Bitcoin private key can be compared with an ID, you give it to send money. Hence, if somebody knows your private key, this somebody can manage your account like you would do, for example. Hence, private keys are rather complex and they aren’t so easy to decode, which is actually the main pride of bitcoin – the security.
Though, in its blog, Ethereum informs users that there is a vulnerability in Mist Browser Beta, and this vulnerability opens a door to thieves. The security team announced in Ethereum blog, that there are vulnerabilities and warned the users that they shall not browse untrusted or suspicious websites.
The team informed as well that wallet apps are not affected by the vulnerability, hence, they can be used freely, without concerns.