Two recommendations come to mind. One is to establish a good security and compliance awareness program. Generally,...
if people are told not to do something -- like access another person's account without proper authorization -- they'll comply.
Second, for extremely sensitive data applications, use two-factor authentication. Username/password authentication in an SSO environment is much riskier than two-factor authentication, as you note. And two-factor authentication doesn't mean all users have to carry a smart card; tokenless two-factor authentication services are readily available and not as expensive as their hard-token counterparts.
But the biggest recommendation I have to offer is to do a market study on whether hardening single sign-on environments is even necessary. (Ask the single sign-on vendor(s) you're working with to help with this.) Generally, organizations are able to implement single sign-on internally with few to no issues of compromise using username/password schemes. Also, be sure to discuss security concerns with the senior managers to better understand what their actual fears are and address them directly.
Dig Deeper on Single-sign on (SSO) and federated identity
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