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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

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Vsftpd Backdoor - Ekoparty Prectf - Amn3S1A Team

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