Vulnerability Discussion

Some routers will send responses to broadcast frames that violate RFC-1122, which fills up a log file system with many useless error messages. An attacker may take advantage of this and attempt to flood the logs with bogus error logs. Ignoring bogus ICMP error responses reduces log size, although some activity would not be logged.

The sysctl --system command will load settings from all system configuration files. All configuration files are sorted by their filename in lexicographic order, regardless of the directories in which they reside. If multiple files specify the same option, the entry in the file with the lexicographically latest name will take precedence. Files are read from directories in the following list from top to bottom. Once a file of a given filename is loaded, any file of the same name in subsequent directories is ignored.

/etc/sysctl.d/*.conf
/run/sysctl.d/*.conf
/usr/local/lib/sysctl.d/*.conf
/usr/lib/sysctl.d/*.conf
/lib/sysctl.d/*.conf
/etc/sysctl.conf

Check

The runtime status of the net.ipv4.icmp_ignore_bogus_error_responses kernel parameter can be queried by running the following command:

$ sudo sysctl net.ipv4.icmp_ignore_bogus_error_responses

net.ipv4.icmp_ignore_bogus_error_responses = 1

If "net.ipv4.icmp_ignore_bogus_error_responses" is not set to "1", this is a finding.

Check that the configuration files are present to enable this network parameter.

$ sudo grep -r net.ipv4.icmp_ignore_bogus_error_response /run/sysctl.d/*.conf /usr/local/lib/sysctl.d/*.conf /usr/lib/sysctl.d/*.conf /lib/sysctl.d/*.conf /etc/sysctl.conf /etc/sysctl.d/*.conf

/etc/sysctl.d/99-sysctl.conf:net.ipv4.icmp_ignore_bogus_error_responses = 1

If "net.ipv4.icmp_ignore_bogus_error_response" is not set to "1" or is missing, this is a finding.

If conflicting results are returned, this is a finding.

Fix

Configure RHEL 9 to not log bogus ICMP errors:

Add or edit the following line in a single system configuration file in the "/etc/sysctl.d/" directory:

net.ipv4.icmp_ignore_bogus_error_responses = 1

Load settings from all system configuration files with the following command:

$ sudo sysctl --system