Context
The goal is to read the ddfc.
By clicking the link, we end up on this page:
The page contains a link to this URL:
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https://ddfc.challenges.404ctf.fr/ddfc?expiry=-5625891076&signature=wawF6dC4Hz9g5NyCc3j1KCDcfztFE/sp
This is the page we obtain:
The link is expired. As the title mentionned, the link is supposed to be valid a week. Here the time stamp indicate the date 21 september 1791:
We need to find a way to modify the expiry parameter in the URL.
After a lot of struggling, trying to understand how the signature is formed, I thought that we could do parameter pollution.
So, I tried to add an expiry parameter:
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https://ddfc.challenges.404ctf.fr/ddfc?expiry=-5625891076&signature=wawF6dC4Hz9g5NyCc3j1KCDcfztFE/sp&expiry=
I get this page:
It looks like the second expiry parameter is used instead of the first one. Does it validate the signature though?
I tried to add a big positive value as expiry parameter:
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https://ddfc.challenges.404ctf.fr/ddfc?expiry=-5625891076&signature=wawF6dC4Hz9g5NyCc3j1KCDcfztFE/sp&expiry=5625891076
We get the page 🎉 !
We can find the flag at the bottom of the page:
I hope you enjoyed this challenge !