This attack could pose a serious security threat. You should take immediate action to stop any damage or prevent further damage from happening.
This signature detects attempts to sniff SMB usernames and passwords through a known challenge key which can then be used to crack the passwords offline.
The SMB sniffer module allows you to capture LM/NTLM hashes that can be cracked later. It uses a known challenge key which allows you to crack the hash offline.