By: unvariant

Tags: pwn AmateursCTF-2023

Problem Description:


Reveal Hints None

Provided Files

  • chal
  • Dockerfile

chal decompilation

00001269  int32_t main(int32_t argc, char** argv, char** envp)
00001275      int32_t var_4c = argc
00001280      void* fsbase
00001280      int64_t rax = *(fsbase + 0x28)
0000129e      setbuf(fp: stdout, buf: nullptr)
000012b2      setbuf(fp: stderr, buf: nullptr)
000012c1      puts(str: "I'm sure you all enjoy doing she…")
000012d0      puts(str: "But have you ever tried ELF golf…")
000012df      puts(str: "Have fun!")
000012f3      int32_t rax_1 = memfd_create("golf", 0)
000012ff      if (rax_1 s< 0) {
0000130b          perror(s: "failed to execute fd = memfd_cre…")
00001315          exit(status: 1)
00001315          noreturn
00001315      }
0000132b      void var_38
0000132b      int32_t rax_2 = read(fd: 0, buf: &var_38, nbytes: 0x20)
00001337      if (rax_2 s< 0) {
00001343          perror(s: "failed to execute ok = read(0, b…")
0000134d          exit(status: 1)
0000134d          noreturn
0000134d      }
00001366      printf(format: "read %d bytes from stdin\n", zx.q(rax_2))
0000137d      int32_t rax_7 = write(fd: rax_1, buf: &var_38, nbytes: sx.q(rax_2))
00001389      if (rax_7 s< 0) {
00001395          perror(s: "failed to execute ok = write(fd,…")
0000139f          exit(status: 1)
0000139f          noreturn
0000139f      }
000013b8      printf(format: "wrote %d bytes to file\n", zx.q(rax_7))
000013d4      if (fexecve(fd: rax_1, argv, envp) s< 0) {
000013e0          perror(s: "failed to execute fexecve(fd, ar…")
000013ea          exit(status: 1)
000013ea          noreturn
000013ea      }
000013f8      *(fsbase + 0x28)
00001401      if (rax == *(fsbase + 0x28)) {
00001409          return 0
00001409      }
00001403      __stack_chk_fail()
00001403      noreturn


In the decompilation, the binary is reading and executing a file using memfd_create and fexecve. This is a method of executing binaries without making a file on the filesystem, it exists only in memory. The challenge only allows binaries of maxiumum size of 32 and making a valid ELF this small should be impossible. The problem is that remote does not validate that the given file is actually an ELF file (it should start with at least b'\x7FELF') and you can put a shebang instead to get a shell.



cat flag.txt


No unintendeds to see here!