Fix for 404 error while updating and editing posts

Yesterday when I was trying to publish a post it was redirecting me to 404 error page. The problem was with only one particular page and it was some way related to the post content. I was able to save the post when I removed some part of the post. I tried a lot of

[Security Risk] Add this fix asap

There is an XSS vulnerability in SWIFT, you should fix it asap.
Even if you are not a swift user you should check your theme for the following vulnerability and fix it.