Exploit NIDS polymorphic evasion - The End?


18 Дек 2022
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|=---------------=[ NIDS polymorphic evasion - The End? ]=---------------=|
|=-----------------=[ Yuri Gushin <[email protected]> ]=-----------------=|

--[ Contents

  1 - Introduction

  2 - Concept details

  3 - Results

  4 - A working detection method

  5 - Conclusion

  6 - Greetings
  7 - References

  8 - Code

--[ 1 - Introduction

Today's Network Intrusion Detection Systems, alarmed of the dangers brought
by polymorphic shellcodes, try to detect them using desperate methods that
eat up CPU cycles.  This is done so the claim can be made that such NIDS
foil even the most devious crackers.  The truth of the matter is,
they don't.

Eventhough nowadays vulnerabilities are detected using advanced methods such
as protocol decoding and protocol anomaly checks, no NIDS is safe from the
unknown, I assure you - 0days are more than myth; they're here, they're
queer, get used to it.  This means even the most advanced NIDS needs generic
rules by which it catches exploitation payloads, which limits the attack
vector of polymorphism to unknown attacks a.k.a 0days.

I won't go on describing how a polymorphic shellcode is built, I'll assume
you know this already, if not, you're welcomed to read CLET team's [1]
excellent article and check out their tool, another resource [2] would be
ADMMutate by K2 of ADM, which is also a nice piece of work.

While I focus on the the 32bit Intel architecture, the same concept applies
to all others, it's simply a matter of logic, IA32 is the most common attack
victim, thus I decided to focus solely on it.

--[ 2 - Concept details

Snort, Prelude-NIDS, the proof-of-concept tool [3] by NGSec, and various
commercial IDS/IPS products, try to detect polymorphic shellcodes by using
the same technique; they look for the nop-sled.  By keeping a list of known
nop-replacements, they look for packets with X consecutive bytes from their
list.  If there are X or more bytes of nop-replacements, an alarm is raised.

The problem, however, is that this method of detection is quite easy to
bypass.  They look for CONTINUOUS sequences of nop instructions meaning that
if you put a byte that isn't in that specific NIDS's list of known nop
replacements, the counter gets reset and it continues to scan further down
the packet.  If we keep putting bytes not appearing in the IDS list, say,
every (X-1) bytes, we will keep resetting the counter before X is ever
reached, and no alarm will be raised.

Essentially a nop is any sequence of bytes that fulfills this criteria:

1) Is a valid, executable instruction

2) After exeuction of the nop, execution will continue at some point later
   in memory than the first byte of the nop but earlier in memory than, or
   exactly at, the byte following the end of the nop sled (i.e. the start of
   the shellcode's decoding loop)

3) Does not affect critical registers, e.g. stack pointer, in such a way
   that would cause the exploit payload to fail

4) A subsequence starting at any byte in the nop must itself be a valid nop
   (if we attackers want to have a 100% error free nop sled)

It is difficult for an IDS to determine whether a sequence of bytes meets
all the above criterions.  For example, in order to test whether some
sequence meets the first criterion, the IDS would have to either attempt to
execute the bytes in a sandbox or else have a full disassembler integrated
in order to test whether the bytes were valid instructions.  Both options
would probably consume too many resources to be feasible in real-time.
Having an exhaustive list of valid nop sequences is thus the only option
left, though anybody can pick up for example the Intel manual [4] and dig
through the opcodes to see what will work for his or hers specific exploit
without damaging the execution of the shellcode while bypassing the remote

The list of nop-replacements that appears in ADMmutate is the biggest public
list of it's kind, consequently it also appears in just about any IDS out
there that aims to detect polymorphic exploitation payloads.
Because of that, I have taken that list of nops and mark it as tainted, this
will be used to our advantage.

As part of this research, I decided to develop a polymorphic nop-sled
generation engine for the Intel architecture.  It will simply fill a given
buffer with random nops in a recursive manner, so that the nop sled will
be a valid set of instructions, while at the same time will reset the IDS
counters and let the packet slip by without raising any alarms.

