via wordfence.com => original post link
On Wednesday afternoon a small percentage of WPEngine websites using a paid version of Wordfence experienced a 500 Internal Server Error or white screen on their sites due to an erroneous firewall rule that we released. If you have experienced this issue, please check your email which contains instructions to fix the issue. You can also find guidance on our Twitter account along with our forums where we have posted a solution. We have also been hard at work in our ticketing system answering support requests from our affected customers. You can open a ticket by signing into this site and visiting your Licenses page, and clicking Get Help on the applicable license. You can also find instructions for the fix in this longer more detailed post.
Please keep in mind only a small percentage of WPEngine users using Wordfence were affected, and these were limited to paid users only due to the way we release firewall rules.
The rest of this post contains an after-action report of the root cause and what we’re doing about the issue.