When you post a link on Facebook, the URL will be acessed by Facebook bot in order to get the details of that page or article.
Sucuri Firewall won’t block good bots such as Google, Bing, Facebook, etc. by default, but there are special filters that could interfere with bot activity on your website.
Note: To check if the solutions described here work without having to post often to Facebook, you can use the Facebook Sharing Debugger.
Geo Blocking
One of the most used WAF features, Geo Blocking allows you to filter which country or region can view or interact with your website. While not by itself a bullet-proof method, it decreases resource usage on the hosting server and can be very useful in blocking a large scale DDoS attack.
To speed up the delivery of content, Facebook has several facilities around the world. According to datacenters.com, Facebook operates datacenters in the United States, Ireland, England, Sweden and Denmark.
If Geo Blocking is set to block any of these countries, links may not be accessible by the Facebook bot, despite our best efforts to allow well known bots. Therefore, we recommend not blocking any of these countries, nor the regions of Europe and the United Kingdom.
Emergency DDoS
There is a security filter called “Emergency DDoS Protection”, it’s located on the Security tab of the WAF dashboard:
This filter adds a JavaScript check to all requests to the website and could prevent legitimate bots from reaching the website. They would see a “You are being redirected” message instead of the page content.
If the redirection message rings a bell, then you must disable the Emergency DDoS Protection setting to prevent the block.
Aggressive Bot Filter
The Sucuri WAF will block bad requests by default, but to increase protection against bad-bots, you can enable the Aggressive Bot Filter setting located in the Security tab of the WAF dashboard.
The Aggressive Bot Filter setting should not cause problems with legitimate bots, such as the Facebook bot, but if the first two options do not resolve your issue, and Aggressive Bot Filter is enabled, disabling this setting could help.