CVE-2026-64648 in Next.js
Summary
by MITRE • 07/27/2026
Next.js is a React framework for building full-stack web applications. In versions 12.0.0 through 15.5.20 and 16.0.0 through 16.2.10, a server-side fetch with a request body may return a cached response body from a different request to the same URL but different body. Confidential data in the POST's response body would then leak to unauthorized requests. Though the request itself will not be deduped. This only applies to fetch calls with a request that has a different init than the one passed to fetch. A safe request would be: fetch(new Request(init), init). An unsafe request would be: fetch(new Request(init), aDifferentInit). This issue has been fixed in versions 15.5.21 and 16.2.11.
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Analysis
by VulDB Data Team • 07/27/2026
This vulnerability affects Next.js applications running within specific version ranges, creating a critical server-side caching flaw that enables unauthorized data leakage through HTTP POST requests. The issue stems from improper cache key generation when processing fetch operations with request bodies, allowing cached responses to be incorrectly returned for different request payloads targeting the same endpoint. This represents a classic cache poisoning vulnerability where the system fails to properly differentiate between distinct requests based on their complete initialization parameters.
The technical flaw manifests when Next.js processes fetch calls that utilize Request objects with differing init configurations. When developers create fetch requests using new Request(init) syntax followed by a different init parameter, the framework's caching mechanism incorrectly treats these as identical requests despite their varying payloads and headers. This behavior specifically impacts POST requests where response bodies contain sensitive information, as the system returns cached data from one request context to another unauthorized context. The vulnerability operates at the HTTP layer, exploiting the framework's assumption that identical URLs equate to identical request contexts when they actually differ in their payload characteristics.
The operational impact of this vulnerability is significant for applications handling confidential data through POST endpoints, particularly those implementing server-side rendering or API route caching mechanisms. Attackers can exploit this weakness by crafting malicious requests that leverage the cache to access response bodies from other users' requests, potentially exposing personal information, session tokens, or business-critical data. This represents a privilege escalation scenario where unauthorized users gain access to data they should not be able to retrieve through normal application behavior, creating potential data breaches and compliance violations.
Security mitigations involve upgrading to patched versions 15.5.21 and 16.2.11 where the caching logic has been corrected to properly account for request body differences when generating cache keys. Organizations should also implement proactive monitoring of their fetch implementations to identify potentially unsafe patterns such as using different init parameters with Request objects. The fix aligns with common security best practices for cache management and follows principles outlined in CWE-204, which addresses improper handling of cached data. Additionally, this vulnerability maps to ATT&CK technique T1566.001 related to credential access through server-side request forgery, as the flaw enables unauthorized data retrieval that could include authentication tokens or session information.
The vulnerability demonstrates the importance of proper cache key generation in web frameworks and highlights how seemingly minor implementation details can create significant security risks. It underscores the necessity for comprehensive testing of caching behaviors, particularly when dealing with HTTP methods that carry request payloads. Organizations should conduct thorough code reviews focusing on fetch API usage patterns to identify potential instances where the same URL might be used with different request parameters, ensuring that cache invalidation occurs appropriately based on complete request characteristics rather than just endpoint URLs.