This was the second RSA challenge and was a simple cube root attack. The premise of the challenge is that the exponent used for encryption is very small as compared to the public key modulus. So the effect of mod(n) in RSA encryption has no effect. Read the writeup for detailed explanation.

Original writeup (https://c12yptonic.github.io/ctf-writeups/ctfs/deconstructf21#rsa-2-).