SharePoint 2013 had new feature called HTML Field Security,It will restrict the user to place any of the websites in the iframe in pages of your site.
You can set the below permissions
- Do not permit contributors to insert iframes from external domains into pages on this site.
- Permit contributors to insert iframes from any external domain into pages on this site.
- Permit contributors to insert iframes from the following list of external domains into pages on this site.
Simply you need to add the site names in the list box to restrict or allow the users to use the iframe.
*This feature will not helpful when user trying to put the iframe in webpart through code.