Implicit sharing on parent-child records
Key Takeaway
If you can access a case, contact, or opportunity through ownership or sharing, you get implicit Read Only access to its parent account, even when the Account org-wide default is Private.
Implicit sharing is a built-in Salesforce mechanism that automatically gives access between an account and its child records, such as cases, contacts, and opportunities. Specific rules decide what level of access implicit sharing grants and where that access comes from. Knowing those rules lets you predict when a user can see an account that a Private Account org-wide default would otherwise hide.
Implicit sharing in both directions
Implicit sharing works in both directions between an account and its child records. In parent implicit access, access to a child record can grant access to the parent account. In child implicit access, access to the parent account can grant access to its child records.
Implicit sharing applies only to standard relationships, not custom ones.