sábado, 6 de junho de 2020

Vsftpd Backdoor - Ekoparty Prectf - Amn3S1A Team

It's a 32bits elf binary of some version of vsftpd, where it have been added a backdoor, they don't specify is an authentication backdoor, a special command or other stuff.

I started looking for something weird on the authentication routines, but I didn't found anything significant in a brief period of time, so I decided to do a bindiff, that was the key for locating the backdoor quickly. I do a quick diff of the strings with the command "strings bin | sort -u" and "vimdiff" and noticed that the backdoored binary has the symbol "execl" which is weird because is a call for executing elfs, don't needed for a ftp service, and weird that the compiled binary doesn't has that symbol.





Looking the xrefs of "execl" on IDA I found that code that is a clear backdoor, it create a socket, bind a port and duplicate the stdin, stdout and stderr to the socket and use the execl:



There are one xrefs to this function, the function that decides when trigger that is that kind of systems equations decision:


The backdoor was not on the authentication, it was a special command to trigger the backdoor, which is obfuscated on that systems equation, it was no needed to use a z3 equation solver because is a simple one and I did it by hand.



The equation:
cmd[0] = 69
cmd[1] = 78
cmd[1] + cmd[2] = 154
cmd[2] + cmd[3] = 202
cmd[3] + cmd[4] = 241
cmd[4] + cmd[5] = 233
cmd[5] + cmd[6] = 217
cmd[6] + cmd[7] = 218
cmd[7] + cmd[8] = 228
cmd[8] + cmd[9] = 212
cmd[9] + cmd[10] = 195
cmd[10] + cmd[11] = 195
cmd[11] + cmd[12] = 201
cmd[12] + cmd[13] = 207
cmd[13] + cmd[14] = 203
cmd[14] + cmd[15] = 215
cmd[15] + cmd[16] = 235
cmd[16] + cmd[17] = 242

The solution:
cmd[0] = 69
cmd[1] = 75
cmd[2] = 79
cmd[3] = 123
cmd[4] = 118
cmd[5] = 115
cmd[6] = 102
cmd[7] = 116
cmd[8] = 112
cmd[9] = 100
cmd[10] = 95
cmd[11] = 100
cmd[12] = 101
cmd[13] = 106
cmd[14] = 97                    
cmd[15] = 118
cmd[16] = 117
cmd[17] = 125


The flag:
EKO{vsftpd_dejavu}

The binary:
https://ctf.ekoparty.org/static/pre-ekoparty/backdoor


More info
  1. Hackerx
  2. Pentest Companies
  3. Pentest Meaning
  4. Pentest Cyber Security
  5. Pentest Vs Red Team
  6. Hacking Linux
  7. Hacking Gif
  8. Hacking Games
  9. Pentest Iso
  10. Pentest Box
  11. Hacking The Art Of Exploitation
  12. Pentest With Kali Linux
  13. Pentest Nmap
  14. Hacking
  15. Pentest Bootcamp
  16. Pentest Partners
  17. Pentest Meaning
  18. Hacking Groups

Sem comentários: