GHSA-v753-g4cw-jm48 - Echo has Open Redirect via Control-Character Prefix Defeating sanitizeURI

## Summary Echo's dedicated open-redirect guard, `sanitizeURI`, decides whether a redirect target is a local path by inspecting exactly two byte positions: `uri[0]` and `uri[1]`. If the value begins with a control character such as a tab or a newline, followed by a separator pair, the guard does not fire and the value is written to the `Location` header unchanged. Browsers remove tab and newline characters from URLs before parsing them, and treat a backslash as a forward slash for HTTP and HTTPS URLs. A `Location` value of ``/\t/attacker.example`` is therefore resolved by the browser as an absolute URL on `attacker.example`. Confirmed end to end in a headless browser. ## Technical background ### The guard `middleware/slash.go:144` (duplicated verbatim at `echo.go:958`): ```go func sanitizeURI(uri string) string { // double slash `\\`, `//` or even `\/` is absolute uri for browsers and by redirecting request to that uri // we are vulnerable to open redirect attack. so replace all slashes from the beginning with single slash if len(uri) > 1 && (uri[0] == '\\' || uri[0] == '/') && (uri[1] == '\\' || uri[1] == '/') { uri = "/" + strings.TrimLeft(uri, `/\`) } return uri } ``` The condition is anchored at absolute offsets 0 and 1. It cannot see a separator that has been shifted by one byte. ### Data flow 1. `RemoveTrailingSlash` (`middleware/slash.go:112-140`) reads `url.Path`, which `net/http` has already percent-decoded, strips the trailing slash, and appends the raw query string: ```go path := url.Path ... if l > 0 && strings.HasSuffix(path, "/") { path = path[:l] uri := path if qs != "" { uri += "?" + qs } if config.RedirectCode != 0 { return c.Redirect(config.RedirectCode, sanitizeURI(uri)) // line 132 } ``` 2. `AddTrailingSlash` does the same in the opposite direction at `middleware/slash.go:67`. 3. A request for `/%09/attacker.example/x/` has `url.Path` equal to `"/\t/attacker.example/x/"` (a literal TAB