[erlang-questions] 'ssh' security issue

OvermindDL1 <>
Tue Dec 10 22:26:22 CET 2013


Another question while I am at it with this exec option, the 'shell'
option gets a User and PeerAddr passed in, any way to do that with
exec?  If I can at least get the User then I can allow them to issue
one-off commands this way too, which would be quite useful for remote
scripting.

On Tue, Dec 10, 2013 at 2:22 PM, OvermindDL1 <> wrote:
> Ah all you are amazing, I never saw that option in the docs indeed and
> looks like it should do what I want.  Thanks much for all the help!
>
> On Tue, Dec 10, 2013 at 10:20 AM, Jakob Cederlund <> wrote:
>> Actually, the sample cli module works quite all right. The problem is that
>> the default implementation in ssh_cli for the "exec" thing in ssh is
>> actually to execute it (using erl_scan and erl_eval and stuff). There is an
>> undocumented option to ssh (actually to the ssh_cli module) that can be used
>> to customize this. The option {exec, {M, F, []}} takes an exported function
>> (M:F/1) that is called with the parameters given to the ssh commands as a
>> string. This function should spawn a process that writes the desired output
>> on stdout.
>>
>> So to avoid the strange eval phenomenon, and provide another function that
>> just echoes the parameters back, you can write a module x:
>> -module(x).
>> -export([exec/1]).
>> exec(A) -> spawn(fun() -> io:format("~p\n", [A]), exit(normal) end).
>>
>> and specify the function x:exec/1 as a call-back for the exec option:
>> B=ssh_sample_cli:listen(8323, [{subsystems, []}, {exec, x, exec, ""]).
>>
>> And then when you do:
>>> ssh -p 8323 to.the.host 'lists:reverse("test").'
>>
>> You get back:
>> "list:reverse(\"test\")."
>>
>> Hope this helps. (And sorry for the mess…)
>> /Jakob
>>
>>
>>
>> 2013/12/10 Ingela Andin <>
>>>
>>> Hi!
>>>
>>> The CLI example in the SSH application must be seen as a hack. We intend
>>> to clean it up and
>>> extend the SSH documentation, when it gets prioritized I can not say. Well
>>> anyway your CLI
>>> implementation must take care of SSH exec request as well.  You can also
>>> look at the ssh_cli.erl  module.
>>> If I remember correctly there was a bug, before ssh-2.1.7, with regards to
>>> the exec request  so that  it was not forwarded to CLI process but rather
>>> always interpreted in the erlang shell environment.
>>>
>>> Regards Ingela Erlang/OTP team - Ericsson AB
>>>
>>>
>>>
>>> 2013/12/7 OvermindDL1 <>
>>>>
>>>> Greetings,
>>>>
>>>> I am attempting to just create an SSH shell to connect to a system by
>>>> users so they can do commands without the web interface, and as such I
>>>> certainly do not want things like port forwarding or being able to run
>>>> arbitrary erlang code, however I do not seem to be able to disable
>>>> running arbitrary erlang code.  An example of the ssh_sample_cli
>>>> included with erlang:
>>>> """
>>>> $ erl
>>>> Erlang R16B02 (erts-5.10.3) [source] [64-bit] [smp:8:8]
>>>> [async-threads:10] [hipe] [kernel-poll:false]
>>>>
>>>> Eshell V5.10.3  (abort with ^G)
>>>> 1> c(ssh_sample_cli).
>>>> ssh_sample_cli.erl:146: Warning: this expression will fail with a
>>>> 'badarith' exception
>>>> {ok,ssh_sample_cli}
>>>> 2> B=ssh_sample_cli:listen(8323, [{subsystems, []}]).
>>>> {ok,<0.67.0>}
>>>> """
>>>>
>>>> And from another shell/computer:
>>>> """
>>>> $ ssh -p 8321 to.the.host
>>>> 's password:
>>>> Enter command
>>>> CLI> help
>>>> CLI Sample
>>>> crash                  crash the cli
>>>> exit                   exit application
>>>> factors    <int>       prime factors of <int>
>>>> gcd        <int> <int> greatest common divisor
>>>> help                   help text
>>>> host                   print host addr
>>>> lcm        <int> <int> least common multiplier
>>>> prime      <int>       check for primality
>>>> primes     <int>       print all primes up to <int>
>>>> rho        <int>       prime factors using rho's alg.
>>>> self                   print my pid
>>>> user                   print name of user
>>>>
>>>> ---> ok
>>>> CLI> exit
>>>> ---> done
>>>> Connection to to.the.host closed.
>>>> """
>>>>
>>>> So far so good (the main program where I have this implemented has a
>>>> well running shell of its own), but lets try a couple other things:
>>>> """
>>>> $ sftp -P 8321 to.the.host
>>>> 's password:
>>>> subsystem request failed on channel 0
>>>> Connection closed
>>>> """
>>>>
>>>> Also good, no file transfers can be done since the option subsystem is
>>>> set to [], but notice:
>>>> """
>>>> $ ssh -p 8323 to.the.host 'lists:reverse("!?ti pots I od woh dna ereh
>>>> gnineppah si tahw woN").'
>>>> 's password:
>>>> "Now what is happening here and how do I stop it?!"
>>>> """
>>>>
>>>> So... I can still run arbitrary erlang commands, how do I stop this?
>>>> Unable to find an option to pass in or anything through a quick code
>>>> perusal to no avail.  Help?
>>>> _______________________________________________
>>>> erlang-questions mailing list
>>>> 
>>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>
>>>
>>>
>>> _______________________________________________
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>>



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