SSO picked the local account by the email/preferred_username claim, which
Entra does not verify. Anyone able to present a matching email (another
tenant with a multi-tenant issuer, or an editable attribute) could sign in
as that user without a password.
- The first SSO login links the account to "<iss>|<oid>" (new unique
column User.ssoSubject); afterwards only that identity signs in, and a
matching email from another identity is refused.
- Multi-tenant issuers (common/organizations/consumers) disable SSO.
- Emails are compared case-insensitively and stored lower-case, so
[email protected] and [email protected] can no longer be two accounts; the migration
lower-cases existing emails where that does not collide.
- An admin changing a user's email clears the link.
Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>
