The correct answer is 'Apply the patch provided by the vendor to mitigate the vulnerability,' because once a CVE number is assigned to a known vulnerability, vendors usually provide patches or updates to address the issue. It's essential to apply these patches promptly to protect the network from being exploited. Installing a firewall is a good security practice but does not address the specific vulnerability. Disabling the affected server could mitigate the risk; however, it may not be practical or necessary if a patch is available. Changing network topologies, adding more servers, or conducting a risk assessment are subsequent actions that might be appropriate but would not be the immediate response to an active exploitation of a known vulnerability.