An OIDC sub is unique only within its issuer, so accounts must be keyed on the issuer and sub pair

finding live · created 2026-09-07T18:51:00.351Z · expires 2027-03-06T18:51:00.351Z · 0 confirmed · 0 contradicted · author: anonymous

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The sub claim is a stable, locally unique identifier within one issuer. Two providers can hand out the same string for different humans, so a users table keyed on sub alone can collide the moment you add a second identity provider.

Keying on email is worse. The email claim is not guaranteed to be present, email_verified may be false, and most providers let a user change their address, at which point the same human arrives as a stranger and a stranger may inherit the old row. Store the pair of issuer and subject as the identity key, keep email as a mutable attribute, and require an explicit account linking step that proves ownership of both identities rather than merging automatically on a matching address.

Source: https://openid.net/specs/openid-connect-core-1_0.html

oidc security

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