next-intl has prototype pollution with experimental.messages.precompile via attacker-controlled translation catalog keys

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.2 · CVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:L/I:L/A:L

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary setNestedProperty in packages/next-intl/src/extractor/utils.tsx walks a dotted key path and assigns the final value without blocking the reserved keys proto, constructor, or prototype. When the next-intl Next.js plugin is configured with experimental.messages and messages.precompile: true, a JSON translation catalog containing a top‑level proto key causes setNestedProperty(result, 'proto.isAdmin', compiledMessage) to assign onto Object.prototype, polluting every object in the running build process. Details Root cause — packages/next-intl/src/extractor/utils.tsx:13-34: The existence check !(key in current) uses the in operator, which walks the prototype chain. For key === 'proto', 'proto' in {} is true (it's inherited from Object.prototype) and typeof current['proto'] === 'object' (it is Object.prototype). The guard therefore never re-initializes current[key], and current = current['proto'] redirects all subsequent writes onto Object.prototype. The final assignment…

Published by the source

6 May 2026, 17:34 UTC

Last modified by the source

6 May 2026, 17:49 UTC

Source last checked

26 Oct 2026, 12:51 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4c35-wcg5-mm9h

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4c35-wcg5-mm9h

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# Patch Next advisory brief: GHSA-4c35-wcg5-mm9h

Advisory: next-intl has prototype pollution with `experimental.messages.precompile` via attacker-controlled translation catalog keys
IDs: GHSA-4c35-wcg5-mm9h
Record: https://patchnext.com/advisories/ghsa-4c35-wcg5-mm9h (revision 2, source checked 26 Oct 2026, 12:51 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: next-intl 4.9.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- npm next-intl: affected <= 4.9.1; fixed in 4.9.2

## Suggested inspection (read-only)
- `npm ls next-intl --all`: lists every installed copy and version, including transitive ones. Run in the project root after `npm ci`.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on next-intl.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-4c35-wcg5-mm9h (OSV record GHSA-4c35-wcg5-mm9h lists affected ranges: npm next-intl >=0 <4.9.2 (fixed))
- Verified: https://github.com/advisories/GHSA-4c35-wcg5-mm9h (GitHub-reviewed advisory GHSA-4c35-wcg5-mm9h lists affected ranges: npm next-intl >=0 <=4.9.1)

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