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In this challenge, we arrive on a web page where we can’t access the file because of the link that is expired:
Just adding a big expiry date after the signature will give us access to the file. This is because it will check that we set the expiry and signature values, then that the signature is valid (and it is, it’s just the expiry that is not). End finally, it will check the expiry:
We just polluted the URL and got the flag: 404CTF{l4_p011uti0n_c_3st_m41}