API: validate payment_info (#5944)

* API: validate payment_info

* improve dict-check

* Apply suggestions from code review

Co-authored-by: Raphael Michel <michel@pretix.eu>

---------

Co-authored-by: Raphael Michel <michel@pretix.eu>

src/pretix/api/serializers/order.py

@@ -19,6 +19,7 @@
 # You should have received a copy of the GNU Affero General Public License along with this program.  If not, see
 # <https://www.gnu.org/licenses/>.
 #
+import json
 import logging
 import os
 from collections import Counter, defaultdict
@@ -1215,6 +1216,18 @@ class OrderCreateSerializer(I18nAwareModelSerializer):
             raise ValidationError('The given payment provider is not known.')
         return pp
 
+    def validate_payment_info(self, info):
+        if info:
+            try:
+                obj = json.loads(info)
+            except ValueError:
+                raise ValidationError('payment_info must be valid JSON.')
+
+            if not isinstance(obj, dict):
+                # only objects are allowed
+                raise ValidationError('payment_info must be a JSON object.')
+        return info
+
     def validate_expires(self, expires):
         if expires < now():
             raise ValidationError('Expiration date must be in the future.')