OpenNext Cloudflare Middleware Bypass via Cache Interception

A security vulnerability exists in @opennextjs/cloudflare versions 0.3.0 through 1.20.1 due to inconsistent percent-encoded path handling when the enableCacheInterception feature is activated. When enabled, the library performs path decoding during cache lookup that differs from the decoding logic used by middleware matchers.

This discrepancy allows an attacker to craft a request with percent-encoded characters that effectively hide the path from security middleware, while subsequently being normalized into a valid cache key that points to a protected resource. Because the middleware fails to trigger, the authorization check is skipped entirely, and the Cloudflare Worker serves the cached response directly to the unauthenticated user. The vulnerability also stems from a secondary issue where malformed segments are partially decoded rather than rejected, allowing for potential obfuscation. This flaw impacts applications relying on middleware for sensitive access control in conjunction with static asset caching.

Impact

The vulnerability allows remote unauthenticated attackers to bypass intended access control mechanisms, resulting in the unauthorized retrieval of cached data from protected routes. The impact is limited to applications that explicitly enable enableCacheInterception and utilize middleware as their primary authorization gatekeeper. No specific victim counts are documented, but the scope includes all deployments of the affected @opennextjs/cloudflare package that meet these specific configuration criteria.

Recommendation

• Upgrade @opennextjs/cloudflare to version 1.20.2 or later to address CVE-2026-100409.
• If immediate patching is not feasible, disable the enableCacheInterception setting in the defineCloudflareConfig object.
• Implement authorization logic directly within route handlers as a defense-in-depth measure, rather than relying solely on middleware.
• Validate and reject anomalous encoded paths at the edge using Cloudflare WAF rules before the request reaches the Worker environment.