GHSA-rf68-8gjr-36q7

## Summary

Nezha v2.2.3 regresses the GHSA-9rc6-8cjv-rcvx host header injection fix for deployments where the new `dashboard_host` setting is empty. In that configuration, `/api/v1/oauth2/{provider}` again reflects the request `Host` header into the OAuth2 `redirect_uri` sent to the identity provider, even if `install_host` is configured.

## Impact

An attacker who can get a victim to start OAuth2 login through a request that reaches Nezha with a forged `Host` header can make Nezha send an attacker-controlled callback URL as the OAuth2 `redirect_uri`. If the configured OAuth2 provider accepts that redirect URI, the victim's authorization code is sent to the attacker origin. The attacker can then complete the OAuth2 binding or login flow described in GHSA-9rc6.

This is configuration-dependent, but it affects a realistic upgrade/default state because `dashboard_host` is a new optional field. The original v2.2.0 fix fell back to `install_host`; current v2.2.3 only falls back when `dashboard_host` is non-empty.

## Reproduction

On current v2.2.3 / master commit `3d74cd9431a48fa89c6489689eac82a872799ea0`, configure OAuth2 and set an existing dashboard host only through `install_host`, leaving `dashboard_host` empty. Then send the OAuth2 redirect request with a forged Host header:

```http GET /api/v1/oauth2/github?type=1 HTTP/1.1 Host: evil.attacker.test X-Forwarded-Proto: https ```

The generated OAuth2 authorization URL contains:

```text redirect_uri=https%3A%2F%2Fevil.attacker.test%2Fapi%2Fv1%2Foauth2%2Fcallback ```

A minimal route-level unit test demonstrating the issue is:

```go func TestOAuth2Redirect_RegressionWhenDashboardHostEmptyReflectsForgedHost(t *testing.T) { defer setupOAuth2Test(t)() singleton.Conf.InstallHost = "legit-dashboard.example.com" singleton.Conf.DashboardHost = "" singleton.Conf.Oauth2["github"] = &model.Oauth2Config{ ClientID: "client-id", ClientSecret: "client-secret", Endpoint: model.Oauth2Endpoint{AuthURL: "https://idp.example.test/authorize", TokenURL: "https://idp.example.test/token"}, Scopes: []string{"openid"}, }

c, _ := newOAuth2Ctx(t) c.Params = gin.Params{{Key: "provider", Value: "github"}} c.Request.Host = "evil.attacker.test" c.Request.URL.RawQuery = "type=1" c.Request.Header.Set("X-Forwarded-Proto", "https")

resp, err := oauth2redirect(c) require.NoError(t, err)

parsed, err := url.Parse(resp.Redirect) require.NoError(t, err) require.Equal(t, "https://evil.attacker.test/api/v1/oauth2/callback", parsed.Query().Get("redirect_uri")) } ```

The corresponding control with `DashboardHost = "panel.example.com"` pins the same forged request to `https://panel.example.com/api/v1/oauth2/callback`, so the issue is specifically the empty `dashboard_host` fallback.

## Root cause

Current `cmd/dashboard/controller/oauth2.go` chooses the request Host unless `DashboardHost` is non-empty:

```go host := c.Request.Host if !singleton.IsReservedDashboardHost(host) && singleton.Conf != nil && singleton.Conf.DashboardHost != "" { host = singleton.Conf.DashboardHost } return scheme + host + "/api/v1/oauth2/callback" ```

The v2.2.0 fix used `InstallHost` as the fallback for untrusted request hosts. After `DashboardHost` was introduced, empty `dashboard_host` now disables host pinning and reintroduces the original Host header injection behavior for those deployments.

## Remediation

Build the OAuth2 callback URL from a deterministic configured origin. For example:

1. Use `DashboardHost` when it is set. 2. Otherwise fall back to `InstallHost` if it is set, preserving the v2.2.0 invariant for existing deployments. 3. If neither value is set, reject OAuth2 redirect initiation with a configuration error instead of reflecting request Host.

Please also add a regression test that `InstallHost` plus empty `DashboardHost` does not reflect a forged Host into `redirect_uri`.