StackPath's Web Application Firewall can present incoming traffic a number of different challenges, depending on triggered rules. You can read more about why these challenges happen here.
A security screen is displayed under any of these conditions, based on the severity of the rule that was triggered, the reputation of the requester's IP, and a number of other considerations.
The Block Screen will be displayed any time a visitor has received a hard block and will be accompanied by a 403 response code, completely denying access to the requester.
The StackPath captcha is designed to allow access to human viewers while stopping automated requests by forcing visitor input before access can be granted.
StackPath may also show in the Tab Title which is normal and is just there to represent that this validation period is occurring. This test is also used to block automated traffic during application-layer DDoS attacks.
If you have any questions or believe any of these security screens were issued by mistake, our support is available 24/7.