Admidio is an open-source user management solution. Prior to version 5.0.9, 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. This issue has been patched in version 5.0.9.

Affected: Packagist: admidio/admidio

## 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 mat…