Vikunja: OIDC email-fallback account linking ignores email_verified, enabling local-account takeover

## Summary With the per-provider OIDC `emailfallback` option enabled, Vikunja links an SSO login to a pre-existing **local** (username+password) account using only the `email` claim — no `email_verified` (or Microsoft `xms_edov`) check and no password check, on the unauthenticated callback. An attacker who can make the configured issuer emit a token bearing a victim's email logs in as that victim with a full session and no victim interaction (the nOAuth / Grafana CVE-2023-3128 class). The 2.3.0 fix for GHSA-8jvc-mcx6-r4cg added a TOTP gate, not an `email_verified` gate, so users without TOTP remain exposed. ## Details References are `pkg/modules/auth/openid/openid.go` at HEAD. Identity is first resolved on the immutable `(issuer, subject)` pair (`openid.go:428`). On a subject miss with `emailfallback` on, `fallbackSearchUsers` adds an email-only lookup against local accounts: ```go // openid.go:413 searches = append(searches, &user.User{Issuer: user.IssuerLocal, Email: cl.Email}) ``` `getUser` resolves this via `s.Get()`, which ANDs non-zero fields -> `WHERE issuer='local' AND email=?`. Local users always have `Issuer="local"` (`user_create.go:38`), so the lookup matches any local account by email alone; `getOrCreateUser` returns it and the caller mints a session — the password is never read. The `claims` struct has no `email_verified` field (`openid.go:80`) and `getClaims` never consults one; a repo-wide grep for `email_verified`/`xms_edov` returns nothing. The code already warns about this at `openid.go:388` ("Discouraged for untrusted providers where someone can set email without verification") — but enforces nothing. ## Impact Unauthenticated takeover of any existing **local** account (read/write/delete its projects, tasks, attachments, shares), bypassing the password. Scope notes: only `issuer='local'` accounts are matched (not pure-SSO users); the attacker's `sub` is not bound to the victim record, but the attack is repeatable; TOTP users are protected by the 2.3.0 `enforceTOTPIfRequired` gate (`openid.go:250`), non-TOTP users are not. ## Preconditions 1. Admin enabled `emailfallback: true` (defaults false — a default install is unaffected). 2. The configured issuer lets the attacker assert the victim's unverified email: a self-service IdP (Keycloak/Authentik/Auth0/Dex with editable email), a mixed federation, or a multi-tenant Entra `/common` app. `iss`/`aud` are pinned, but the attacker controls `email`, not the issuer. 3. The victim has a local account. Not reachable against a single-tenant IdP that verifies email and disallows self-set addresses. ## Recommended Fix Add `email_verified` to the `claims` struct and require it `true` on the email-fallback branch before linking to a local account; reject when absent/false. For Entra also require `xms_edov` and pin multi-tenant configs to an allowed-tenant list. Fail closed on an email collision not backed by a verified email from a trusted single-tenant issuer rather than silently logging the caller in.