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OpenSSH cutting edge
Federico Biancuzzi, 2005-12-19

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Looking at the stats on your website, it seems that more than 70% of the servers print the banner SSH-1.5 or SSH-1.9. This means that they accept SSHv1. I think this is a bad thing. Why don't you remove the support for version 1 of the protocol from OpenSSH?

Damien Miller: We definitely need support for protocol 1 in the client, as lots of devices still only support that version. It is useful to have it in the server, at least so we can test the client. I would like to turn protocol 1 off by default in the server some time in the next few years, but we don't have any desire to remove it altogether.

Another statistic suggests that more than 80% of the SSH servers on the Internet run OpenSSH. I'm wondering if you have ever verified which version they are running, and what is the average behaviour of an OpenSSH administrator. Does people update the server as soon as a new release is available?

Damien Miller: Funny you mention this, we just completed another version survey with the assistance of Mark Uemura from OpenBSD Support Japan. The results of this should be going up on OpenSSH.com soon.

I don't have detailed OpenSSH version histories for usage surveys before last year's. Certainly the use of paleolithic versions (such as 2.x) is very infrequent, but beyond this it is difficult to tell how quickly users update - many vendors will keep relatively ancient versions (such as 3.1p1) on life-support with spot security fixes. This will avoid known security problems, but it doesn't give their users the benefit of any of the proactive work that we do, nor any of the new features.

It is worth noting that OpenBSD, which has a very conservative policy on its stable trees, typically updates supported OpenBSD releases to the latest OpenSSH version when it is released.

Talking about Microsoft Windows, we often hear the theory that the most used software is targeted by exploit writers because of its market share. However looking back at OpenSSH history I don't see a lot of big exploits, so I'm wondering if this happened because you focus on code quality and security, or maybe because OpenSSH is open source and for some reasons there is less honor in announcing a public exploit for it?

Damien Miller: I don't think being open source is any discouragement to exploit writers, quite the opposite - it is easier to find bugs and exploit them when you have the code. OpenSSH is a very attractive target too - it is ubiquitous and exploit-writers take some pride (or at least schadenfreude) in finding OpenBSD holes.

We are painfully cognisant of impact that any security vulnerability would have across our users, so we take a lot of care to avoid them. Beyond the obvious practicalities of not making mistakes, we have procedural safeguards (every commit must be cross-checked and OKed by at least one other developer) and we use technical safeguards such as privilege separation.

Being very popular means also being a good platform for a worm. Did you adopt any specific measures to fight automated attacks?

Damien Miller: Privilege separation alone probably makes a worm targeting a bug in sshd impractical. An attacker would need to break into the unprivileged sshd process that deals with network communications and, because this just gives them access to an unprivileged and chrooted account, then exploit a second vulnerability to either break the privileged monitor sshd or escalate privilege via a kernel bug. This would add a fair amount of complexity, fragility and size to a worm - it would probably need to implement a fair chunk of the SSH protocol just to propagate.

We also implemented self re-execution at the c2k4 Hackathon. This changes sshd so that instead of forking to accept a new connection, it executes a separate sshd process to handle it. This ensures that any run-time randomizations are reapplied to each new connection, including ProPolice/SSP stack canary values, shared library randomizations, malloc randomizations, stack gap randomizations, etc.

Without re-exec, all sshd child processes would share the same randomizations. This would allow an attacker to exhaustively search for the right offsets and values for their exploit by making many connections (millions probably) to the server. With re-exec, each time they connect the values will all be different so there is no guarantee that they will ever stumble upon the right combination.

Another security improvement, just introduced in openssh-4.2 was the "zlib@openssh.com" compression method. This was an idea that Markus Friedl had after the last zlib vulnerability was published.

The SSH protocol has supported zlib compression for a long time, but the standard "zlib" protocol method requires this to be started early in the protocol: after key exchange, but (critically) before user authentication successfully completed. This exposes the compression code to unauthenticated users.

Our solution is to define a new compression method that still performs zlib compression, but delays its start until after user authentication has finished, so only authenticated users get to see it. This is another significant reduction in attack surface with effectively zero performance impact. This also makes the writing of a worm that targets the zlib code in OpenSSH impossible.

What about the hashing of host names and addresses added to known_hosts files?

