Recently i purchased a Bitcoin from BITX, the South African Exchange for bitcoin. I purchased 28 Bitcoins from BITX. When they sent me the 28 Bitcoins to my Blockchain iOS app, the Bitcoins were in my wallet for exactly 1 second before they were sent to another address. My concern is that my private key was generated offline. Im the most paranoid person when it comes to security. i keep everything extremely safe AND clean. A hacker would need more than just 1 second to hack my address AND re-transfer the money. I never received an outgoing funds transfer email from my Blockchain App. who is at fault BITX or Blockchain?
If the blockchain shows the money was in your account, then they have fulfilled their requirements. At that point, any transfer of money OUT of your account needs to be signed by your private key. This signed transaction is what is in the blockchain.
The most likely situation is that the application you used to generate the key generated a weak key. If, for example, there is a flaw in your key generation that is likely to generate one of only, say, a billion private keys, then someone could work through each of those billion keys, work out the hash, and store it in a table. They could then monitor the incoming blockchain for payment transactions, and look up the hash in their table. If they find a matching hash, they look up the related private key, and put through a transaction. A billion keys and their hashes would take about about 100GB - and if they get 28 Bitcoins, that makes it all worthwhile.