Nasdaq OMX, the company that manages a number of stock exchanges including the US Nasdaq and others in Copenhagen, Stockholm, Helsinki, and the Baltic region underwent a number of hack attempts over the last year. However, the stock exchange servers were safe from the attack and were not compromised.
Federal authorities are investigating the matter, which has been revealed a bit late. One of the sources on this matter was spotted saying,
So far, [the perpetrators] appear to have just been looking around.
Last year, spikes were seen in the stock price activities of companies, twice with a gap of 4 months in between. This was quite alarming and it has been a major cause behind the ongoing investigation. However, the cause for this matter seems to be erroneous handling and not hacking.
The incident can be seen in two ways. Either, Nasdaq OMX computers were compromised or, the attackers were targeting the stock exchange. Either way, a thorough check of both the company and the exchange networks are necessary before jumping into conclusions. The recent Stuxnet worm has proven beyond doubt that these hacks have reached an uber level of sophistication and we cannot assume any security with current measures.