SEC-T CTF - Confusion Writeup

This time I participated in the SEC-T CTF event and it was pretty fun! I played with a group of people from my university and we managed to get quite some points. But I didn't manage to solve some of the challenges on time. However this didn't stop from trying to solve them once the event was over!

One of the challenges I was hooked on was called Confusion. The reason why was because it seemed like an "easy" challenge but for some reason I couldn't figure it out! The challenge began by downloading the following image:

confusion

I ran binwalk on the file and got the following results:

DECIMAL       HEXADECIMAL     DESCRIPTION
--------------------------------------------------------------------------------
0             0x0             PNG image, 422 x 92, 8-bit/color RGBA, non-interlaced
91            0x5B            Zlib compressed data, compressed
2128          0x850           GIF image data, version "87a",
2259          0x8D3           gzip compressed data, from FAT filesystem (MS-DOS, OS/2, NT), NULL date (1970-01-01 00:00:00)
2600          0xA28           JPEG image data, EXIF standard

Alright, it looks like we got some hidden files within the image. I ran strings as well and found some interesting stuff:

1337.pdf
.1337
WYSIWYG: Does not compute,what would Phil Katz say?

According to google, Phil Katz is the co-creator of the Zip file format, which got me thinking that there has to be an embedded .zip file containing something called 1337. Easy peasy!

I went to Wikipedia to lookup the magic number used for the zip format. Turns out it can be one of the following:

PK\x03\x04 , PK\x05\x06 (empty archive), or PK\x07\x08

I loaded the image file in my HEX editor (Hex Fiend) and begun searching for these magic numbers. I found 1 occurrences of PK which was followed by 1337. Looks like I cracked the case....or not. I tried copying different parts of the data after the first occurrence of PK to a new file but no joy.

Screen Shot 2016 09 10 at 23 28 52

Figure 2: First occurrence of PK

I was stuck at this point and begun researching more about zip files and found these two resources, 1 and 2. I learned quite a lot about the structure of .zip files. According to 1 the .zip file should begin with PK and end with Pk. However I couldn't find any other Pk so obviously I was missing something. I even tried inserting the correct trailer but that didn't work as well.

After some hours of testing and reading I finally noticed the UEs= which can be seen in figure 2. Initially I thought it was base64 encoded data but didn't really act on that instinct, until later. It actually was base64 encoded data and it turned out to be PK! Finally some progress.

I tried to copy the data between the first occurrence of PK and UEs= (PK) but that didn't work either. My next idea was to search for more occurrences of UEs= and lo and behold, I found one.

Screen Shot 2016 09 10 at 23 57 32

Figure 3: Second occurrence of UEs=

I decoded the second UEs= found and copied the data between that one and the first PK because according to 1, the .zip file should start with PK and end with Pk. I created a new file with the copied data, saved as file.zip and tried to unzip but no joy! I knew I had to be on the correct path so I continued working with the HEX values for quite some time but I couldn't unzip the file without errors. I started to google how to recover corrupted zip files and found this command:

$ zip -FF file.zip --out result.zip

I ran the command and it successfully recovered the .zip file and dropped a .pdf file containing the following text:

Screen Shot 2016 09 11 at 00 14 03

Figure 4: A font consisting of symbols

I copied the text into my URL bar and BAM, the flag was: sect{I_4m_n0t_3ncrypt3d}

Conclusion

SEC-T CTF was fun and I will most definitely play again in the future. I was a little bit disappointed that I couldn't solve this challenge in time, but hey, next time there won't be any .zip files that can hide from me ;)

Never doubt your instincts!

References

[1] http://resources.infosecinstitute.com/steganography-what-your-eyes-dont-see/

[2] https://users.cs.jmu.edu/buchhofp/forensics/formats/pkzip.html