# icu-minify prototype pollution denial of service in select branch lookup

Source: https://junglewise.ai/threats/icu-minify-prototype-pollution-denial-of-service-in-select-branch-ad42cd1c  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-06. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

Severity: low · CVSS 3.1 · Published 2026-05-06

icu-minify is a JavaScript library used for internationalization message formatting. When user input is passed into a select branch placeholder and the precompile feature is enabled, an attacker can crash the application by supplying property names that exist on Object.prototype (like "toString" or "constructor"), causing a denial-of-service condition that renders affected web pages inaccessible.

Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is This vulnerability exploited in the wild?** No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

**How severe is This vulnerability?** Low severity, with a CVSS score of 3.1, published 6 May 2026.

**Which products and versions are affected?** icu-minify (npm) SEMVER:introduced=0,fixed=4.9.2.

**Is there a fix for This vulnerability?** Yes. icu-minify (npm): upgrade to 4.9.2 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is This vulnerability attacked?** It is attacked over the network, in a software library.

**Am I affected?** Check whether you run icu-minify (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- icu-minify (npm): upgrade to 4.9.2 or later

  ```sh
  npm install icu-minify@4.9.2
  ```


## Technical details

The vulnerability is a prototype pollution-adjacent denial of service in icu-minify's select branch resolver. The `formatSelect()` function performs an unsanitized property lookup on an options object (which inherits from Object.prototype) using user-supplied string values as keys. When a user provides input that matches Object.prototype property names (toString, constructor, __proto__, hasOwnProperty, valueOf, etc.), the lookup returns the inherited function instead of undefined, bypassing the `?? options.other` fallback. The truthy function is then passed to `formatNodes()` which expects an iterable array, resulting in `TypeError: nodes is not iterable` and crashing the renderer. This affects direct icu-minify consumers and next-intl users with precompile mode enabled. No authentication is required; the attack vector is network-accessible form input or URL parameters. Patch version 4.9.2 is available.

## Affected products

- amannn icu-minify ≤ 4.9.1
- amannn next-intl versions using icu-minify ≤ 4.9.1 with experimental.messages.precompile enabled

## Timeline

- 2026-04-27: disclosed
- 2026-05-06: patched: icu-minify 4.9.2
- 2026-05-06: advisory

## References

- [https://github.com/amannn/next-intl/security/advisories/GHSA-r27j-894h-3w3p](https://github.com/amannn/next-intl/security/advisories/GHSA-r27j-894h-3w3p)
- [https://github.com/amannn/next-intl](https://github.com/amannn/next-intl)