Improper Encoding or Escaping of Output Affecting next package, versions >=13.0.0 <15.5.21>=16.0.0 <16.2.11
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-JS-NEXT-18230947
- published23 Jul 2026
- disclosed22 Jul 2026
- creditUnknown
Introduced: 22 Jul 2026
NewCVE-2026-64647 (opens in a new tab)How to fix?
Upgrade next to version 15.5.21, 16.2.11 or higher.
Overview
next is a react framework.
Affected versions of this package are vulnerable to Improper Encoding or Escaping of Output via the IncrementalCache.generateCacheKey function in next/dist/server/lib/incremental-cache. An attacker can make a server-side fetch return a cached response body from a different request by sending POST requests to the same URL with different non-UTF-8 request bodies that collapse to the same cache key. The affected cache key generation decodes request bodies as strings, so distinct byte sequences with invalid UTF-8 can hash to the same entry even though the requests are not deduped. This can leak confidential response data from one POST to an unauthorized request that uses a colliding body, breaking response isolation for server-side cached fetches.
Notes
- Applications using Pages Router are not affected; the advisory’s cache-key confusion is in the server-side fetch cache path used by the App Router/
IncrementalCacheflow. - The issue is only reachable when request bodies are sent with a non-UTF-8 charset or otherwise contain byte sequences that do not round-trip as UTF-8; default UTF-8 request bodies do not trigger the collision described in the advisory.