In order to maintain a 100% error free nop-sled, I've taken all instructions
that require immediate values as arguments, and used them as part of the nop
sled, arguments given to those instructions are nops by themselves, which
ensures execution will flow no matter where we land on the nop sled (which
is an important aspect for an attacker).  Error checking is also in place,
for such instructions such as the JMP family not to jump over the decoding
routine, or jumping back and effectively cause an endless loop (the argument
is signed, thus any byte over 0x80 represents a negative value).

Another important feature is blacklisting forbidden instructions, this can
be achieved both by using the per-character blacklist, and by using the more
advanced per-register blacklist, where instructions can be forbidden by the
register(s) they affect.  For example sometimes unique shellcodes rely upon
certain registers for successful execution, this way you restrict registers
not to be changed so exploit still works.  The same logic applies when you
don't want JMP family instructions to be used - you restrict eip, or when
you don't want the stack to be touched - restrict esp, etc.

The only 'flaw' (if you might call it that) in ADMmutate and the likes, is
in the nop sled, since it's predictable, and considered general-knowledge
amongst IDS vendors.  So using our engine to pad the nops, and any
polymorphic shellcode engine to generate the decipher routine + shellcode,
the exploit payload will easily evade today's NIDS.

The obvious questions:

"What if they simply add your decoy-nops to their list of nop replacements?"

There are a lot of more potential nops out there, they don't have to be one
byte instructions, and they don't HAVE to expect immediate values as
arguments to remain error free, one can find a decent amount of more usable
nop instructions out there.  So yes, if they add the nops published here -
this tool will be redundant, but it's merely a PoC, to show you how easy it
is to bypass advanced NIDS by simply opening up a manual.

IDS vendors will not dwelve into researching what's considered a NOP when
looking for polymorphic patterns, they just take that information from the
attackers' arsenal, which is what makes this, and generally most security
attacks possible.

"What if they add ALL POSSIBLE NOPS to their list?"

If they add *all* bytes usable as nops, + instruction prefixes, basically
ANYTHING that might appear in the nop-sled of a polymorphic exploitation
payload, it will:

1) Produce quite a few false-positives (the more nop-replacements you find,
   the wider the false-positive range grows)

2) Be even more (!) resource-exhaustive

3) Be easily avoided by inserting a non-NOP byte(or bytes) as an argument to
   some instruction, the risk taken would be negligible since we could solve
   this simply by incrementing the return address, the nop-sled will not be
   100% error-free, so it may take a few tries, but cleaning up the logs
   after exploiting a service isn't too hard

--[ 3 - Results

Alright, so we know that the only pattern of detection IDS's have against
any kind of polymorphism is by the nop-sled, which means we can use the
currently available tools to deal with the decoding routine and encryption
of the shellcode, and just use our engine to fill in the initial nops.

In poly.tar.gz you'll find the examples/ directory, server.c is the
vulnerable server used in this demonstration, the two directories inside
examples/ are made to differentiate between the exploit using only ADMmutate
and the new one using polynop to pad the nops and ADMmutate to create the
decipher engine + encrypted shellcode, see USAGE for compilation info.

ADMmutate-only results:

[yuri@alcatraz ~/work/code/lab/polynop/mutated]$ ./exp

<---- NIDSfindshellcodes, it detects the exploit ---->
alcatraz nidsfindshellcode-0.2 # ./nidsfindshellcode -d lo
nidsfindshellcode 0.2 by Fermin J. Serna <[email protected]>
Next Generation Security Technologies

IA32 shellcode found: Protocol TCP localhost:35407 -> localhost:1337

<---- Prelude-NIDS, same deal, shellcode detected ---->
alcatraz log # grep IA32 prelude.log
* Classification: IA32 shellcode found

polynop+ADMmutate results:

[yuri@alcatraz ~/work/code/lab/polynop/mutated+polynoped]$ ./exp

Nothing, no alarms were raised.

The results show us that we managed to successfully bypass both NIDS's, but
I wanted to go further and test whether a detection engine that tries to
detect all nop replacements I found + instruction prefixes would yield too
many false-positives, which is what the next section is about.

