We have discovered a potential Denial of Service / Amplification Attack in nbdkit. Lifecycle --------- Reported: 2019-09-11 Fixed: 2019-09-11 Published: 2019-09-12 There is no CVE number assigned for this issue yet, but the bug is being categorized and processed by Red Hat's security team which may result in a CVE being published later. Credit ------ Reported and patched by Richard W.M. Jones <rjones(a)redhat.com&gt;. Reviewed by Eric Blake <eblake(a)redhat.com&gt; and Daniel Berrangé <berrange(a)redhat.com&gt;. Description ----------- nbdkit is a Network Block Device (NBD) server with multiple plugins. An attacker, by connecting to nbdkit’s TCP port, is able to make the current plugin’s .open() callback run. For some plugins this callback does a considerable amount of work, for example connecting to a remote machine (nbdkit-ssh-plugin) or launching a small VM (nbdkit-guestfs-plugin). Thus for a small amount of work done by the attacker (opening many TCP connections but not sending any data), a large amount of work is done by nbdkit (eg. making many full ssh connections to a remote machine or launching hundreds of VMs). A related problem is that if nbdkit is configured to use TLS for client authentication, even clients which are not authorized are able to attack nbdkit in this way because all they have to do is open a TCP connection. Mitigating this is that the current plugin is NOT selected by the attacker, but is part of the server configuration. Therefore the effectiveness of the attack depends on the particular configuration. There are two situations in which you will NOT be affected: * You are not exposing nbdkit over TCP (eg. using nbdkit -U), or the TCP port is not accessible to untrusted attackers (eg. private LAN, firewall rules). * The plugin you are using does not do much work in its .open() callback. For example the memory plugin does nothing in its callback so is not vulnerable: https://github.com/libguestfs/nbdkit/blob/afbe7f11098df256acf8eafa9d4ec1e... Test if nbdkit is a vulnerable version -------------------------------------- Run the following command (requires bash): nbdkit -fv null --run 'sleep 1 >/dev/tcp/localhost/10809' 2>&1 | grep -q 'open readonly' && echo vulnerable || echo patched Workarounds ----------- It is recommended to upgrade to a fixed version of nbdkit (see next section). However if this cannot be done, apply network filtering (eg. firewall, TCP wrappers, etc.) to ensure that untrusted clients cannot connect to nbdkit’s public TCP port. Or use a Unix domain socket instead of a loopback TCP connection. Fixes ----- This affects all versions of nbdkit. A fix is available for 1.12, 1.14 and the current development branch. * development branch (1.15) https://github.com/libguestfs/nbdkit/commit/c05686f9577fa91b6a3a4d8c06595... or use nbdkit >= 1.15.1 from http://download.libguestfs.org/nbdkit/1.15-development/ * stable branch 1.14 https://github.com/libguestfs/nbdkit/commit/e06cde00659ff97182173d0e33fff... or use nbdkit >= 1.14.1 from http://download.libguestfs.org/nbdkit/1.14-stable/ * stable branch 1.12 https://github.com/libguestfs/nbdkit/commit/22b30adb796bb6dca264a38598f80... or use nbdkit >= 1.12.7 from http://download.libguestfs.org/nbdkit/1.12-stable/ -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-df lists disk usage of guests without needing to install any software inside the virtual machine. Supports Linux and Windows. http://people.redhat.com/~rjones/virt-df/