Practical Binary Analysis – CTF Walkthrough – Level 3, 4
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
At offset 0x20 we have the
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  .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.
0x1504 we change the type from SHT_NOBITS to SHT_PROGBITS.
After we run the binary we get the valid flag:
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!