Vulnerability Discussion

Denial of service (DoS) is a condition when a resource is not available for legitimate users. When this occurs, the organization either cannot accomplish its mission or must operate at degraded capacity.

This requirement addresses the configuration of the operating system to mitigate the impact of DoS attacks that have occurred or are ongoing on system availability. For each system, known and potential DoS attacks must be identified and solutions for each type implemented. A variety of technologies exist to limit or, in some cases, eliminate the effects of DoS attacks (e.g., limiting processes or establishing memory partitions). Employing increased capacity and bandwidth, combined with service redundancy, may reduce the susceptibility to some DoS attacks.

Check

Verify an application firewall is configured to rate limit any connection to the system.

Check all the services listening to the ports with the following command:

$ sudo ss -l46ut

Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
tcp LISTEN 0 128 [::]:ssh [::]:*

For each entry, verify that the Uncomplicated Firewall is configured to rate limit the service ports with the following command:

$ sudo ufw status

Status: active

To Action From
-- ------ ----
22/tcp LIMIT Anywhere
22/tcp (v6) LIMIT Anywhere (v6)

If any port with a state of "LISTEN" is not marked with the "LIMIT" action, this is a finding.

Fix

Configure the application firewall to protect against or limit the effects of DoS attacks by ensuring the Ubuntu operating system is implementing rate-limiting measures on impacted network interfaces.

Check all the services listening to the ports with the following command:

$ sudo ss -l46ut

Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
tcp LISTEN 0 128 [::]:ssh [::]:*

For each service with a port listening to connections, run the following command, replacing "[service]" with the service that needs to be rate limited.

$ sudo ufw limit [service]

Rate-limiting can also be done on an interface. An example of adding a rate-limit on the eth0 interface follows:

$ sudo ufw limit in on eth0