CVE-2020-25685

A flaw was found in dnsmasq. When getting a reply from a forwarded query, dnsmasq checks in forward.c:reply_query(), which is the forwarded query that matches the reply, by only using a weak hash of the query name. Due to the weak hash (CRC32 when dnsmasq is compiled without DNSSEC, SHA-1 when it is) this flaw allows an off-path attacker to find several different domains all having the same hash, substantially reducing the number of attempts they would have to perform to forge a reply and get it accepted by dnsmasq. This is in contrast with RFC5452, which specifies that the query name is one of the attributes of a query that must be used to match a reply. This flaw could be abused to perform a DNS Cache Poisoning attack. If chained with CVE-2020-25684 the attack complexity of a successful attack is reduced. The highest threat from this vulnerability is to data integrity.

Mitigation

The impact of this flaw can be reduced by disabling the dnsmasq cache by adding --cache-size=0 when calling dnsmasq or by adding a line with cache-size=0 to the dnsmasq configuration file (/etc/dnsmasq.conf by default).

When using Red Hat Enterprise Linux 8.3 with libvirt through a virt:rhel module, use virsh net-edit <network-name> and reference <https://libvirt.org/formatnetwork.html#elementsNamespaces&gt; to add the suggested option cache-size=0.

There is no way to customize the dnsmasq configuration generated by libvirt, when using versions of Red Hat Enterprise Linux prior to version 8.3. If dnsmasq is being run through NetworkManager, create a new file in /etc/NetworkManager/dnsmasq.d/ and add cache-size=0 to it.

In all cases, by disabling the cache, you may experience a performance loss in your environment due to all DNS queries being forwarded to the upstream servers. Please evaluate if the mitigation is appropriate for the system’s environment before applying.