Asking users for social security

What are the guidelines behind asking users for social security? And yes, its to enhance user experience.
As a US citizen I would say that asking for someone’s SSN is never acceptable outside of a US government use.

Even for government use the full number is rarely requested. I normally see requests for the last 4 digits along with several other bits of information to prove identity.
Asking users for social security
 
 
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