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 by using the following command:

$ ss -l46ut
Netid State Recv-Q Send-Q Local Address:Port Peer Address:Port Process
tcp LISTEN 0 511 *:http *:*
tcp LISTEN 0 128 [::]:ssh [::]:*
tcp LISTEN 0 128 [::]:ipp [::]:*
tcp LISTEN 0 128 [::]:smtp [::]:*


For each entry, verify that the ufw is configured to rate limit the service ports by using the following command:

$ sudo ufw status
Status: active

To Action From
-- ------ ----
80/tcp LIMIT Anywhere
25/tcp LIMIT Anywhere
Anywhere DENY 240.9.19.81
443 LIMIT Anywhere
22/tcp LIMIT Anywhere
80/tcp (v6) LIMIT Anywhere
25/tcp (v6) LIMIT Anywhere
22/tcp (v6) LIMIT Anywhere (v6)

25 DENY OUT Anywhere
25 (v6) DENY OUT Anywhere (v6)

If any port with a state of "LISTEN" that does not have an action of "DENY", 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 Ubuntu 22.04 LTS is implementing rate-limiting measures on impacted network interfaces.

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

$ sudo ufw limit

Rate-limiting can also be done on an interface. An example of adding a rate limit on the "ens160" interface follows:

$ sudo ufw limit in on ens160