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Shirley Mali
Shirley Mali

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Recovering a Flag from an RDP Cache

#cybersecurity #forensics #ctf #digitalforensics

Description: Learn how I solved the Job Interview challenge on Root-Me by converting an EnCase image, detecting hidden archives, and uncovering sensitive RDP cache screenshots using open-source tools.

🧠 Root-Me Forensics Challenge: Job Interview

The “Job Interview” challenge from Root-Me's Forensic section is an exciting test of your ability to work with forensic images and uncover hidden artifacts.

In this walkthrough, I’ll show how I:

  • Extracted a hidden archive from a forensic .E01 image
  • Identified and unpacked an RDP bitmap cache
  • Analyzed screenshots for sensitive information
  • Ultimately recovered the flag

🧰 Tools I Used

Tool Use Case
ewfexport Convert EnCase .E01 image to .raw
file, tar Identify file types and extract archives
bmc-tools Decode .bmc RDP bitmap cache
eog View extracted .bmp screenshots
binwalk (optional) Analyze file internals for signatures

🪪 Step 1: Convert .E01 to .raw

The challenge provides an EnCase image file: image_forensic.e01. This needs to be converted into a raw binary format.

Use the following command:

ewfexport image_forensic
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When prompted, input the following:

  • Export format: raw
  • Target path and filename: image
  • Segment size: (just press Enter for default)

This will generate:

image.raw
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⚠️ Don't add the .e01 again — the tool detects it automatically.

🔍 Step 2: Investigate the File Type

Now, don’t just assume that image.raw is a true raw disk image. Use the file command:

file image.raw
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Output:

image.raw: POSIX tar archive (GNU)
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🎯 It’s not a disk image — it’s a .tar archive disguised with a .raw extension.

📦 Step 3: Extract the Archive

Unpack the tar file:

tar -xvf image.raw
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This extracts:

bcache24.bmc
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🧠 Step 4: What Is a .bmc File?

.bmc files are bitmap cache files used by Windows Remote Desktop Protocol (RDP).

These files contain screen fragments cached during an RDP session. They can reveal:

  • Screenshots of documents
  • Passwords or flags displayed
  • Session activity logs

Since this format is not natively supported, we’ll use an open-source Python tool called bmc-tools.

🛠️ Step 5: Extract .bmp Screenshots Using bmc-tools

5.1 Clone the Repository 

git clone https://github.com/ANSSI-FR/bmc-tools.git
cd bmc-tools
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5.2 Create Output Directory 

mkdir ../bcache24bmc
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5.3 Run the Tool 

./bmc-tools.py -s ../bcache24.bmc -d ../bcache24bmc/ -v
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  • -s:Source .bmc file
  • -d: Output directory for .bmp files
  • -v: Verbose mode

This creates .bmp images in the output folder.

🖼️ Step 6: Review the Extracted Screenshots

To browse the extracted screenshots:

eog ../bcache24bmc/*.bmp
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Manually inspecting the images reveals three screenshots:

- Yeah (RdP)

  • this is the (l3av3s_Tra)
  • flag (c3s)_

🏁 Final Flag

RdP_l3av3s_Trac3S
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🎉 This is the flag displayed in three of the RDP session screenshots!

🧠 Forensic Takeaways

  • Always use file to verify content types
  • Don't trust extensions — .raw can be .tar
  • RDP .bmc files can leak visual data from remote sessions
  • Screenshots are evidence, even if they’re fragments
  • Open-source tools like bmc-tools are vital in DFIR work

📋 Summary of Commands

Step 1: Convert E01 to raw 

ewfexport image_forensic
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Step 2: Inspect the file type 

file image.raw
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Step 3: Extract tar archive 

tar -xvf image.raw
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Step 4: Clone BMC tools and set up 

git clone https://github.com/ANSSI-FR/bmc-tools.git
cd bmc-tools
mkdir ../bcache24bmc
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Step 5: Decode bitmap cache 

./bmc-tools.py -s ../bcache24.bmc -d ../bcache24bmc/ -v
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Step 6: View extracted images 

eog ../bcache24bmc/*.bmp
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🙌 Let’s Connect
If this write-up helped or inspired you:

💻 GitHub:
🔗 LinkedIn:

✍️ Follow me on Dev.to for more CTF and DFIR content

Thanks for reading — and happy hunting! 🧩🕵️‍♀️

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