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🧪 cat.jpg Challenge Report – picoCTF

1. Identification

Description:\ The challenge asks us to find the flag in a JPEG image named cat.jpg. The hint mentions that files can always be changed in a secret way, suggesting hidden metadata or steganography.

Tools used:

  • file

  • exiftool

  • base64

Procedure:

$ file CatTheCTF
CatTheCTF: JPEG image data, JFIF standard 1.02, aspect ratio, density 1x1, segment length 16, baseline, precision 8, 2560x1598, components 3

Result:\ The file is a standard JPEG image (2560x1598 pixels), which can be displayed normally. However, it may contain hidden information in its metadata.

2. Acquisition

Procedure:\ The image was downloaded from the provided link using wget and copied for analysis under a new name to preserve the original:

$ wget https://mercury.picoctf.net/static/d1375e383810d8d957c04eef9e345732/cat.jpg
$ cp cat.jpg CatTheCTF

Result:

  • Original file: cat.jpg

  • Working copy: CatTheCTF

The original file remains unchanged for verification purposes.

3. Preservation

Goal:\ Ensure the file's integrity during analysis.

Steps:

  • Create a copy for analysis (CatTheCTF)

  • Keep the original (cat.jpg) untouched

  • Optionally, verify the file hash:

$ sha256sum cat.jpg CatTheCTF 
ab639e3a45a4f76efe104953a77b709e441e0546ec9fb08780e57fe3644d4250  cat.jpg
ab639e3a45a4f76efe104953a77b709e441e0546ec9fb08780e57fe3644d4250  CatTheCTF

Result:\ The file is preserved and ready for analysis without any changes to its original content.

4. Analysis

Goal:\ Look for hidden data inside the image (metadata or embedded information).

Steps:

  1. Check metadata using ExifTool:
$ exiftool CatTheCTF 
ExifTool Version Number         : 11.88
File Name                       : CatTheCTF
Directory                       : .
File Size                       : 858 kB
File Modification Date/Time     : 2025:08:20 23:42:31+00:00
File Access Date/Time           : 2025:08:20 23:43:09+00:00
File Inode Change Date/Time     : 2025:08:20 23:42:31+00:00
File Permissions                : rw-rw-r--
File Type                       : JPEG
File Type Extension             : jpg
MIME Type                       : image/jpeg
JFIF Version                    : 1.02
Resolution Unit                 : None
X Resolution                    : 1
Y Resolution                    : 1
Current IPTC Digest             : 7a78f3d9cfb1ce42ab5a3aa30573d617
Copyright Notice                : PicoCTF
Application Record Version      : 4
XMP Toolkit                     : Image::ExifTool 10.80
License                         : cGljb0NURnt0aGVfbTN0YWRhdGFfMXNfbW9kaWZpZWR9
Rights                          : PicoCTF
Image Width                     : 2560
Image Height                    : 1598
Encoding Process                : Baseline DCT, Huffman coding
Bits Per Sample                 : 8
Color Components                : 3
Y Cb Cr Sub Sampling            : YCbCr4:2:0 (2 2)
Image Size                      : 2560x1598
Megapixels                      : 4.1

Important output:

License : cGljb0NURnt0aGVfbTN0YWRhdGFfMXNfbW9kaWZpZWR9

  1. Analyze the encoded data:\ The License field contains a Base64-encoded string.

  2. Decode Base64:

$ echo cGljb0NURnt0aGVfbTN0YWRhdGFfMXNfbW9kaWZpZWR9 | base64 -d

Result:\ The flag hidden in the metadata is:

picoCTF{the_m3tadata_1s_Try_it_yourself}

5. Reporting

Summary of Analysis:

Phase Description
Identification File identified as a standard JPEG. Metadata could hide information.
Acquisition File downloaded and copied for safe analysis.
Preservation Original file kept intact, working copy used for analysis.
Analysis ExifTool revealed Base64 text in the License field. Decoding revealed the flag.
Reporting Final flag: picoCTF{the_m3tadata_1s_Try_ityourself}

✅ Flag:

picoCTF{the_m3tadata_1s_Try_it_yourself}

💬 "Control the code, and you control the world." 🔐 From wiping metadata to gaining root access — every step is documented and my goal is to deeply understand the system, not just hack!

Abdelwahab Shandy

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AS Cyber “)).