CVE-2021-25218

A flaw was found in bind. An assertion failure is triggered, resulting in termination of the named server process, if named attempts to respond over UDP with a response that is larger than the current effective interface maximum transmission unit (MTU), and if response-rate limiting (RRL) is active. The highest threat from this vulnerability is to system availability.

An attacker may abuse the Path MTU discovery (PMTUD) protocol to trick bind into exceeding the interface MTU. Response Rate Limiting (RRL) is not enabled by default for user defined views nor the builtin one, but it is enabled by default for the default builtin CHAOS class view, which bind uses to provide various information.

This issue did not affect the versions of bind as shipped with Red Hat Enterprise Linux 6, 7, and 8 as they provide an older version of the code which does not allow to trigger the assertion.

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.

Disabling RRL in all views, including the builtin CHAOS class view, prevents the faulty assertion from being reached in the vulnerable versions of bind. To do that you can remove rate-limit from your named.conf files and provide a replacement for the builtin CHAOS view, like the one below:

view override_bind chaos {
        recursion no;
        notify no;
        allow-new-zones no;
        max-cache-size 2M;

        zone "version.bind" chaos {
                type primary;
                database "_builtin version";
        };
        zone "hostname.bind" chaos {
                type primary;
                database "_builtin hostname";
        };
        zone "authors.bind" chaos {
                type primary;
                database "_builtin authors";
        };
        zone "id.server" chaos {
                type primary;
                database "_builtin id";
        };
};