How do I know if mod_security is running?

mod_security is NOT part of standard apache2…it's a 3rd-party pay-for product. If you're running an apache2 from a vanilla Linux distro, you most likely don't have it.

To see if it (or any module) is enabled, do:

ls /etc/apache2/mods-enabled | grep mod_security

mod_security will be EOL'd on 7/1/2024:

https://www.trustwave.com/en-us/resources/security-resources/software-updates/end-of-sale-and-trustwave-support-for-modsecurity-web-application-firewall/

-- sw

I understood, thank you very much.

I followed a guide here at Linode, thought everything was fine (I think they needed to updated such things).

Well, the fault was mine, of course, because I didn't search before. :(