Challenge RE #12

Hi there, let's solve the 12th challenge on RE, from challenges.re. This time it's quite exiting we have a binary file, and always the code. Having the binary will allow us to use Ida in order to see the code, and also it will mark us loops involved in the program. Yes, I'm aware that you can do quite more things with Ida. Let's start analyzing this binary. In case you want to know how to install Ida or other tool that will allow you to disassembly the binary, Google it's your friend.

Analysis

We have to be careful all the time when we are analyzing a binary file, in case the binary it's a malicious program. In this case looking at the binary file, I don't see anything suspicious. The program make use of simple functions from Linux like puts, xtats, time and utime. I mean there's no use of write to file or anything that could affect the files on your computer. Let's assume my guess it's correct, and let's try to run this program first without argument to see it's output. If the source of the binary file it's a trustful one, it's always super helpful to interact with the program. This will help us to describe more precisely what the program does.

In this case a call without argument produces the following output:

[COMMAND]

$ ./e12

[OUTPUT]

Usage: <filename>
error #1!

Immediately that we call the program we get a clue of what the program it's expecting, in this case, we will need to supply a filename. Let's try to supply a non existing filename to see what we get.

[COMMAND]

$ ./e12 lalala

[OUTPUT]

error #1!

We just get the error #1! output. Then ok, let's try now to supply a filename that exists.

[COMMAND]

$ ./e12 example

[OUTPUT]

Interesting, we don't get any output. Ok, it's time to dig on the program, we already explored how it's supposed to be used, let's see in detail what it's doing.

Logic

The first thing we see at the start of the program are these instructions

main:
    push    rbx
    mov     rbx, rsi
    sub     rsp, 160
    cmp     edi, 2
    je      .L2
    mov     edi, OFFSET FLAT:.LC0
    call    puts

Looking at this code we can say that we have only the main method, and we perform a check on the number of parameters supplied to the program in the lines cmp edi, 2 and je .L2. In case we don't supply a file, automatically we output the usage, otherwise we print the error message. Nothing new, that we haven't investigate already by running the program. Let's keep digging.

When we supply the correct arguments, the program make a call to _xstat which it's equivalent to stat. Looking at the doc of this method, you can infer that the program it's storing in rdx the information about the stats of the file. In case of error taking file stats we output the single line error #1!. On the other hand, what happens if the stats are collected successfully? We don't see any output to console, so something must be happening. I mean the program should use these stats for something. The code that handle this parts it's the following:

mov     rax, QWORD PTR [rsp+88]
xor     edi, edi
mov     QWORD PTR [rsp], rax
call    time

mov     rdi, QWORD PTR [rbx+8]
mov     rsi, rsp
mov     QWORD PTR [rsp+8], rax
call    utime

test    eax, eax
js      .L11

Here we have one call to time and another one to utime. The first one retrieves the current time in seconds, storing it in rsp as a time_t structure. While the second one, use this structure to overwrite the last time and modification times of the file. With this we can infer that yes, the program it's not outputting anything to the console, but it's modifying the last time that this file was accessed.

Let's try our hypothesis. To see this information normally in the console, let's run ls -lsh to see the last time the example file, the one we created previously, was accessed. In my case I have

ls -lsh

[OUTPUT]
4.0K -rw-rw-r-- 1 gulolio gulolio    6 Sep 21 18:27 example

We have that the last time, was 21 of September on 18:27. Now let's run the program and right after ls -lsh again, to see if this data changed.

./e12 example && ls -lsh

[OUTPUT]
4.0K -rw-rw-r-- 1 gulolio gulolio    6 Sep 21 18:30 example

Indeed the time changed, so now we can summarize what the program does in a single line.

Formal description

The program modified the last access and modification times of the file specified in the argument.

Conclusion

Quite simple program, also analyzing the program with Ida helped a lot. For example it marks you the flow of the whole program, so you don't have to lose time drawing arrows on a notebook. It's marks you in the program where strings are been used, so you don't have to go back where the string was defined to see it. It even added useful name to variables, like in the following case

mov     rax, [rsp+0A8h+stat_buf.st_atim.tv_sec]

I would highly recommend you to use it, there's a free version for almost every OS(except some exotic option I imagine). Take the time to learn how to use it, could save you a lot of time.