ssh-add: local private keys added to forwarded agents

[Nicolas.Williams at ubsw.com]

The behaviour you describe does not violate the [draft] specification. Clearly, many (most!) SSH users do not store keys in smartcards or any other kind of removable media, and noone claims such behaviour to be in violation of the [draft] spec. Note the word "ideally" in the spec text you quote.

Also, your statement about the agent socket names is incorrect. It is "ls -F" that is adding that '=' to the end of the socket name.

Having said this, I would second any proposal that ssh-add require an additional argument to force the addition of keys to forwarded agents; this would require a mechanism to tell the difference between local and forwarded agents and, in practice, I imagine either a socket naming pattern or an environment variable's setting would be used to tell the difference, though neither approach would be foolproof.

But I am not making such a proposal. I'm big enough to keep track of and know which sessions are which and which sessions have forwarded agents and which don't.

Nico
--  

> -----Original Message-----
> From: Dave Ryan [mailto:dave at ugc.org.uk]
> Sent: Wednesday, June 05, 2002 6:08 AM
> To: openssh-unix-dev at mindrot.org
> Subject: ssh-add: local private keys added to forwarded agents
> 
> 
> Hi,
> 
> This may or may not cause concern for some people 
> (considering a lot of 
> people store all of their keys on a single client system).
> 
> Snippet from draft-ietf-secsh-agent-00.txt:
> 
> 2. Security Considerations
> 
>    This protocol is designed only to run as a channel of the SSH
>    protocol.
> 
>    The goal of this extension is to ensure that the users private keys
>    never leave the machine they are physically at.  Ideally 
> the private
>    keys should be stored on a password protected removable 
> media such as
>    a smartcard.
> 
> I noticed that ssh-add will add a private key to a forwarded agent, if
> there are no local agents started by that user - this breaks the draft
> specification as private keys on a local host are added to an agent 
> running on a remote host. 
> 
> For example,
> 
> USERA starts ssh-agent on HOSTA. USERA then ssh's to HOSTB, USERA then
> runs ssh-add on HOSTB, the private keys from HOSTB are then 
> added to the
> ssh-agent on HOSTA.
> 
> If USERA had started ssh-agent on HOSTB and then ran ssh-add, 
> the keys 
> would have remained on local to the system. 
> 
> I also noticed that if there are no local agents running a 
> remote agent
> socket will show up in /tmp/ssh-XXXXXXXX/ as agent.$PID= whereas if a 
> local agent IS running the "=" is dropped.
> 
> I'm not sure if it is appropriate to apply mechanisms to ssh-add to 
> prevent it adding local keys to a forwarded agent or if a quick 
> addition to the man pages will suffice.
>  
> If this has been discussed before I apologise, couldn't find any 
> references to anything similar.
> 
> Cheers,
> Dave.
> 
> -- 
> ugc Security Research
> http://www.ugc.org.uk/~dave
> _______________________________________________
> openssh-unix-dev at mindrot.org mailing list
> http://www.mindrot.org/mailman/listinfo/openssh-unix-dev
> 

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