Re: one of my servers has been compromized

[Gage Bystrom <themadichib0d () gmail com>]

Well in that case it becomes fairly sane, assuming you've safeguarded
against the one of the worst case scenario like Valdis previously
mentioned. There are a handful of things I can think of however that
could still work, at which point depends on the attackers goals.

But at that point it'd be a complete loss for the defender, and only a
half victory for the attacker. After all the defender only wins if the
attacker fails to accomplish his goals. The minute he changes his
goals into something you've already been forced to concede to him the
minute he concedes the following: "I'm not getting the kernel" and one
of the following: "I'm not modifying critical files" or "The intrusion
has a high chance of being detected".

But meh, at the point it is an unrealistic scenario anyways. An
attacker who can recognize that, while going through with the
decision, while being able to plan ahead, while being skilled enough
to actually prepare for the plan, while actually encountering the
scenario needed for the per-requisites for this to occur is perhaps
the very scenario behind the "everything can be hacked" possibility we
all inherently recognize.

Oh well, anyways this thread has been very interesting to me, and I'm
glad that I'm not the only one who could see how over-responding would
have been completely useless to the OP. That and he likely has more
than he needs to put an end to his current circumstance.

On Tue, Dec 6, 2011 at 5:33 PM, John Jacobs <flamdugen () hotmail com> wrote:
> > Sounds pretty neat to be honest. But one thing I'm wondering is that if
> > they have root, what's stopping them from turning that off? After all
> > they need root to load the modules in the first place, so if they are
> > in a position to want to do that, then they are in a position to turn
> > that off. Granted they probably wouldn't be able to load modules till
> > next boot(at least Id probably cry if that wasn't the case) but even
> > that can be a win scenario depending on how they want to execute the
>
> Hi Gage, thank you for your reply.  What you are missing is that by disabling kernel module loading you are applying 
> a defense-in-depth strategy to prevent a *vulnerable* module from automatically loading in the first place resulting 
> in root compromise.  I believe you may not be aware that some modules are loaded automatically if a corresponding 
> special device is accessed.  Usually the userspace modprobe utility is executed though this can be controlled by the 
> value of /proc/sys/kernel/modprobe
>
> Preventing module loading has historically be a valuable way to prevent privilege escalation or further root 
> compromise.  Such an example would be the 'ptrace' exploit, see 
> http://www.sans.org/security-resources/malwarefaq/Ptrace.php
>
> Historically there have been various kernel modules that are vulnerable that could be loaded by userland non-root 
> programs or access.  Ubuntu likes to automatically load modules.
>
> Removing CAP_SYS_MODULE or kernel.modules_disabled=1 make good security sense.  See 
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=3d43321b7015387cfebbe26436d0e9d299162ea1
>  and 
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=25354c4fee169710fd9da15f3bb2abaa24dcf933
>  and https://wiki.ubuntu.com/Security/Features#block-modules
>
> The goal here is defense in depth.  Revocation of loading the kernel modules cannot be undone unless a system reboot 
> is effected which should be highly suspicious.
>
> The goal isn't about protecting ones boxens from a theoretical boogie-man it is to leverage all available and sane 
> methods for properly securing ones box.  I see no point to to use these options.
>
> Thanks,
> John

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