This is the most serious bug you'll hear about this week: the issues identified and fixed in OpenSSH are dubbed CVE-2016-0777 and CVE-2016-0778.

An early heads up came from Theo de Raadt in this mailing list posting.

Until you are able to patch affected systems, the recommended workaround is to use

# echo -e 'Host *\nUseRoaming no' >> /etc/ssh/ssh_config

That is, add the option UseRoaming no to your /etc/ssh/ssh_config (or your user's ~/.ssh/config) file, or start your ssh client with -oUseRoaming=no included on the commandline.

We will be updating this article with more information as it becomes available.

UPDATE: This affects OpenSSH versions 5.4 through 7.1.

UPDATE: The following commit from deraadt@ has just gone in:

CVSROOT:        /cvs
Module name:    src
Changes by:     deraadt@cvs.openbsd.org 2016/01/14 07:34:34

Modified files:
        usr.bin/ssh    : readconf.c ssh.c

Log message:
Disable experimental client-side roaming support.  Server side was
disabled/gutted for years already, but this aspect was surprisingly
forgotten. Thanks for report from Qualys

UPDATE: Errata patches for 5.8 and 5.7 have been published.

UPDATE: Portable OpenSSH 7.1p2 has been released:

 * SECURITY: ssh(1): The OpenSSH client code between 5.4 and 7.1
   contains experimential support for resuming SSH-connections (roaming).

   The matching server code has never been shipped, but the client
   code was enabled by default and could be tricked by a malicious
   server into leaking client memory to the server, including private
   client user keys.

   The authentication of the server host key prevents exploitation
   by a man-in-the-middle, so this information leak is restricted
   to connections to malicious or compromised servers.

   MITIGATION: For OpenSSH >= 5.4 the vulnerable code in the client
   can be completely disabled by adding 'UseRoaming no' to the global
   ssh_config(5) file, or to user configuration in ~/.ssh/config,
   or by passing -oUseRoaming=no on the command line.

UPDATE: Fixed versions are available for OpenBSD snapshots dated 2016-01-12 and later. M:Tier has binpatches for OpenBSD 5.7-stable and 5.8-stable. Debian, Ubuntu, RHEL, and many other Linux distros have it now or will soon.

UPDATE: The roaming code has been stripped out of OpenBSD -current:

CVSROOT:	/cvs
Module name:	src
Changes by:	markus@cvs.openbsd.org	2016/01/14 09:17:40

Modified files:
	usr.bin/ssh    : clientloop.c kex.c kex.h monitor.c 
	                 monitor_wrap.c opacket.c opacket.h packet.c 
	                 packet.h readconf.c readconf.h serverloop.c 
	                 ssh.c ssh2.h sshconnect.c sshconnect2.c sshd.c 
	usr.bin/ssh/lib: Makefile 
	usr.bin/ssh/ssh: Makefile 
	usr.bin/ssh/ssh-keyscan: Makefile 
	usr.bin/ssh/ssh-keysign: Makefile 
	usr.bin/ssh/sshd: Makefile 
Removed files:
	usr.bin/ssh    : roaming.h roaming_client.c roaming_common.c 
	                 roaming_dummy.c roaming_serv.c 

Log message:
remove roaming support; ok djm@

UPDATE: The FreeBSD port has been updated, but the version in their base system remains vulnerable.

UPDATE: Qualys Security has posted their full report on the issues.

UPDATE: While the information leak is much more difficult to exploit on systems with ASLR, like OpenBSD, some users may want to consider rotating their key pairs. If you use ssh-agent(1), however, the man page offers some good news:

The agent will never send a private key over its request channel. Instead, operations
that require a private key will be performed by the agent, and the result will be
returned to the requester. This way, private keys are not exposed to clients using the
agent.

UPDATE: For Mac OS X, the version of OpenSSH in MacPorts has been updated. Since Apple typically delays security fixes, you're advised to apply the workaround if using the bundled OpenSSH instead.