Echo static file middleware route protection bypass via encoded dot-dot traversal

Echo is a Go web framework. Prior to 4.16.0 and 5.4.0, Echo's static file handlers can normalize dot segments after route matching and serve a file through a public static route even though the corresponding direct path is protected by route-level middleware. An unauthenticated attacker can read files that an application intended to protect with route-level middleware, when those files are also reachable through a static filesystem mount after path normalization. This vulnerability is the result of an incomplete fix for CVE-2026-55677. This issue is fixed in 4.16.0 and 5.4.0.