add support for response_mode=form_post

backend/frontend/frontend_included.go

@@ -91,6 +91,15 @@ func RegisterFrontend(router *gin.Engine, rateLimitMiddleware gin.HandlerFunc) e
 		}
 
 		if path == "index.html" {
+			// Check if this is an OAuth2 authorization request with response_mode=form_post
+			// In that case, we need to allow form submissions to the redirect_uri
+			responseMode := c.Query("response_mode")
+			redirectURI := c.Query("redirect_uri")
+			if responseMode == "form_post" && redirectURI != "" {
+				// Set the allowed form-action in CSP to include the redirect URI
+				middleware.SetAllowedFormAction(c, redirectURI)
+			}
+
 			nonce := middleware.GetCSPNonce(c)
 
 			// Do not cache the HTML shell, as it embeds a per-request nonce

backend/internal/controller/oidc_controller.go

@@ -94,6 +94,11 @@ func (oc *OidcController) authorizeHandler(c *gin.Context) {
 		return
 	}
 
+	// Set the allowed form-action in CSP when response_mode is form_post
+	if input.ResponseMode == "form_post" && input.CallbackURL != "" {
+		middleware.SetAllowedFormAction(c, input.CallbackURL)
+	}
+
 	code, callbackURL, err := oc.oidcService.Authorize(c.Request.Context(), input, c.GetString("userID"), c.ClientIP(), c.Request.UserAgent())
 	if err != nil {
 		_ = c.Error(err)

backend/internal/dto/oidc_dto.go

@@ -71,6 +71,7 @@ type AuthorizeOidcClientRequestDto struct {
 	CodeChallenge         string `json:"codeChallenge"`
 	CodeChallengeMethod   string `json:"codeChallengeMethod"`
 	ReauthenticationToken string `json:"reauthenticationToken"`
+	ResponseMode          string `json:"responseMode"`
 }
 
 type AuthorizeOidcClientResponseDto struct {

backend/internal/middleware/csp_middleware.go

@@ -23,17 +23,39 @@ func GetCSPNonce(c *gin.Context) string {
 	return ""
 }
 
+// SetAllowedFormAction sets the allowed form-action for the CSP header.
+// This is used for OAuth2 response_mode=form_post to allow form submissions to the client's redirect URI.
+func SetAllowedFormAction(c *gin.Context, uri string) {
+	c.Set("csp_allowed_form_action", uri)
+}
+
 func (m *CspMiddleware) Add() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		// Generate a random base64 nonce for this request
 		nonce := generateNonce()
 		c.Set("csp_nonce", nonce)
 
+		// Get any allowed form-action from context (set by the authorize endpoint)
+		// Also check query parameters for response_mode=form_post (for both frontend and API requests)
+		formAction := "'self'"
+		if v, ok := c.Get("csp_allowed_form_action"); ok {
+			if uri, ok := v.(string); ok && uri != "" {
+				formAction = "'self' " + uri
+			}
+		} else {
+			// If not set by the authorize endpoint, check query parameters
+			responseMode := c.Query("response_mode")
+			redirectURI := c.Query("redirect_uri")
+			if responseMode == "form_post" && redirectURI != "" {
+				formAction = "'self' " + redirectURI
+			}
+		}
+
 		csp := "default-src 'self'; " +
 			"base-uri 'self'; " +
 			"object-src 'none'; " +
 			"frame-ancestors 'none'; " +
-			"form-action 'self'; " +
+			"form-action " + formAction + "; " +
 			"img-src * blob:;" +
 			"font-src 'self'; " +
 			"style-src 'self' 'unsafe-inline'; " +