add support for response_mode=form_post

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'; " +