Author: 1nt3rc3pt0r
Initial Analyisis
Looking at the challenge you can observe it geting redirected http://gc1.eng.run:32357/index.php?loop=0 to http://gc1.eng.run:32357/index.php?loop=10 and there exist a encoded flag(ivyiw{1m5_p3773e_70_h1l3_50i3_71b3}) and and key vigenere is set to 1 at the localstorage of Browser which gives us the hint that used cipher to decode flag is vignere.
solution
So from our initial analysis we know we have to get a key to decode the vigenere cipher and the key is hidden in one of the requests.Now using the burpsuit we can navigate throught each and every requests starting from /index.php?loop=0 to /index.php?loop=10 at /index.php?loop=5 in the Script tag the key getting added to localstorage.
Now we have key and cipher text decoding them gives us the flag.