python recv() and eval()

Question

can someone please help me understand this code

clientsock.send("Welcome to Maths_Server 1.0\n")

try:
    clientsock.send("Enter the first number, so I can EVALuate it:\n")
    firstNum = eval(clientsock.recv(1024))
    firstNum = firstNum + firstNum + ord(flag[4]) + ord(flag[8]) + ord(flag[5])
    clientsock.send("Enter the second number, so I can EVALuate it:\n")
    secondNum = eval(clientsock.recv(1024))
    if secondNum == firstNum:
        clientsock.send("The flag is: " + flag + "\n")
        firstNum = 0
        secondNum = 0
except:
    pass

clientsock.close()

i want to know what does recv() does and how can i make the if statement true

this code is full of errors and security flaws. DON'T USE IT. — Daniel, Feb 11 '16 at 17:26
i am not using it, actually my mission is to know how to crack it (homework) but i don't understand what does recv do — AdoobII, Feb 11 '16 at 17:37
`clientsock` is obviously a socket object. So https://docs.python.org/2/library/socket.html#socket.socket.recv. Note, Googling `python recv` leads you to this exact page. — kindall, Feb 11 '16 at 17:47
ok how can i know make the if statement true without knowing the value of firstnum — AdoobII, Feb 11 '16 at 17:53
You don't have to crack anything, simple do `server.sendall('clientsock.sendall("Flag: %s" % flag)')` and read the result. — Daniel, Feb 11 '16 at 18:00
i can't change the code, it's a webserver and i am accessing it from netcat — AdoobII, Feb 11 '16 at 18:02
they have provided me with a piece of the source code of the webserver — AdoobII, Feb 11 '16 at 18:03

score 0 · Accepted Answer · answered Feb 12 '16 at 09:55

Well, you can't determine the firstNum value because it's inputted by the user, so you input a random number into the firstNum, then for the secondNum you enter firstNum, so what this does is the eval() takes the firstNum value and convert it into string for the secondNum which will make the if-statement true. Hope this works out.

score -2 · Answer 2 · edited Jan 17 '23 at 08:30

-2

The answer to this riddle is this is actually the server code, but you need to make a client which will connect to that server, and produce equal results to get the flag.

edited Jan 17 '23 at 08:30

Eric Aya

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answered Jan 16 '23 at 23:00

Guy

1

python recv() and eval()

2 Answers2