ctf-writeup/UMass CTF 2023/Deepfried

Deepfried

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About the Challenge

We have been given a source code (You can download the file here)

How to Solve?

If we check the the source code, we will see there is a folder called restriced_memes and inside the folder there is an image called TheFlag.jpg so I'm assumming we need to access /restricted_memes/TheFlag.jpg to obtain the flag

But we can't access that endpoint directly because that endpoint can only be accessed locally.

router.all('/restricted_memes/:img', async (req,res, next)=>{
    if(req.ip === '::ffff:127.0.0.1') {
        next();
    } else {
       return res.status(403).send("Unauthorized Request");
    }
})

So, the idea here. We need to input http://127.0.0.1:3000/restricted_memes/TheFlag.jpg in the Image URL form

As we can see in the image, the flag was located in flag.txt directory. So we need to input http://127.0.0.1:3000/restricted_memes/flag.txt in the Image URL form

Press CTRL + U to check the source code

You will see there is a Base64 string in img tag. Decode the msg using Base64 Decoder to obtain the flag

UMASS{v@Mo$_APr0nTaR_1!i!I!}