feat(op): Add response_mode: form_post (#551)

* feat(op): Add response_mode: form_post

* Fix to parse the template ahead of time

* Fix to render the template in a buffer

* Remove unnecessary import

* Fix test

* Fix example client setting

* Make sure the client not to reuse the content of the response

* Fix error handling

* Add the response_mode param

* Allow implicit flow in the example app

* feat(rp): allow form_post in code exchange callback handler

---------

Co-authored-by: Tim Möhlmann <tim+github@zitadel.com>

pkg/op/auth_request.go

@@ -1,9 +1,12 @@
 package op
 
 import (
+	"bytes"
 	"context"
+	_ "embed"
 	"errors"
 	"fmt"
+	"html/template"
 	"log/slog"
 	"net"
 	"net/http"
@@ -464,6 +467,17 @@ func AuthResponseCode(w http.ResponseWriter, r *http.Request, authReq AuthReques
 		Code:  code,
 		State: authReq.GetState(),
 	}
+
+	if authReq.GetResponseMode() == oidc.ResponseModeFormPost {
+		err := AuthResponseFormPost(w, authReq.GetRedirectURI(), &codeResponse, authorizer.Encoder())
+		if err != nil {
+			AuthRequestError(w, r, authReq, err, authorizer)
+			return
+		}
+
+		return
+	}
+
 	callback, err := AuthResponseURL(authReq.GetRedirectURI(), authReq.GetResponseType(), authReq.GetResponseMode(), &codeResponse, authorizer.Encoder())
 	if err != nil {
 		AuthRequestError(w, r, authReq, err, authorizer)
@@ -484,6 +498,17 @@ func AuthResponseToken(w http.ResponseWriter, r *http.Request, authReq AuthReque
 		AuthRequestError(w, r, authReq, err, authorizer)
 		return
 	}
+
+	if authReq.GetResponseMode() == oidc.ResponseModeFormPost {
+		err := AuthResponseFormPost(w, authReq.GetRedirectURI(), resp, authorizer.Encoder())
+		if err != nil {
+			AuthRequestError(w, r, authReq, err, authorizer)
+			return
+		}
+
+		return
+	}
+
 	callback, err := AuthResponseURL(authReq.GetRedirectURI(), authReq.GetResponseType(), authReq.GetResponseMode(), resp, authorizer.Encoder())
 	if err != nil {
 		AuthRequestError(w, r, authReq, err, authorizer)
@@ -535,6 +560,43 @@ func AuthResponseURL(redirectURI string, responseType oidc.ResponseType, respons
 	return mergeQueryParams(uri, params), nil
 }
 
+//go:embed form_post.html.tmpl
+var formPostHtmlTemplate string
+
+var formPostTmpl = template.Must(template.New("form_post").Parse(formPostHtmlTemplate))
+
+// AuthResponseFormPost responds a html page that automatically submits the form which contains the auth response parameters
+func AuthResponseFormPost(res http.ResponseWriter, redirectURI string, response any, encoder httphelper.Encoder) error {
+	values := make(map[string][]string)
+	err := encoder.Encode(response, values)
+	if err != nil {
+		return oidc.ErrServerError().WithParent(err)
+	}
+
+	params := &struct {
+		RedirectURI string
+		Params      any
+	}{
+		RedirectURI: redirectURI,
+		Params:      values,
+	}
+
+	var buf bytes.Buffer
+	err = formPostTmpl.Execute(&buf, params)
+	if err != nil {
+		return oidc.ErrServerError().WithParent(err)
+	}
+
+	res.Header().Set("Cache-Control", "no-store")
+	res.WriteHeader(http.StatusOK)
+	_, err = buf.WriteTo(res)
+	if err != nil {
+		return oidc.ErrServerError().WithParent(err)
+	}
+
+	return nil
+}
+
 func setFragment(uri *url.URL, params url.Values) string {
 	uri.Fragment = params.Encode()
 	return uri.String()

pkg/op/auth_request_test.go

@@ -1027,9 +1027,10 @@ func TestAuthResponseCode(t *testing.T) {
 		authorizer func(*testing.T) op.Authorizer
 	}
 	type res struct {
-		wantCode           int
-		wantLocationHeader string
-		wantBody           string
+		wantCode               int
+		wantLocationHeader     string
+		wantCacheControlHeader string
+		wantBody               string
 	}
 	tests := []struct {
 		name string
@@ -1111,6 +1112,33 @@ func TestAuthResponseCode(t *testing.T) {
 				wantBody:           "",
 			},
 		},
+		{
+			name: "success form_post",
+			args: args{
+				authReq: &storage.AuthRequest{
+					ID:            "id1",
+					CallbackURI:   "https://example.com/callback",
+					TransferState: "state1",
+					ResponseMode:  "form_post",
+				},
+				authorizer: func(t *testing.T) op.Authorizer {
+					ctrl := gomock.NewController(t)
+					storage := mock.NewMockStorage(ctrl)
+					storage.EXPECT().SaveAuthCode(context.Background(), "id1", "id1")
+
+					authorizer := mock.NewMockAuthorizer(ctrl)
+					authorizer.EXPECT().Storage().Return(storage)
+					authorizer.EXPECT().Crypto().Return(&mockCrypto{})
+					authorizer.EXPECT().Encoder().Return(schema.NewEncoder())
+					return authorizer
+				},
+			},
+			res: res{
+				wantCode:               http.StatusOK,
+				wantCacheControlHeader: "no-store",
+				wantBody:               "<!doctype html>\n<html>\n<head><meta charset=\"UTF-8\" /></head>\n<body onload=\"javascript:document.forms[0].submit()\">\n<form method=\"post\" action=\"https://example.com/callback\">\n<input type=\"hidden\" name=\"state\" value=\"state1\"/>\n<input type=\"hidden\" name=\"code\" value=\"id1\" />\n\n\n\n\n</form>\n</body>\n</html>",
+			},
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -1121,6 +1149,7 @@ func TestAuthResponseCode(t *testing.T) {
 			defer resp.Body.Close()
 			assert.Equal(t, tt.res.wantCode, resp.StatusCode)
 			assert.Equal(t, tt.res.wantLocationHeader, resp.Header.Get("Location"))
+			assert.Equal(t, tt.res.wantCacheControlHeader, resp.Header.Get("Cache-Control"))
 			body, err := io.ReadAll(resp.Body)
 			require.NoError(t, err)
 			assert.Equal(t, tt.res.wantBody, string(body))

pkg/op/form_post.html.tmpl

@@ -0,0 +1,14 @@
+<!doctype html>
+<html>
+<head><meta charset="UTF-8" /></head>
+<body onload="javascript:document.forms[0].submit()">
+<form method="post" action="{{ .RedirectURI }}">
+{{with .Params.state}}<input type="hidden" name="state" value="{{ index . 0 }}"/>{{end}}
+{{with .Params.code}}<input type="hidden" name="code" value="{{ index . 0 }}" />{{end}}
+{{with .Params.id_token}}<input type="hidden" name="id_token" value="{{ index . 0 }}"/>{{end}}
+{{with .Params.access_token}}<input type="hidden" name="access_token" value="{{ index . 0 }}" />{{end}}
+{{with .Params.token_type}}<input type="hidden" name="token_type" value="{{ index . 0 }}" />{{end}}
+{{with .Params.expires_in}}<input type="hidden" name="expires_in" value="{{ index . 0 }}" />{{end}}
+</form>
+</body>
+</html>