TFC CTF 2023 - SOME TRAFFIC

Description

• Category: Forensic (+ Steganography)
• Difficulty: MEDIUM
• Points: 500 → 50

Our SOC analysts said that in the last few days, some of our employees started to upload a lot of photos on random sites. Check it out.

Flag Format: TFCCTF{…}.

Disclaimer (forensics+stegano)

Download:

• sus.pcapng

Resolution

We have the file sus.pcapng¹ which contains network packet capture.

The challenge talks about photos being uploaded on random site. So let’s find those images using the filter functionality in Wireshark (CTRL+F):

We found 3 PNGs and we export them (File -> Export Objects -> HTTP and Content-Typ: multipart/form-data for images only):

Now we have 3 “partial” PNGs because we still need to remove the form data boundaries (here with ImHex):

After we fixed those 3 images, we can use steganography tools to find hidden data.

• we first used stegoVeritas to find any information in transformed images (LSB, RGB) but we cound not find any useful information;

• then we used zsteg to detect any stegano-hidden and we found something interesting:

  $ zsteg -a 1.png 
  imagedata           .. file: MacBinary, Mon Feb  6 07:28:16 2040 INVALID date, modified Mon Feb  6 07:28:16 2040 "F"
  ...
  b4,g,msb,yx         .. file: OpenPGP Public Key
  b8,g,lsb,yx         .. text: "FCCTF{H1dd3n_d4t4_1n_p1x3ls_i5n't_f4n_4nd_e4sy_to_f1nd!}TFCCTF{H1dd3n_d4t4_1n_p1x3ls_i5n't_f4n_4nd_e4sy_to_f1nd!}TFCCTF{H1dd3n_d4t4_1n_p1x3ls_i5n't_f4n_4nd_e4sy_to_f1nd!}TFCCTF{H1dd3n_d4t4_1n_p1x3ls_i5n't_f4n_4nd_e4sy_to_f1nd!}TFCCTF{H1dd3n_d4t4_1n_p1x3ls_"
  ...
  

And this time we get the flag: TFCCTF{H1dd3n_d4t4_1n_p1x3ls_i5n't_f4n_4nd_e4sy_to_f1nd!}.

Additional ressources

1. PcapNg - Wireshark ↩