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CVE-2026-6556 is a middleware bypass vulnerability in @fastify/express affecting version 4.0.6 and earlier. Within prefixed plugin scopes, the package rewrites the plugin prefix for middleware mount paths only when the path argument is a string. Non-string mount paths, specifically arrays of paths and regular expressions, are not rewritten with the plugin prefix. As a result, middleware registered with those non-string path forms does not match the actual prefixed request path, even though Fastify still matches and serves the target route. In applications that rely on path-scoped middleware inside prefixed plugins for security-sensitive controls such as authentication, authorization, rate limiting, or auditing, requests to the prefixed route can bypass the intended middleware entirely.
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Cross-references every affected SKU, including bundled OEM variants.
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