DCTF 2021 - Bell

Read number and run throught known function


Blaise's friends like triangles too! nc dctf-chall-bell.westeurope.azurecontainer.io 5311


The function gives us a number and then waits for multiple inputs.


Loading the file into ghidra we can take a look at what happens.

undefined8 main(void)
  int iVar1;
  uint uVar2;
  time_t tVar3;

  tVar3 = time((time_t *)0x0);
  iVar1 = rand();
  uVar2 = iVar1 % 5 + 8;
  return 0;
undefined8 process(uint param_1)
  long lVar1;
  bool bVar2;
  long lVar3;
  long in_FS_OFFSET;
  uint local_24;
  long local_20;

  lVar1 = *(long *)(in_FS_OFFSET + 0x28);
  bVar2 = true;
  local_24 = 1;
  while ((int)local_24 <= (int)param_1) {
    lVar3 = triangle((ulong)param_1,(ulong)local_24,(ulong)local_24);
    if (lVar3 != local_20) {
      bVar2 = false;
    local_24 = local_24 + 1;
  if (bVar2) {
    system("cat flag.txt");
  else {
    puts("Better luck next time.");
  if (lVar1 != *(long *)(in_FS_OFFSET + 0x28)) {
                    /* WARNING: Subroutine does not return */
  return 0;
long triangle(uint param_1,int param_2)
  long lVar1;
  long lVar2;

  if ((int)param_1 < param_2) {
    lVar1 = 0;
  else {
    if ((param_1 == 1) && (param_2 == 1)) {
      lVar1 = 1;
    else {
      if (param_2 == 1) {
        lVar1 = triangle((ulong)(param_1 - 1),(ulong)(param_1 - 1),(ulong)(param_1 - 1));
      else {
        lVar2 = triangle((ulong)param_1,(ulong)(param_2 - 1U),(ulong)(param_2 - 1U));
        lVar1 = triangle((ulong)(param_1 - 1),(ulong)(param_2 - 1U),(ulong)(param_2 - 1U));
        lVar1 = lVar1 + lVar2;
  return lVar1;

From this I could see, I can win, if I just run the same operations and print my solutions. So I rewrote the triangle function in Python3 and integrated a read of the random number. My final script was:

#!/usr/bin/env python3
from pwn import *

context.arch = 'amd64'
context.kernel = 'amd64'
#context.log_level = "DEBUG"
context.log_level = "INFO"

def triangle(a,b):
    if a < b:
        return 0
        if a == 1 and b == 1:
            return 1
            if b == 1:
                return triangle(a-1,a-1)
                return triangle(a,b-1) + triangle(a-1,b-1)

p = remote("dctf-chall-bell.westeurope.azurecontainer.io", 5311)

random = int(p.readuntil('\n')[:-1].decode('ASCII'))

for i in range(1,(random)+1):


Running this gives the flag.


DCTF 2021 - Pinch me

Buffer overflow to overwrite variable

Description This should be easy! nc dctf1-chall-pinch-me.westeurope.azurecontainer.io 7480 Preface We got a binary file which asked us Am I dreaming? and with basic input prints then Pinch me! Overview Loading the binary into ghidra we can see, that the interaction happens in the function vuln void vuln(void) { char local_28 [24]; int local_10; int local_c Read More

DCTF 2021 - Just In Time

Using frida to get decrypted flag.

Description Don't fall in (rabbit) holes Preface We get a binary which just prints Decryption finished. Overview Using ghidra, we can analyse the binary. Inside the main of the binary we can see, that their is some binary content and multiple functions called with strncpy in between. undefined8 main(int argc,char **argv) { char *key_text; char Read More

DCTF 2021 - Baby bof

Buffer overflow and ret2libc

Description It's just another bof. nc dctf-chall-baby-bof.westeurope.azurecontainer.io 7481 Preface We got a simple binary with output plz don't rop me and after our input plz don't rop me Also we got a Dockerfile, which showed us the used image was Ubuntu:20.04 Overview Based on the output, we know it was a rop challenge. Also checksec baby_bof gave us. Arch: amd64-64-little RELRO: Partial RELRO Read More

DCTF 2021 - Pwn sanity check

Simple buffer overflow with ret2win.

Description This should take about 1337 seconds to solve. nc dctf-chall-pwn-sanity-check.westeurope.azurecontainer.io 7480 Preface We get a simple binary, with simple input and output. Overview Looking at the binary in ghidra, I found these functions. void vuln(void) { char local_48 [60]; int local_c; puts("tell me a joke"); fgets(local_48,0x100 Read More

DCTF 2021 - Hotel rop

ROP chain with multiple function and then ret2win

Description They say programmers' dream is California. And because they need somewhere to stay, we've built a hotel! nc dctf1-chall-hotel-rop.westeurope.azurecontainer.io 7480 Preface We got a binary file with simple input and some output related to hotel checkIn. Overview Based on the name of the challenge, we can be certain, that some sort of rop is needed. Loading the binary into ghidra we can see our function vuln. void vuln(void) { char Read More

DCTF 2021 - Readme

Format String to dump the memory and get flag.

Description Read me to get the flag. nc dctf-chall-readme.westeurope.azurecontainer.io 7481 Preface We get a binary which asks for our name and then prints hello + input. But in order for the binary to run, a file flag.txt needs to be created in the working directoy. Overview Decompiling the binary in ghidra, we see a function vuln where the logic happens. The decompiled function with some renaming of the variables looks like this: void vuln(void) { Read More