I just started Burp and analyzed the traffic! this is the result:
There are some data that server respond, but the app doesn't show them; phone numbers.
I didn't know what should I do with these phone numbers to get the flag. This is the captured server response:
HTTP/1.1 200 OK
Server: nginx/1.18.0 (Ubuntu)
Content-Type: application/json
Cache-Control: no-cache, private
Date: Mon, 26 Oct 2020 16:17:47 GMT
X-RateLimit-Limit: 60
X-RateLimit-Remaining: 57
Access-Control-Allow-Origin: *
Connection: close
Content-Length: 1388
[{"id":1,"name":"Bugs Bunny","avatar":"bugs_bunny.jpg","email":"gexeve3991@ngo1.com","address":"4617 Goodwin Avenue","gender":"male","age":"22","phone":"36213893021"},{"id":2,"name":"Mickey Mouse","avatar":"mickey_mouse.jpg","email":"aufgetretenen@iatarget.com","address":"3844 Stiles Street","gender":"male","age":"28","phone":"12369532255"},{"id":4,"name":"Bart Simpson","avatar":"bart_simpson.jpg","email":"buccinator@usabrains.us","address":"2418 Loving Acres Road","gender":"male","age":"35","phone":"55634559910"},{"id":3,"name":"Popeye","avatar":"popeye.jpg","email":"hypnotoxin@lerepo.ga","address":"New Jersey popeye Street","gender":"male","age":"43","phone":"22361255893"},{"id":5,"name":"Patrick Star","avatar":"patrick_star.jpg","email":"patrick_star@gmail.com","address":"Richmond 2136 Queens Lane","gender":"male","age":"18","phone":"41223365236"},{"id":6,"name":"Homer Simpson","avatar":"homer_simpson.jpg","email":"vakantietij@walmartshops.com","address":"Timber Oak Drive","gender":"male","age":"47","phone":"99632531930"},{"id":7,"name":"Olive Oyl","avatar":"olive_oyl.jpg","email":"senpakuh@vidred.tk","address":"Ocala Rhapsody Street","gender":"female","age":"52","phone":"89633366552"},{"id":8,"name":"Sylvester","avatar":"sylvester.jpg","email":"emergera@didacvidal.com","address":"Tigard 2285 Kincheloe Road","gender":"male","age":"31","phone":"77632351752"}]
...
It's ridiculous. just putting the numbers together:
RaziCTF{3621389302112369532255556345599102236125589341223365236996325319308963336655277632351752}