I sent out 2 invitations to all of my friends for my birthday! I’ll know if they get stolen because the two invites look similar, and they even have the same md5 hash, but they are slightly different! You wouldn’t believe how long it took me to find a collision. Anyway, see if you’re invited by submitting 2 PDFs to my website. http://mercury.picoctf.net:55343/
🎯 Challenge Files & Infrastructure
Provided Files
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Files:None
🔍 Initial Analysis
First Steps
Initially, the website appears as follows:
Site Presentation
With an upload form for two files. From the description of the challenge, I understood that two different files must be uploaded but with the same MD5. This is a vulnerability called MD5 Collision, where it’s possible to change some sections of the file and obtain the same md5 for both files, even though they are different. After doing some research, I found a tool https://github.com/brimstone/fastcoll that allows generating two files with the same md5 but actually different, exploiting a collision. Let’s proceed with the exploitation.
🔬 Vulnerability Analysis
Potential Vulnerabilities
MD5 Collision Attack
🎯 Solution Path
Exploitation Steps
Initial setup
First, I installed fastcoll and generated the files with the collision:
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git clone https://github.com/upbit/clone-fastcoll fastcoll
cd fastcoll
make
cd ..
./fastcoll/fastcoll md5_data
Fastcoll
This created two files, md5_data1 and md5_data2. In fact, when trying to use md5sum, which generates the hash of the file:
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md5sum md5_data*
Md5Sum
As we can see, they are identical. Now, let’s check if the content is also identical, since there is a check during the upload phase to see if the files are exactly the same. We can do this with the diff command:
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diff md5_data1 md5_data2
Diff
As we can see, there are differences (otherwise, the output would be empty). Now that we’ve created the files, let’s move on to the exploitation.
Exploitation
The files created are binary files without an extension. However, I discovered through a test that by changing the extension of any file to .pdf, the server accepted it. Since there is no actual check on the file’s header, we can directly rename the two files created earlier:
The automatic exploit uses subprocess to generate two files with the same MD5 but different content (thus creating a collision) via the tool fastcoll. Once the two files are generated, they are renamed with the .pdf extension since there is a file extension check during the upload process. Then, a POST request is made to upload the files, and the flag is extracted from the response using a regex.
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# Requirementsgit clone https://github.com/upbit/clone-fastcoll fastcoll
cd fastcoll
make