Damien Miller: Well, that is a defence against worms using SSH as a vector rather than exploiting bugs in OpenSSH itself. This came out of a study from Schechter, Jung, Stockwell, and McLain early this year that found that the hostnames in known_hosts files could be used to build a topological worm, in other words one that can find a fairly optimal path from an infected system to another vulnerable system. In this case, the worm would just use normal trust relationships (Kerberos/GSSAPI trusts, public keys or harvested passwords) to spread from system to system - no bugs in SSH implementations were required.

Hashing the hostnames in known_hosts removes a good source of target information for such potential worms at the cost of some convenience - it is difficult to manually edit the known_hosts file when the hostnames are gibberish. To reduce this inconvenience, we added some extra commands to ssh-keygen so it can lookup, remove or rehash known_hosts. The HashKnownHosts is still off by default, but we might consider turning it on by default if we hear enough success stories.

Note that there are plenty more sources of information to a worm that are outside of our control - shell histories, netstat output, etc. Once again, good fundamental security practices such as not sharing accounts and limiting the range of account trust (especially transitive trust) are still required.

Did you develop any measure to fight timing based attacks?

Damien Miller: There are two classes of timing attacks, one of which matters and the other is not so important.

The not so important timing attacks allow active detection of which usernames are valid by differing timings in authentication failure, e.g. a valid username might take a little while to return (as the authentication method does the work of verifying their supplied credentials) where an invalid username might return quickly (as the authentication method returns early because it knows the username is invalid and destined to fail). We implement defences against these attacks by sending a fake username and credentials to the authentication backends. This hasn't been 100% effective when we delegate authentication to external libraries (e.g. PAM in portable OpenSSH) as they can do their own checks which return early anyway. I don't think these "attacks" matter that much because all they do is reveal the existence of something that isn't much of a secret anyway.

The other class of timing attacks are more scary - these are attacks that allow a passive observer to recover information relating to authentication secrets such as passwords. Attacks of this type have been found by Solar Designer and independently by Song, Wagner and Tian.

A simple attack of this type is watching the early parts of the protocol for a packet which contains a response to a password request. With some knowledge of the protocol these are fairly easy to spot or guess and once an attacker has obtained one, they can directly recover the length of the password. To prevent this, OpenSSH pads passwords up to a minimum of 64 characters. After 64 characters, brute forcing attacks are infeasible anyway, unless you have picked an utterly stupid password like "a" x 64.

A stronger attack involves watching the protocol, *after* the user has authenticated and established a session for occasions where they type a password in, such as running "su" or ssh'ing to another host. Without countermeasures, these can be clearly distinguished in the protocol as a one-way stream of short packets (keystrokes) without replies because the server will disable TTY echo. This will give a passive observer information about password length and inter-keystroke timing. To defeat this attack, OpenSSH sends back fake replies when TTY echo is turned off.

There have been some attacks based on timing. For example you could spot a valid username, and then start a password guessing attack. Then a lot of system administrators started to get their logs full of failed login. This is still an annoying problem and it's solved in very different ways; some people use the firewall to limit the number of active tcp connections to the box, others filter by source IP, and then there are all those tricks you can do on the system itself. Is there a plan to stop these annoying bots from being so efficient? Why don't you delay the login prompt by an increasing time like a standard console does?

Damien Miller: I wouldn't say that these attacks are too efficient - OpenSSH only allows a couple of guesses before the session is hung up and this is only after a fairly computationally expensive key exchange, so there is something of a natural rate limit already. As such, I don't believe that the protocol supports effective brute-force attacks against remotely sane passwords.

So what we see are worms that go for low hanging fruit - stupid password combinations like passwords that are the same as the username and are simple dictionary words - in other words, the stuff that people have been warned repeatedly about for over 20 years. These worms are a only a nuisance for any site that implements even basic password complexity requirements, or implements retrospective password auditing (e.g. using John the Ripper).

As far as defences, I'm more interested in adding better authentication controls (such as, untrusted hosts need to authenticate with a key before a password) than failure delays. Failure delays don't help much anyway - a smart attacker will just run multiple attacks in parallel to defeat them. There might be a case for forcing some sort of client-puzzle to be solved between failed password authentication attempts, but that is more complexity.



Federico Biancuzzi is freelancer; in addition to SecurityFocus he also writes for ONLamp, LinuxDevCenter, and NewsForge.
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