--[ 4 - A working detection method

The above attack will flawlessly bypass most common NIDS systems.  All of
the opensource NIDS's I've seen are vulnerable, some commercial ones I've
tested are vulnerable as well.

There is one feasible solution to this - reading through the Intel manual to
obtain a definitive list of NOP replacements.  The currently available
solutions of this sort are blindly dependant upon ADMmutate, it was quite
funny to see that they even copied the structures from it, which is why they
are so easy to bypass, and generally are quite inefficient.

To address the issue of yet even more unknown nops or junk arguments that
reset the IDS counter we could use an adjustable value for skipping forward
without resetting our counter, for example with a value of 5 it will allow
the NIDS to keep counting consecutive nops while allowing 5 non-nops to
appear without resetting the counter.

The detection engine can be found along with polynop, it's advantage over
current such implementations is that it's able to catch polymorphic payloads
that contain "unknown" nop replacements as well.  Another advantage would be
it's wide list of detectable nop replacements, while maintaining efficiency.
The detection engine is pretty straightforward, peek at polydeteng.h for any
changes and off you go :)

Keep in mind that this "ultimate" solution still falls under the
disadvantages discussed in section 2 of this paper, the countermeasures we
employed are as follows:

1) The false-positives issue (discussed below)
2) The resources-exhaustion issue is solved (to an extent) by use of hash
3) The SKIP variable solves the non-nops injection issue, but should be
   treated as a sensitivity variable.

Even when a big list of 128 bytes is given, in theory, the probability of a
false positive is extremely low:

( (128/256)^ALARM ) * 100 =  49.09e-90 %

This probability chance is alot lower than that of you winning the lottery!

In the calculation above we used an ALARM threshold of 300 bytes (which is
a respectable length for a nop sled).  As we increase ALARM the probability
drastically decreases, in real-world scenarios exploit payloads commonly
use atleast that many bytes of nops.

In practice, the false positives number is dependant upon the type of data
transferred over the wire, mp3 files & various binaries will sometimes
raise an alarm, rarely gif files will too, but other than that, no false
positives are raised - for a corporate environment where the only kind of
traffic flow is generally mail & web browsing this presents little threat,
and poses a viable solution to the polymorphic exploit payload problem.

--[ 5 - Conclusion

When I started this research I was sure that once I find a sufficient amount
of nop-replacements their detection would produce too many false-positives
to be feasible, well, I was wrong.  The outcome of this research has shown
me that theory and practice can sometimes be very distinct from one another.

This is how polydetect was born - I saw the opportunity for a solid solution
to the problem; having a big list of known nop replacements, all possible
instruction prefixes - and a simple fine-tuning variable such as SKIP is
enough to detect any polymorphic exploit payload out there, after studying
the network at use and properly fine-tuning the detection engine, catching
such exploitation attempts becomes trivial.  The detection engine developed
here is good enough to be considered definite and can catch any attempt of
polymorphic evasion for IA32, and should be used as a building block for a
complete solution.

This paper was written to trigger your curiousity regarding this subject,
this is how new methods are born, this is merely the beginning - you decide
how this ends.

--[ 6 - Greetings

Greets go out to the ECL crew, Alex Behar, Valentin Slavov, blexim,
stranger, Dimiter Manevski, Azrael, elius, shrink, cntz, tanin00... and all
the rest of the hippies I forgot to mention :)

Thank you for reading this paper :>

--[ 7 - References

[1] - http://www.phrack.org/show.php?p=61&a=9
      Polymorphic Shellcode Engine Using Spectrum Analysis - CLET Team

[2] - http://www.ktwo.ca/ADMmutate-0.8.4.tar.gz

[3] - http://www.ngsec.com/downloads/misc/NIDSfindshellcode.tgz

[4] - http://developer.intel.com/design/pentium4/manuals/index_new.htm
      IA-32 Intel Architecture Software Developer's Manual
      Volume 2: Instruction Set Reference

--[ 8 - Code

The source code and the paper can both be found at:

# milw0rm.com [2006-04-08]

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