Anonymous systems if exists on both ends of the network, exit relay and entry relay can sniff the packets and statistically correlate the traffic which can point out the destination or the source address of the packet. This also includes eavesdropping at the exit nodes of the network. The data that is between the exit node and the destination server is not encrypted. These data packets can be captured easily. This captured data can reveal the source information, and as suggested by Egersta these nodes are costly to run and require a lot of bandwidth which could suggest that some intelligence agencies might be running them. There have been tested techniques with one third of control nodes to compromise the Tor network.
