CVE-2025-40758 is a high-severity vulnerability affecting Siemens Mendix SAML modules compatible with Mendix versions 9.24, 10.12, and 10.21. The vulnerability stems from improper verification of cryptographic signatures (CWE-347) within the SAML authentication process. Specifically, affected versions of the Mendix SAML module insufficiently enforce signature validation and binding checks. This flaw allows unauthenticated remote attackers to potentially bypass SAML signature verification, enabling them to hijack user accounts in certain Single Sign-On (SSO) configurations. The vulnerability impacts all versions prior to V4.0.3 for Mendix 10.12 compatible, prior to V4.1.2 for Mendix 10.21 compatible, and prior to V3.6.21 for Mendix 9.24 compatible modules. The CVSS v3.1 score is 8.7 (High), reflecting network attack vector, high complexity, no privileges required, no user interaction, and a scope change with high confidentiality and integrity impacts but no availability impact. The vulnerability allows attackers to forge or manipulate SAML assertions due to insufficient cryptographic signature enforcement, undermining the trust model of SAML-based SSO. This can lead to unauthorized access to applications relying on Mendix SAML for authentication, potentially exposing sensitive data and critical business functions. No known exploits are currently reported in the wild, but the severity and nature of the flaw make it a significant risk once exploited. Siemens has published patches in versions V4.0.3, V4.1.2, and V3.6.21 to address this issue, though no direct patch links are provided in the data. Organizations using affected Mendix SAML modules should prioritize upgrading to patched versions to mitigate this risk.

For European organizations, this vulnerability poses a substantial risk, especially those leveraging Mendix applications integrated with SAML-based SSO for identity federation. Successful exploitation can result in unauthorized account takeover without requiring user interaction or prior authentication, potentially leading to data breaches, unauthorized transactions, and disruption of business operations. Given the widespread adoption of Mendix low-code platforms across various sectors in Europe—including finance, manufacturing, healthcare, and public services—the impact could be broad. Confidentiality and integrity of user identities and sensitive data are at high risk, which may also lead to regulatory non-compliance issues under GDPR due to unauthorized access and data exposure. The vulnerability's exploitation could undermine trust in federated authentication systems, complicate incident response, and increase the attack surface for further lateral movement within networks. The lack of known exploits currently provides a window for proactive mitigation, but the high CVSS score indicates that once weaponized, the threat could be severe.

European organizations should immediately assess their use of Mendix SAML modules and identify affected versions. The primary mitigation is to upgrade affected Mendix SAML modules to the fixed versions: V4.0.3 or later for Mendix 10.12 compatible, V4.1.2 or later for Mendix 10.21 compatible, and V3.6.21 or later for Mendix 9.24 compatible. Where immediate upgrade is not feasible, organizations should implement compensating controls such as: 1) Enforce strict SAML binding and signature validation policies at the identity provider (IdP) and service provider (SP) levels to detect and reject malformed or unsigned assertions. 2) Monitor authentication logs for anomalous SSO activity indicative of signature bypass attempts. 3) Employ multi-factor authentication (MFA) as an additional layer to reduce the risk of account compromise. 4) Conduct thorough security reviews of SAML configurations to ensure adherence to best practices, including validating all SAML responses and requests. 5) Limit the exposure of SAML endpoints to trusted networks and implement network segmentation to reduce attack surface. 6) Stay updated with Siemens security advisories and apply patches promptly. These targeted actions go beyond generic advice by focusing on the specific cryptographic signature verification weakness and SSO context.