xfce.org/lunar-linux.org server nearly hacked

Brian J. Tarricone bjt23 at cornell.edu
Wed Jul 27 20:46:11 CEST 2005

Hash: SHA1

Auke Kok wrote:
> Brian J. Tarricone wrote:
>>Auke Kok wrote:
>>>- Mount /tmp and world-writeable mountpoints with noexec, this will stop
>>>most OOTB exploits immediately as the rootkit or backdoor will fail to
>>Though this really doesn't help all *that* much, since the attacker can
>>just do:
>>$ /lib/libc.so.6 /tmp/really_bad_program
>>And voila - you can execute stuff on /tmp.
> Supposedly that doesn't work anymore, with my systems I don't get that
> result even:
> /tmp # /lib/libc.so.6 /tmp/ls
> GNU C Library stable release version 2.3.2, by Roland McGrath et al.
> Copyright (C) 2003 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.
> There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
> Available extensions:
>         GNU libio by Per Bothner
>         crypt add-on version 2.1 by Michael Glad and others
>         linuxthreads-0.10 by Xavier Leroy
>         BIND-8.2.3-T5B
>         libthread_db work sponsored by Alpha Processor Inc
> Report bugs using the `glibcbug' script to <bugs at gnu.org>.
> /tmp #
> and supposedly it's the linker libdl.so that should work according to refs:
> /tmp # /lib/ld-linux.so.2 /tmp/ls
> /tmp/ls: error while loading shared libraries: /tmp/ls: failed to map
> segment from shared object: Operation not permitted
> /tmp #
> no go for executing binaries thus, unless I'm missing another way around
> that. I'd sure like to hear about it ;^)

Gah, yeah, I meant ld-linux.  My mistake.  And you're right, it doesn't
appear to work on volumes mounted noexec, though I'm pretty sure it used
to at some point.

On the other hand, you can still run perl scripts on noexec volumes, and
considering that you can do a ridiculous amount of things with perl
(such as a network-accessible daemon that spawns backdoor shells), I'd
say that still represents a security risk, though admittedly not as much
of one.

I suppose this whole point is moot if someone's managed to compromise a
real user's account, though, since they could just run binaries from
that user's home directory.  I imagine not too many hosts (where people
do useful work, anyway) have /home mounted noexec.

>>Out of idle curiosity, can
>>you safely remove the execute bit on libc and have a functioning system?
>> I know in general on Linux you don't need to make shared libs
>>executable, but I dunno, libc may be an exception.
> only to have 'ldd' functioning AFAIK, but I'm not gonna try this on a
> live system just yet, maybe with something safe first ;^)

Yeah... not something you want to test out on a production system ^_^.
And since the ld-linux trick doesn't work on noexec mounts, there
probably isn't much (any?) benefit.


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