If WHMCS tells you to upgrade your server's version of cURL to prevent security issues, do not be alarmed.
cURL 7.35.0 is the most recent and secure version for Ubuntu 14.04.
cURL 7.47.0 is the most recent and secure version for Ubuntu 16.04.
cURL 7.58.0 is the most recent and secure version for Ubuntu 18.04.
cURL 7.68.0 is the most recent and secure version for Ubuntu 20.04.
cURL 7.81.0 is the most recent and secure version for Ubuntu 22.04.
Like most Linux distributions, Ubuntu backports security patches so that software like cURL can stay secure without breaking other software by changing the version.
You can find the latest version number of cURL for your Ubuntu version below.
Ubuntu 14.04:
https://launchpad.net/ubuntu/trusty/+source/curl/+changelog.
Ubuntu 16.04:
https://launchpad.net/ubuntu/xenial/+source/curl/+changelog.
Ubuntu 18.04:
https://launchpad.net/ubuntu/bionic/+source/curl/+changelog.
Ubuntu 20.04:
https://launchpad.net/ubuntu/focal/+source/curl/+changelog.
Ubuntu 22.04:
https://launchpad.net/ubuntu/jammy/+source/curl/+changelog.
Ubuntu 24.04:
https://launchpad.net/ubuntu/noble/+source/curl/+changelog.