Is it possible, that the network administrator (wpa2 wifi network) can log all the data the users send and then get password out of it?
I know it is not possible to get the password if I'm surfing with https but what if I only use http pages?
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On any Wi-Fi network - encrypted or not, given today's Wi-Fi encryption protocols - any sufficiently skilled and equipped user of the network (and especially the network administrator) could easily access any data you transmit or receive via cleartext protocols. This includes usernames and passwords as well as web pages, documents, and other data sent or obtained via http, ftp, telnet, etc.
For open networks, gathering cleartext data is as easy as sniffing the traffic in the air. WEP security adds a slight barrier, but is still easily decipherable by even unauthenticated users.
WPA and WPA2 require a good bit more computational power for outsiders to crack, and much more time. For these, an attacker would most likely monitor traffic for awhile and then take the data home for offline cracking. As with just about any cryptography, brute force will always win if given enough time. With WPA and WPA2, that just means a lot of time.
There are side-channel attacks to WPA and WPA2 though. Currently, the Wi-Fi Protected Setup (or similar) features in most SOHO routers has a weakness that will allow an attacker to gain access to your network in fairly short time. Once they've cracked your key through this method, they can join the network like any other user (provided you don't have other protections - most of which are trivially bypassable - in place).
For WPA and WPA2, there are known weaknesses that allow authenticated users (or attackers who have broken into the network) to sniff traffic as if it were unprotected. At this point, the only defense you have is encryption at higher levels of the network stack (i.e.: HTTPS). Even then, many of these higher-level protocols can be subjected to man-in-the-middle (MitM) attacks if the victim is less than vigilant in verifying their SSL certificates (or the attacker has a certificate from a compromised CA).
The only real additional threat that a malicious network administrator would pose, is that they have access to the wired side of the network also. On the wire, traffic is not protected by the same encryption (WEP/WPA/WPA2) that applies to the wireless connection. Anyone on the wire could then sniff your traffic as if it had been sent across an open (unprotected) network on the air.
For example if the administrator terminates the HTTPS on his side and you accept his faulty certificate, the administrator can then decrypt and view your requests in plaintext.
Tools like SSLStrip can also be usefull when attacking HTTPS as MITM. The only difference for the user is usually that a supposed HTTPS site is instead provided as HTTP. Would you notice?