Effects much worse for other distributions than expected [LWN.net]

Effects much worse for other distributions than expected

Posted May 15, 2008 10:48 UTC (Thu) by erich (subscriber, #7127)
Parent article: Debian vulnerability has widespread effects

What concerns me most is that other distribution users are likely to assume they're safe. They're not necessarily so. They're only safe if none of their users is/was running Debian or Ubuntu.
It's very simple:

Server A runs some 'unaffected' Linux distribution
User B is runinng an 'affected' Linux distribution
User B enables key-based logins on Server A to his account / maybe even the root account
Since his key is weak, Logins to Server A can be bruteforced easily.

So if any of your users might be running Debian or Ubuntu - so if he might have a weak key - you should update OpenSSH to a version with the blacklist of known weak keys shipped by Debian and Ubuntu.

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Effects much worse for other distributions than expected

Posted May 15, 2008 11:47 UTC (Thu) by nix (subscriber, #2304) [Link]

I *believe* that logins *from* server A to server B are safe, even if server A is using a DSA
key, because server B never knows anything but the public half of that key (which is, well,
public).

Am I right?

Effects much worse for other distributions than expected

Posted May 15, 2008 15:45 UTC (Thu) by rfunk (subscriber, #4054) [Link]

I think you're backwards.  Or maybe I am.  Referring to two servers rather 
than a server and a client makes this more confusing; in any ssh 
connection, one side is acting as a server and the other side is acting as 
a client, no matter what other purpose the two machines have.

When using public-key authentication, the ssh server knows the public half 
of the key, and the ssh client knows the private half of key (and also the 
public half).

If the key is vulnerable, then any client given a bunch of tries can guess 
the private half of the key.

Effects much worse for other distributions than expected

Posted May 15, 2008 19:19 UTC (Thu) by nix (subscriber, #2304) [Link]

Er, yeah, sorry, bad phrasing. If the client (from whom you're connecting, 
which has the secret key) is not vulnerable, and the server (to which 
you're connecting, and which has the public key) is vulnerable, you are 
safe: otherwise, you are not.

Effects much worse for other distributions than expected

Posted May 15, 2008 20:24 UTC (Thu) by rfunk (subscriber, #4054) [Link]

Actually I wouldn't say you're entirely safe if the server is vulnerable and you're not.  
There's still the issue of the host key, which is used to prevent the bad guys from 
pretending to be the server.  If that host key is compromised, then someone can pretend 
to be the server.  Then you're in a little trouble if they can also get your public key (it's 
treated as public, shouldn't be horribly hard), and more trouble if you're using password 
authentication.