = yield tpl.render(ctx, emitter)
...
32:    }

The check at line 20 lives inside the for (const tpl of templates) body. When templates.length === 0, the loop body never executes, so the limiter is never consulted on that invocation.

The for tag re-enters renderTemplates once per collection item with no independent time check:

// src/tags/for.ts
70:    for (const item of collection) {
71:      scope[this.variable] = item
72:      ctx.continueCalled = ctx.breakCalled = false
73:      yield r.renderTemplates(this.templates, ctx, emitter)
74:      if (ctx.breakCalled) break
75:      scope.forloop.next()
76:    }

When {%- for i in (1..N) -%}{%- endfor -%} is parsed, this.templates is []. Each of the N calls to r.renderTemplates(this.templates, ctx, emitter) therefore performs zero renderLimit.check() calls and zero template work — it just spins the JS-level for loop and the generator boilerplate. With N = 30_000_000 this still costs ~2.26 s of CPU, and N = 100_000_000 costs ~9.6 s, fully bypassing whatever wall-clock budget the integrator configured.

The range expression itself is bounded only by memoryLimit:

// src/render/expression.ts:67-72
function * evalRangeToken (token: RangeToken, ctx: Context) {
  const low: number = yield evalToken(token.lhs, ctx)
  const high: number = yield evalToken(token.rhs, ctx)
  ctx.memoryLimit.use(high - low + 1)
  return range(+low, +high + 1)
}

So the maximum bypass is governed by the (separate) memoryLimit, not by renderLimit. Integrators following the docs/source/tutorials/dos.md guidance — which positions renderLimit as the time-based defense — get no time-based defense at all on this code path.

PoC

Reproduced against liquidjs@10.25.7 (HEAD 34877950):

# Empty for-body bypasses renderLimit (50 ms) and runs for ~2.26 s:
$ node -e "const { Liquid } = require('liquidjs');
  const engine = new Liquid({ memoryLimit: 1e9, renderLimit: 50 });
  const t = Date.now();
  engine.parseAndRenderSync('{%- for i in (1..30000000) -%}{%- endfor -%}', {});
  console.log('Took', Date.now()-t, 'ms');"
Took 2255 ms

# Same template with a single-character body is correctly bounded:
$ node -e "const { Liquid } = require('liquidjs');
  const engine = new Liquid({ memoryLimit: 1e9, renderLimit: 50 });
  try { engine.parseAndRenderSync('{%- for i in (1..30000000) -%}.{%- endfor -%}', {}); }
  catch(e) { console.log('correctly threw:', e.message); }"
correctly threw: template render limit exceeded, line:1, col:1

Scaling N:

• N = 30_000_000 → 2255 ms (≈ 45× over the 50 ms limit)
• N = 100_000_000 → 9581 ms (≈ 191× over the 50 ms limit)

Time grows linearly with N, capped only by memoryLimit (default Infinity, so the only cap by default is process memory).

Impact

Any liquidjs integrator who follows the upstream DoS guidance and sets a finite renderLimit to bound per-render CPU — typical for SaaS / multi-tenant environments where end users author templates (themes, email templates, snippets) — does not get the bound they configured. A single template submission can keep an event-loop thread busy for seconds, which on a Node.js server is sufficient to stall all in-flight requests on that worker. With a large enough range and a permissive memoryLimit, the wedge time is attacker-controlled. No data is exposed and no integrity is harmed; impact is availability only.

Recommended Fix

Move the renderLimit check to a location that runs unconditionally per renderTemplates invocation, so a zero-template body still triggers it; alternatively (or additionally) have iteration tags that invoke renderTemplates per element check the limiter themselves once per iteration.

// src/render/render.ts — check at function entry, before the templates loop
public * renderTemplates (templates: Template[], ctx: Context, emitter?: Emitter): IterableIterator<any> {
  if (!emitter) {
    emitter = ctx.opts.keepOutputType ? new KeepingTypeEmitter() : new SimpleEmitter()
  }
  ctx.renderLimit.check(getPerformance().now())   // <-- runs even when templates is empty
  const errors = []
  for (const tpl of templates) {
    ctx.renderLimit.check(getPerformance().now())
    ...
  }
  ...
}

And/or, defensively, in the iteration tags themselves so the guard cost is paid once per element rather than only at re-entry:

// src/tags/for.ts (around line 70)
for (const item of collection) {
  ctx.renderLimit.check(getPerformance().now())   // <-- per-iteration time check
  scope[this.variable] = item
  ctx.continueCalled = ctx.breakCalled = false
  yield r.renderTemplates(this.templates, ctx, emitter)
  if (ctx.breakCalled) break
  scope.forloop.next()
}

// src/tags/tablerow.ts (around line 54) — analogous addition
for (let idx = 0; idx < collection.length; idx++, tablerowloop.next()) {
  ctx.renderLimit.check(getPerformance().now())
  ...
}

The same hardening should be applied anywhere a tag drives an attacker-influenced loop count over a (potentially empty) templates array.