Validate the from email for invalid characters

This might not be sufficient for all cases. If some other strange
IndexErrors appear, more validation has to be done. For now, it catches
all observed cases.

server/openslides/users/models.py

@@ -271,12 +271,26 @@ class User(RESTModelMixin, PermissionsMixin, AbstractBaseUser):
         except KeyError as err:
             raise ValidationError({"detail": "Invalid property {0}", "args": [err]})
 
+        from_email = config["users_email_sender"].strip()
+        blacklist = ("[", "]", "\\")
+        if any(x in from_email for x in blacklist):
+            blacklist_str = '"' + '", "'.join(blacklist) + '"'
+            raise ValidationError(
+                {
+                    "detail": "Invalid characters in the sender name configuration. "
+                    + f"Not allowed: {blacklist_str}"
+                }
+            )
+        if from_email:
+            from_email += " "
+        from_email += f"<{settings.DEFAULT_FROM_EMAIL}>"
+
         # Create an email and send it.
         email = mail.EmailMessage(
-            subject,
-            message,
-            config["users_email_sender"] + " <" + settings.DEFAULT_FROM_EMAIL + ">",
-            [self.email],
+            subject=subject,
+            body=message,
+            from_email=from_email,
+            to=[self.email],
             reply_to=[config["users_email_replyto"]],
         )
         try: