GHSA-r7w9-592q-9vg4 - Echo has CSRF Protection Bypass via MethodOverride Rewriting POST to Safe Method

## Summary The `MethodOverride` middleware accepts any non-empty string as the override target method — including safe methods (GET, HEAD, OPTIONS, TRACE) — when rewriting a POST request. When `MethodOverride` is registered before `CSRF` in the post-routing middleware chain (`e.Use()`), it rewrites the request method to a safe method before `CSRF` evaluates it. The CSRF middleware then skips token validation for the now-safe method, allowing state-changing POST handlers to execute without any CSRF check. Both the legacy token path and the modern `Sec-Fetch-Site` protection path are bypassed simultaneously. ## Affected Component - `middleware/method_override.go` — `MethodOverrideConfig.ToMiddleware()` (lines 55–71): no whitelist on the override target method - `middleware/csrf.go` — `CSRFConfig.ToMiddleware()` (lines 188–189): safe-method bypass branch reads the already-rewritten method - `middleware/csrf.go` — `checkSecFetchSiteRequest()` (line 283): `Sec-Fetch-Site` safe check also reads the rewritten method ## Description ### MethodOverride accepts safe methods as override targets `MethodOverrideConfig.ToMiddleware()` (`method_override.go:55-71`) restricts the **source** method (only POST requests are overrideable) but applies no whitelist to the **target**: ```go // method_override.go:61-67 req := c.Request() if req.Method == http.MethodPost { m := config.Getter(c) if m != "" { req.Method = m // ← any non-empty string accepted, including "GET" } } ``` The docstring's security claim — "For security reasons, only POST method can be overridden" — refers only to the source-method restriction. There is no guard preventing POST from being rewritten to `GET`, `HEAD`, `OPTIONS`, or `TRACE`. The test suite (`method_override_test.go`) only tests POST→DELETE and POST→PUT; POST→GET is never tested or blocked. ### CSRF safe-method branch reads the already-rewritten method `CSRFConfig.ToMiddleware()` (`csrf.go:188-189`) makes its validation deci