Moved discussion to Security > IPS forum for correct placement.
The "wannacry" ransomware is an exploit which is addressed in Cisco IPS Signature file 982, as noted in this Cisco security response bulletin:
https://tools.cisco.com/security/center/content/CiscoSecurityResponse/cisco-sr-20170515?vs_f=Cisco%20Security%20Response&vs_cat=Security%20Intelligence&vs_type=RSS&vs_p=MS17-010%20(Ransomware%20WannaCry)%20Impact%20to%20Cisco%20Products&vs_k=1.
To update your IPS 4255 appliance you require a current subscription / support contract. With that, you can download the signature and install it on the IPS using cli, Cisco IPS Device Manager (IDM), IPS Manager Express (IME) or Cisco Security Manager (CSM).
The download can be found here:
https://software.cisco.com/download/release.html?mdfid=286129615&flowid=49062&softwareid=282549755&release=S982&relind=AVAILABLE&rellifecycle=&reltype=latest
Additional details for installation are in the readme file included on the signature file download page:
http://www.cisco.com/web/software/282549755/138269/IPS-sig-S982.readme.txt
