Gitea: OAuth2 sign-in reactivates an administrator-deactivated account on auth sources without refresh tokens (incomplete fix of #38009)

Description

Gitea's OAuth2 sign-in callback reactivates a deactivated user account (IsActive=false) when the user signs in through an authentication source that does not issue refresh tokens (notably GitHub, and any OIDC/OAuth2 source configured without offline_access). PR #38009 added a gate intended to reactivate users only when the OAuth2 auto-sync cron had disabled them, using "the stored refresh token is empty" as the signal. That signal is wrong: for sources that never issue refresh tokens, an empty refresh token is the normal state of every user, so the gate cannot distinguish a cron-disabled account from one an administrator deliberately deactivated. The next time the administrator-deactivated user signs in through the provider, Gitea sets IsActive=true and grants a full session, silently undoing the administrator's action. This is the exact behavior #38009 was written to prevent. (ProhibitLogin, the hard ban, is enforced separately and is not affected.)

No special privileges are required beyond being the deactivated user and being able to sign in through the source.

Root Cause

routers/web/auth/oauth.go (the handleOAuth2SignIn reactivation gate):

if !u.IsActive {
    extLogin, hasExt, err := user_model.GetExternalLogin(ctx, authSource.ID, gothUser.UserID)
    if err != nil { ctx.ServerError("GetExternalLogin", err); return }
    isDisabledByAutoSync := hasExt && extLogin.RefreshToken == ""   // wrong signal
    if isDisabledByAutoSync {
        opts.IsActive = optional.Some(true)                          // reactivates the account
    }
}

The assumption that RefreshToken == "" is produced only by the auto-sync cron is false:

• The cron's disable path is unreachable for sources without refresh tokens. services/auth/source/oauth2/source_sync.go returns early: if !provider.RefreshTokenAvailable() { return ... }, so it never disables (or touches the tokens of) such users.
• The stored token is exactly what the provider returned, with no synthesizing: services/externalaccount/user.go stores RefreshToken: gothUser.RefreshToken. When the provider issues none, this is "" from the first login.
• GitHub never issues a refresh token: goth hardcodes func (p *Provider) RefreshTokenAvailable() bool { return false } (providers/github/github.go). OIDC/OAuth2 without offline_access likewise store "".

So for a GitHub (or no-refresh-token) source, RefreshToken == "" is the state of every user, including one an administrator deactivated, and the gate reactivates them.

Proof of Concept

Setup:

• A Gitea instance with a GitHub authentication source (Admin Panel -> Authentication Sources -> OAuth2 -> GitHub), or any OAuth2/OIDC source configured without offline_access.
• Account V: a normal user who has signed in at least once through that source (an external_login_user row exists with empty refresh_token).

Steps:

1. As an administrator, open Admin Panel -> Users -> V and uncheck "Activated" (is_active=false). Confirm V's requests now bounce to the activation page.
2. As V, sign in again via "Sign in with GitHub" and complete the provider flow.
3. V lands in the application with a working session. SELECT is_active FROM "user" WHERE lower_name='v'; now returns true.

Expected (intended by #38009): V stays is_active=false and is routed to the activation page. Actual: V is is_active=true with a full session — the administrator's deactivation is undone.

- GitHub user, ADMIN deactivated                 refreshToken=""    -> REACTIVATED + session granted   <<< admin action undone
- OIDC user w/ refresh token, ADMIN deactivated  refreshToken="..." -> stays disabled  (control)
- OIDC user, AUTO-SYNC cron disabled             refreshToken=""    -> REACTIVATED (intended)
RESULT: BYPASS CONFIRMED.

Gitea's own regression test TestOAuth2CallbackReactivationGating ("auto-sync-disabled user is reactivated") sets RefreshToken="" and asserts reactivation after a full OIDC callback — that state is identical to a GitHub-source user an administrator deactivated.

Impact

Any Gitea instance using a GitHub authentication source (one of the most common) or an OIDC/OAuth2 source without refresh tokens, that relies on the "Activated" toggle to disable accounts, is affected. A deactivated user restores their own account to active and obtains a session, regaining whatever access the account had. Deactivation does not clear IsAdmin, so a deactivated administrator regains admin access. Bound: accounts disabled with "Prohibit Login" stay blocked; this defeats the IsActive=false deactivation only.