GitHub Security Advisory: GHSA-p9w9-87c8-m235
Package: admidio/admidio (COMPOSER)
Severity: HIGH (CVSS 8.2)
CWE: CWE-20: Improper Input Validation, CWE-601: URL Redirection to Untrusted Site ('Open Redirect')
Vulnerable versions: <= 5.0.8
Patched in: 5.0.9

Summary: Admidio Sends SAML Response to Unvalidated Assertion Consumer Service URL from AuthnRequest

Details:
## Summary

The SAML IdP implementation in Admidio's SSO module uses the `AssertionConsumerServiceURL` value directly from incoming SAML AuthnRequest messages as the destination for the SAML response, without validating it against the registered ACS URL (`smc_acs_url`) stored in the database for the corresponding service provider client. An attacker who knows the Entity ID of a registered SP client can craft a SAML AuthnRequest with an arbitrary `AssertionConsumerServiceURL`, causing the IdP to send the signed SAML response -- containing user identity attributes (login name, email, roles, profile fields) -- to an attacker-controlled URL.

## Details

The vulnerability is in `src/SSO/Service/SAMLService.php`, in `handleSSORequest()` at lines 439-465:

```php
// Line 439: ACS URL extracted directly from the AuthnRequest (attacker-controlled)
$clientACS = $request->getAssertionConsumerServiceURL();

// ...

// Line 456: Used as the Destination of the SAML Response
$response->setDestination($clientACS);

// Lines 463-465: Also used as the Recipient in SubjectConfirmationData
$subjectConfirmationData = new \LightSaml\Model\Assertion\SubjectConfirmationData();
$subjectConfirmationData
    ->setRecipient($clientACS) // Required recipient URL
```

There is no code that compares `$clientACS` against the client's registered `smc_acs_url` column value. The SAML 2.0 specification and OASIS security considerations explicitly require IdPs to verify that the AssertionConsumerServiceURL matches one of the SP's registered ACS endpoints.

**Signature validation is conditional and does not prevent this attack:**

At lines 417-419:
```php
if ($client->getValue('smc_require_auth_signed') || $client->getValue('smc_validate_signatures')) {
    $this->validateSignature($client, $request, $client->getValue('smc_require_auth_signed'));
}
```

If neither `smc_require_auth_signed` nor `smc_validate_signatures` is enabled for the SP client (which is the default when creating a new client), the AuthnRequest is processed without any signature verification. This means an attacker only needs to know the SP's Entity ID (which is often publicly discoverable from the SP's metadata endpoint) to craft a malicious AuthnRequest.

Even when signatures ARE validated, the ACS URL should still be checked against the registered value, because:
- Signature validation proves the request came from the SP, not that the ACS URL is authorized
- If the SP's signing key is compromised, the ACS URL becomes the last line of defense
- This follows the defense-in-depth principle mandated by SAML 2.0 Profiles Section 4.1.4.1

**The same pattern exists in the error response path** at lines 323-326:
```php
} elseif (method_exists($request, 'getAssertionConsumerServiceURL')) {
    $response->setDestination($request->getAssertionConsumerServiceURL());
} else {
    $response->setDestination($client->getValue('smc_acs_url'));
}
```

**Attack flow:**
1. Attacker discovers the Entity ID of a SAML client registe