CVE-2026-91039

Authentication Bypass by Spoofing vulnerability in team-alembic ashauthentication allows an attacker who operates one identity-provider connection of a dynamicoidc strategy to be signed in as a local user established through a different connection.

The strategy is meant to keep each connection in its own identity namespace by writing every UserIdentity row's strategy field as "<name>/<connectionid>", but that namespacing never takes effect. connectionid is populated only on the ephemeral runtime struct built per request in dynamicoidc/plug.ex, and DynamicOidc.IdentityChange.change/3 re-fetches the strategy from the compile-time DSL through Info.strategyforaction, yielding the persisted struct whose connectionid is its defstruct default of nil. OAuth2.identitystrategyname/1 therefore falls back to the bare strategy name for both the identity write and the reads in oauth2/userresolver.ex and oauth2/signin_preparation.ex. Since the identity resource's unique key is (uid, strategy), one row exists per sub across every connection, and the identity-match branch runs before any email check. Neither strategy handles iss, so nothing else distinguishes the issuers: OpenID Connect Core section 5.7 makes sub unique only within an issuer, so two connections numbering subjects independently share one subject space.

This issue affects ash_authentication: from 5.0.0-rc.10 before 5.0.0-rc.14.