Earlier today the freenode infra team noticed an anomaly on a single IRC server. We have since identified that this was indicative of the server being compromised by an unknown third party. We immediately started an investigation to map the extent of the problem and located similar issues with several other machines and have taken those offline. For now, since network traffic may have been sniffed, we recommend that everyone change their NickServ password as a precaution.
Since traffic may have been sniffed, you may also wish to consider any channel keys or similar secret information exchanged over the network.
Freenode Staff Blog: https://blog.freenode.net/2014/09/server-issues-2/