Entries tagged ssh

Related tags: afl, boo-boo, daemons, debian, git, madduck, mysql, openssh, scp, security.

Patching scp and other updates.

Sunday, 8 January 2017

I use openssh every day, be it the ssh command for connecting to remote hosts, or the scp command for uploading/downloading files.

Once a day, or more, I forget that scp uses the non-obvious -P flag for specifying the port, not the -p flag that ssh uses.

Enough is enough. I shall not file a bug report against the Debian openssh-client page, because no doubt compatibility with both upstream, and other distributions, is important. But damnit I've had enough.

apt-get source openssh-client shows the appropriate code:

    fflag = tflag = 0;
    while ((ch = getopt(argc, argv, "dfl:prtvBCc:i:P:q12346S:o:F:")) != -1)
          switch (ch) {
            case 'P':
                    addargs(&remote_remote_args, "-p");
                    addargs(&remote_remote_args, "%s", optarg);
                    addargs(&args, "-p");
                    addargs(&args, "%s", optarg);
            case 'p':
                    pflag = 1;

Swapping those two flags around, and updating the format string appropriately, was sufficient to do the necessary.

In other news I've done some hardware development, using both Arduino boards and the WeMos D1-mini. I'm still at the stage where I'm flashing lights, and doing similarly trivial things:

I have more complex projects planned for the future, but these are on-hold until the appropriate parts are delivered:

  • MP3 playback.
  • Bluetooth-speakers.
  • Washing machine alarm.
  • LCD clock, with time set by NTP, and relay control.

Even with a few LEDs though I've had fun, for example writing a trivial binary display.



So about that idea of using ssh-keygen on untrusted input?

Monday, 12 October 2015

My previous blog post related to using ssh-keygen to generate fingerprints from SSH public keys.

At the back of my mind was the fear that running the command against untrusted, user-supplied, keys might be a bad plan. So I figured I'd do some fuzzing to reassure myself.

The most excellent LWN recently published a piece on Fuzzing with american fuzzy lop, so with that to guide me I generated a pair of SSH public keys, and set to work.

Two days later I found an SSH public key that would make ssh-keygen segfault, and equally the SSH client (same parser), so that was a shock.

The good news is that my Perl module to fingerprint keys is used like so:

my $helper = SSHKey::Fingerprint->new( key => "ssh ...." );
if ( $helper->valid() ) {
   my $fingerprint = $helper->fingerprint();

The validity-test catches my bogus key, so in my personal use-cases this OK. That said it's a surprise to see this:

skx@shelob ~ $ ssh -i key.trigger.pub steve@ssh.steve.org.uk 
Segmentation fault

Similarly running "ssh-keygen -l -f ~/key.trigger.pub" results in an identical segfault.

In practice this is a low risk issue, hence mentioning it, and filing the bug-report publicly, even if code execution is possible. Because in practice how many times do people fingerprint keys from unknown sources? Except for things like githubs key management page?

Some people probably do it, but I assume they do it infrequently and only after some minimal checking.

Anyway we'll say this is my my first security issue of 2015, we'll call it #roadhouse, and we'll get right on trademarking the term, designing the logo, and selling out for all the filthy filthy lucre ;)



Generating fingerprints from SSH keys

Wednesday, 7 October 2015

I've been allowing users to upload SSH public-keys, and displaying them online in a form. Displaying an SSH public key is a pain, because they're typically long. That means you need to wrap them, or truncate them, or you introduce a horizontal scroll-bar.

So rather than displaying them I figured I'd generate a fingerprint when the key was uploaded and show that instead - This is exactly how github shows your ssh-keys.

Annoyingly there is only one reasonable way to get a fingerprint from a key:

  • Write it to a temporary file.
  • Run "ssh-keygen -lf temporary/file/name".

You can sometimes calculate the key via more direct, but less obvious methods:

awk '{print $2}' ~/.ssh/id_rsa.pub | base64 -d | md5sum

But that won't work for all key-types.

It is interesting to look at the various key-types which are available these days:

mkdir ~/ssh/
cd ~/ssh/
for i in dsa ecdsa ed25519 rsa rsa1 ; do
  ssh-keygen -P "" -t $i -f ${i}-key

I've never seen an ed25519 key in the wild. It looks like this:

$ cat ~/ssh/ed25519-key.pub
ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIMcT04t6UpewqQHWI4gfyBpP/ueSjbcGEze22vdlq0mW skx@shelob

Similarly curve-based keys are short too, but not as short:

ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBLTJ5+  \
 rWoq5cNcjXdhzRiEK3Yq6tFSYr4DBsqkRI0ZqJdb+7RxbhJYUOq5jsBlHUzktYhOahEDlc9Lezz3ZUqXg= skx@shelob

Remember what I said about wrapping? Ha!

Anyway for the moment I've hacked up a simple perl module SSH::Key::Fingerprint which will accept a public key and return the fingerprint, as well as validating the key is well-formed and of a known-type. I might make it public in the future, but I think the name is all wrong.

The only code I could easily find to do a similar job is this node.js package, but it doesn't work on all key-types. Shame.

And that concludes this weeks super-happy fun-time TODO-list item.



Dwayne, I think you might be colorblind.

Tuesday, 13 April 2010

It is unfortunate that most server-packages don't seperate out their init scripts into separate packages:

  • foo
    • Contains the server binary, associated config files, and libraries.
  • foo-run or foo-server
    • Contains the init script(s).

Right now its a real pain to have to modify things like /etc/init.d/ssh to launch two daemons, running on two different ports, with two different configuration files.

Running multiple copies of SMTP daemons, databases, and similar things is basically more complex than it has to be, because our packages aren't setup for it.

If you maintain a daemon please do consider this, failing that honoring a flag such as "DISABLED=true" in /etc/default/foo would allow people to use their own /etc/init.d/foo.local initscript. (That's not perfect, but it is a step in the right direction.)

ObFilm: Little Miss Sunshine.



Yo lunchbox, hurry it up.

Tuesday, 9 September 2008

Today I received two kittens. They rock.

Somebody else who rocks? Martin F. Krafft.

Martin rocks because of this gem I came across today managing ~/.ssh/known_hosts via git.

The idea is that instead of having a single file known_hosts you have a known_hosts.d/ directory containing multiple entries that are concatenated together.

So neat. So obvious. So nice. Rock award.

ObFilm: Jay and Silent Bob Strike Back



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