GHSA-3jp4-mhh4-gcgr

### Summary

The SAML authentication success handler in Kimai returns the `RelayState` POST parameter as a redirect destination without validating the host or scheme. After a user successfully authenticates via SAML, they are redirected to an attacker-controlled URL if the IdP includes a malicious `RelayState` value. This enables phishing attacks that steal credentials or session tokens post-SSO.

*Requires SAML to be enabled (non-default configuration).*

### Details

Vulnerable file: `src/Saml/Security/SamlAuthenticationSuccessHandler.php`

```php // Line 27-33 $relayState = $request->request->get('RelayState', $request->query->get('RelayState')); if (\is_scalar($relayState)) { $relayState = (string) $relayState; if ($relayState !== $this->httpUtils->generateUri($request, (string) $this->options['login_path'])) { return $relayState; // No host/scheme validation — any URL accepted } } ```

The only check is that `RelayState` does not equal the configured `login_path`. Any external URL (e.g., `https://attacker.com`) passes this check and is returned as the redirect destination.

The existing unit test `SamlAuthenticationSuccessHandlerTest::testRelayState()` confirms this behavior — an absolute URL in `RelayState` results in a redirect to that URL with no restriction.

### Steps to Reproduce

``` 1. Enable SAML authentication in Kimai 2. Configure a SAML IdP (e.g., SimpleSAMLphp) 3. Initiate IdP-initiated SSO with RelayState=https://attacker.com — or intercept the ACS POST and modify RelayState to https://attacker.com 4. Complete SAML authentication at the IdP 5. Observe: after the SAMLResponse POST to /saml/acs, Kimai issues: HTTP/1.1 302 Found Location: https://attacker.com ```

Code-confirmed via unit test (`testRelayState`): `onAuthenticationSuccess` with `RelayState=http://localhost/relayed` redirects directly to that URL. External URLs follow the same code path.

### Impact

While this bug exists it has low practical possibilities and the attacker needs to be able to create a SAML request, meaning either admin access to an IdP supporting such an action OR access to the private SAML keys / certificates.

In other words: only exploitable in IdP-initiated SSO flows where the IdP includes a `RelayState` value supplied by the attacker (e.g., via a malicious link to the IdP).

### Fix

The `RelayState` is validated before redirecting, see #5878

- It may not contain a host or port and cannot start with `//`. - If it contains a host, it must match the current host.