CVE-2019-6477

A flaw was found in the way bind limited the number of TCP clients that can be connected at any given time. A remote attacker could use one TCP client to send a large number of DNS requests over a single connection, causing exhaustion of the pool of file descriptors available to named, and potentially affecting network connections and the management of files such as log files or zone journal files.

The patch for CVE-2018-5743 introduced a change in the way bind calculated the number of concurrent connections, from counting the outstanding TCP queries to counting the TCP client connections. However this functionality was not correctly implemented, a attacker could use a single TCP connection to send large number of DNS requests causing denial of service. As per upstream the fix does not help in a situation where a TCP-pipelining client is sending queries at an excessive rate, allowing a backlog of outstanding queries to build up. More details about this is available in the upstream advisory.

This bind flaw can be exploited by a remote attacker (AV:N) by opening large number of simultaneous TCP client connections with the server. The attacker needs to use a server which has TCP-pipelining capability to use one TCP connection to send large number of requests. (AC:L and PR:N) No user interaction is required from the server side (UI:N). The attacker can cause denial of service (A:H) by exhausting the file descriptor pool which named has access to. (S:U)

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 vulnerability can be mitigated by disabling server TCP-pipelining:

       keep-response-order { any; };

and then restarting BIND. The server restart is necessary because neither a 'reload' nor a 'reconfig' operation will properly reset currently pipelining TCP clients. Disabling TCP-pipelining entirely is completely effective at mitigating the vulnerability with minimal impact to clients that use pipelined TCP connections and with no impact to clients that do not support TCP-pipelining. The majority of Internet client DNS queries are transported over UDP or TCP without use of TCP-pipelining. Note: This mitigation will only work with bind-9.11 and above.