IBM has issued a Security Bulletin for Notes Client single logon feature.
Notes Client Single Logon uses an operating system communication
mechanism for password transmission between Windows and Notes that can
be attacked by malicious code planted on the user workstation to reveal
the user password. To prevent the potential for attack, disable Notes
Client Single Logon and instead opt to use the more secure Notes Shared
Login or Notes Federated Login.
For more details see here