CVE-2020-25705

A flaw in the way reply ICMP packets are limited in the Linux kernel functionality was found that allows to quickly scan open UDP ports. This flaw allows an off-path remote user to effectively bypassing source port UDP randomization. The highest threat from this vulnerability is to confidentiality and possibly integrity, because software that relies on UDP source port randomization are indirectly affected as well.

This issue is rated as having Moderate impact because of the attack scenario limitation. It is possible to harm the networking services only, but not for the overall system under attack, and impossible to get access to this remote system under attack.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

The mitigation is to disable ICMP destination unreachable messages.

The commands to disable UDP port unreachable ICMP reply messages:

iptables -I OUTPUT -p icmp --icmp-type destination-unreachable -j DROP service iptables save

For additional information about "service iptables save" please read https://access.redhat.com/solutions/1597703 It is not recommended to apply this rule if host being used as forwarder (router) of IP packets.

Or it is possible to use this firewall-cmd instead of iptables and the result is similar: firewall-cmd --permanent --direct --add-rule ipv4 filter OUTPUT 0 -p icmp --icmp-type destination-unreachable -j DROP