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OpenSSH: client bug CVE-2016-0777

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Re: OpenSSH: client bug CVE-2016-0777

#41

Post by kardos »

DrRobinson wrote: post #40
kardos wrote: post #30

Earlier quoted context omitted.

So the real question is, can a MITM intercept connections to boxen you frequent to exploit this? Or is it limited to connecting to hostile honeypots?

Apparently not possible with MITM. "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." https://lists.mindrot.org/pipermail/openssh-unix-dev/2016-Ja...

... and those that you haven't established TOFU with yet

Re: OpenSSH: client bug CVE-2016-0777

#42

Post by ryanlol »

jlgaddis wrote: post #39

ETA: I was way off... you can ignore this... :-) cf. section 3.3.5 [0], which describes "Roaming (Suspend/Resume)". This is documentation for an application by a company called AppGate (later acquired by Cryptzone) that wrote {some|most} of the code in OpenSSH's "roaming_client.c". This gives a hint of what the ramifications may be: basically, a MITM, who observed the initial session negotiation, can disconnect the c…

That's not correct though, MITM cannot exploit this. A malicious SSH server can exploit this during a connection though, but only after host key verification.

Re: OpenSSH: client bug CVE-2016-0777

#43

Post by agwa »

StefanKarpinski wrote: post #34

Does it strike anyone else as bizarre / poor form for an experimental feature to be enabled by default in OpenSSH, which is normally very conservative with option defaults?

What's more, it was not documented in the ssh_config(5) man page. If it had been, I probably would have disabled it long ago when hardening my SSH config.

Time to head to the source to look for other undocumented options...

Update: my findings are here (scroll to bottom for the upshot): https://gist.github.com/AGWA/e92d4f5343be1f7a941d

UseRoaming is the only one to be concerned about. There are many other undocumented options, but they're all aliases for a documented option or are deprecated/unsupported.

Re: OpenSSH: client bug CVE-2016-0777

#45

Post by noja »

agwa wrote: post #43
StefanKarpinski wrote: post #34

Does it strike anyone else as bizarre / poor form for an experimental feature to be enabled by default in OpenSSH, which is normally very conservative with option defaults?

What's more, it was not documented in the ssh_config(5) man page. If it had been, I probably would have disabled it long ago when hardening my SSH config. Time to head to the source to look for other undocumented options... Update: my findings are here (scroll to bottom for the upshot): https://gist.github.com/AGWA/e92d4f5343be1f7a941d UseRoaming is the only one to be concerned about. There are many other undocumente…

Please report back what you find!

Re: OpenSSH: client bug CVE-2016-0777

#46

Post by ralphm »

jlgaddis wrote: post #2

Workaround (yes, it's client-side): # echo -e "Host *\n\tUseRoaming no\n" >> /etc/ssh/ssh_config Disclaimer: won't work on all operating systems, shells, etc. YMMV. Consult a doctor before following any advice you get from the Internet. Void where prohibited. Restrictions may apply. Edited per comments below

Don't ever fix your `ssh_config` by appending stuff to the end of the file. The configuration syntax allows for block constructs without an explicit end marker (like `Match` and `Host`). Appending will cause all kinds of sadness.

Re: OpenSSH: client bug CVE-2016-0777

#48

Post by ryanlol »

agwa wrote: post #43
StefanKarpinski wrote: post #34

Does it strike anyone else as bizarre / poor form for an experimental feature to be enabled by default in OpenSSH, which is normally very conservative with option defaults?

What's more, it was not documented in the ssh_config(5) man page. If it had been, I probably would have disabled it long ago when hardening my SSH config. Time to head to the source to look for other undocumented options... Update: my findings are here (scroll to bottom for the upshot): https://gist.github.com/AGWA/e92d4f5343be1f7a941d UseRoaming is the only one to be concerned about. There are many other undocumente…

https://github.com/openssh/openssh-portable/blob/e6c85f8889c... Here you go

Re: OpenSSH: client bug CVE-2016-0777

#49

Post by mapgrep »

DrRobinson wrote: post #23
merqurio wrote: post #17

Earlier quoted context omitted.

Is there any way to update openssh on my side, without waiting to apple ? Is this enough ? https://mochtu.de/2015/01/07/updating-openssh-on-mac-os-x-10...

You can patch it with: # echo 'UseRoaming no' >> /etc/ssh/ssh_config or $ echo "UseRoaming no" >> ~/.ssh/config

The second one. There is no /etc/ssh on a stock Mac. (Although there is a /private/etc/ssh_config on mine, without the intervening ssh dir)

Re: OpenSSH: client bug CVE-2016-0777

#50

Post by sneak »

fulafel wrote: post #33
rnhmjoj wrote: post #31

Does this affect mosh?

No, the roaming in Mosh is unrelated to OpenSSH's roaming feature.

Irrelevant; it still affects mosh as mosh does an ssh connection first to do auth and key exchange. That ssh session is potentially vulnerable when connecting to a malicious or compromised host.
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