Vikunja is an open-source self-hosted task management platform. In versions 1.0.0 through 2.3.0, when an administrator enables the per-provider `emailfallback` option on an OpenID Connect provider, Vikunja links an SSO login to a pre-existing local (username+password) account using only the `email` claim from the IdP. The fallback never checks an `email_verified` (or Microsoft `xms_edov`) signal and never requires the matched account's password. An attacker who can obtain a token from the configured issuer carrying a victim's email logs in as that victim with a full session, with no consent or interaction from the victim. Version 2.4.0 fixes the issue.

Affected: Go: code.vikunja.io/api

## 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 al…