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

Related links

1. Computer Hacker
2. Hacker Security Tools
3. Nsa Hack Tools Download
4. Free Pentest Tools For Windows
5. Pentest Tools For Ubuntu
6. New Hack Tools
7. Github Hacking Tools
8. Hacking Tools Github
9. Growth Hacker Tools
10. Hacking Tools Usb
11. Hack Website Online Tool
12. Tools For Hacker
13. Hacking Tools Github
14. Hack Rom Tools
15. Hacking Tools For Pc
16. Tools For Hacker
17. Hack Tools Pc
18. How To Install Pentest Tools In Ubuntu
19. Pentest Tools Alternative
20. Hacker Tools Online
21. Hacker Tools Apk Download
22. Hacker Techniques Tools And Incident Handling