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Hack.LU 2016 CTF CthCoin Writeup

Introduction

I participated in the Hack.LU CTF again this year with, just like in 2013, but now together with the great Team HacknamStyle from KU Leuven. We ended up 24th of 220 active teams by solving the CthCoin challenge (20 solves), among others:

CthCoin (Category: Crypto/Web)

Cthulhu awakens and all worshippers will be rewarded greatly! A new Cryptocurrency was created, and Cthulhu generous gives away free coins. Can you break it, but be careful do not provoke him.

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How My Rogue Android App Could Monitor & Brute-force Your App’s Sensitive Metadata

TL;DR: A rogue Android app could read any other App’s file metadata: filename, size, last modification date. If a filename contained sensitive predictable data, the rogue Android app could locally brute-force this, which was the case for Instagram on Android. Through the leakage of filesize and last modification date, a rogue Android app could monitor real-time usage of others apps. The file system permissions bug has been present in Android since the very beginning. Google rated this vulnerability as a low risk issue and paid out a $500 bug bounty.

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How I Could Steal Money from Instagram, Google and Microsoft

TL;DR: Instagram ($2000), Google ($0) and Microsoft ($500) were vulnerable to direct money theft via premium phone number calls. They all offer services to supply users with a token via a computer-voiced phone call, but neglected to properly verify whether supplied phone numbers were legitimate, non-premium numbers. This could have allowed a dedicated attacker to steal thousands of EUR/USD/GBP/… . Microsoft was exceptionally vulnerable to mass exploitation by supporting virtually unlimited concurrent calls to one premium number. The vulnerabilities were submitted to the respective Bug Bounty programs and properly resolved.

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