Practical Binary Analysis – CTF Walkthrough – Level 3, 4

Hello,

In this article I'll present you my solution on the Chapter 5 CTF from the book Practical Binary Analysis.

For this binary, the hint is to fix four broken things.

Running file gives us the following response:

binary@binary-VirtualBox:~/ctf$ file ./lvl3 
./lvl3: ERROR: ELF 64-bit LSB executable, Motorola Coldfire, version 1 (Novell Modesto) error reading (Invalid argument)

And the readelf command gives us:

binary@binary-VirtualBox:~/ctf$ readelf -h ./lvl3 
ELF Header:
  Magic:   7f 45 4c 46 02 01 01 0b 00 00 00 00 00 00 00 00 
  Class:                             ELF64
  Data:                              2's complement, little endian
  Version:                           1 (current)
  OS/ABI:                            Novell - Modesto
  ABI Version:                       0
  Type:                              EXEC (Executable file)
  Machine:                           Motorola Coldfire
  Version:                           0x1
  Entry point address:               0x4005d0
  Start of program headers:          4022250974 (bytes into file)
  Start of section headers:          4480 (bytes into file)
  Flags:                             0x0
  Size of this header:               64 (bytes)
  Size of program headers:           56 (bytes)
  Number of program headers:         9
  Size of section headers:           64 (bytes)
  Number of section headers:         29
  Section header string table index: 28
readelf: Error: Reading 0x1f8 bytes extends past end of file for program headers

At this moment, it was clear that the ELF header is broken, in order to fix it I opened up Wikipedia and the elf specification.

As I went through each field manually, with Binary Ninj. As I was checking the offset of the current byte, at 0x07, Wikipedia says: It is often set to 0 regardless of the target platform. I've changed it to 0x00. (Note: I think this field was probably ok as it is)

At offset 0x12, the value Specifies target instruction set architecture and is currently invalid. From googling, I found an article titled: Novell's Next Generation OS Will Natively Support Intel's Future IA-64 Architecture so I set the value to 0x3E.

At offset 0x20 we have the e_phoff which Points to the start of the program header table. It usually follows the file header immediately, making the offset 0x34 or 0x40 for 32- and 64-bit ELF executables, respectively. The value de ad be ef is clearly invalid. I replaced the value with 40 00 00 00.

At this moment I thought I fixed the binary and ran it, it ran and it gave me an invalid flag.

If you run the following command:

binary@binary-VirtualBox:/media/sf_Dropzone$ readelf -S ./lvl3 | grep .text
  [14] .text             NOBITS           0000000000400550  00000550

You'll see that the .text section is marked as 0x8 - NOBITS and it should be 0x1 - PROGBITS. To make the change I've used Binary Ninja as a hex editor, opening the binary in raw mode.

From the readelf command:

  Start of section headers:          4480 (bytes into file)

The start of the section header is 4480 bytes. A section header has the length of 0x40 bytes. 4480 to hex -> 0x1180. 0x40 * 14 + 0x1180 = 0x1500.

At offset 0x1504 we change the type from SHT_NOBITS to SHT_PROGBITS.

After we run the binary we get the valid flag:

3a5c381e40d2fffd95ba4452a0fb4a40  ./lvl3

---

After finishing level 3 I wanted to go to sleep and instead I thought of running ltrace, strace on the binary and I got this:

binary@binary-VirtualBox:~/ctf$ ltrace ./lvl4
__libc_start_main(0x4004a0, 1, 0x7ffd6fb460e8, 0x400650 <unfinished ...>
setenv("FLAG", "656cf8aecb76113a4dece1688c61d0e7"..., 1)             = 0
+++ exited (status 0) +++

Didn't expect this, very nice tho.

Thanks for reading!