Echoのmiddleware.Staticに認証を回避される脆弱性

Summary

When middleware.Static is mounted globally (e.Use(middleware.Static(...))) and an application relies on a route- or group-level middleware guard to restrict a path that also falls under the served static root, a %2F-encoded request bypasses that guard while the file is still served. This is the same encoding/decoding mismatch fixed for StaticFS/StaticDirectoryHandler in CVE-2026-55677 / GHSA-vfp3-v2gw-7wfq, but the fix did not cover the middleware.Static path. In addition, the StaticConfig.EnablePathUnescaping docstring states the default configuration prevents exactly this bypass, which is inaccurate for the global-mount case.

This is not path traversal: no file outside the static root is accessed. The served file lives inside the static root; the issue is that access-control expectations set by a route guard are inconsistent between the literal and %2F-encoded forms of the same path.

Details

Root cause — the router and the static handler disagree on how to interpret %2F:

• The router matches routes on the raw, still-encoded path, so GET /admin%2Fsecret.txt does not match a /admin/... route or group; any middleware attached to that route/group is therefore skipped.
• middleware.Static selects its path source in middleware/static.go (~L219): with a global e.Use mount there is no wildcard route, so it uses c.Request().URL.Path, which net/http has already decoded (%2F → /). It then path.Cleans (~L235)…