CVE-2025-66578 affects xmlseclibs, a widely used PHP library for XML encryption and digital signatures. The vulnerability stems from a flaw in the interaction between xmlseclibs and the underlying libxml2 library's canonicalization process. Specifically, when libxml2 attempts to canonicalize an invalid XML input, it may return an empty string instead of a canonicalized node set. xmlseclibs fails to detect this failure and proceeds to compute the DigestValue over this empty string, effectively bypassing the intended authentication mechanism. This results in an authentication bypass vulnerability categorized under CWE-248 (Uncaught Exception). The impact is that attackers can craft malicious XML documents that appear valid and signed, allowing them to bypass signature verification and potentially inject unauthorized data or commands. The vulnerability affects all xmlseclibs versions prior to 3.1.4, which addresses the issue by treating canonicalization failures as fatal and aborting validation. Workarounds include adding explicit checks to reject empty or nil canonicalization outputs and handling exceptions properly. The CVSS 3.1 base score is 6.0, reflecting network attack vector, low attack complexity, required privileges, no user interaction, and partial impact on confidentiality, integrity, and availability. No public exploits are known at this time, but the flaw poses a significant risk to systems relying on xmlseclibs for secure XML processing.

For European organizations, this vulnerability can undermine the trustworthiness of XML-based authentication and encryption mechanisms, which are commonly used in web services, SAML assertions, and secure communications. An attacker exploiting this flaw could bypass signature verification, leading to unauthorized access, data tampering, or injection of malicious payloads. This threatens confidentiality by exposing sensitive data, integrity by allowing unauthorized modifications, and availability by potentially disrupting services dependent on XML validation. Sectors such as finance, government, healthcare, and critical infrastructure that rely on XML security standards are particularly vulnerable. Given the network attack vector and the requirement for high privileges, insider threats or compromised accounts could leverage this vulnerability to escalate attacks. The absence of user interaction lowers the barrier for automated exploitation once privileges are obtained. Failure to patch or implement mitigations could result in data breaches, regulatory non-compliance (e.g., GDPR), and operational disruptions.

European organizations should immediately upgrade xmlseclibs to version 3.1.4 or later, which contains the official fix for this vulnerability. If upgrading is not immediately feasible, implement strict validation checks to treat any canonicalization failure—such as exceptions, nil, or empty outputs—as fatal errors that abort XML signature validation. Code audits should be conducted to ensure no silent acceptance of empty canonicalization results occurs. Additionally, restrict privileges to minimize the risk of high-privilege account compromise, and monitor XML processing logs for anomalies indicative of signature bypass attempts. Employ defense-in-depth by validating XML inputs at multiple layers and using alternative signature verification libraries where possible. Regularly update libxml2 to the latest secure versions to reduce underlying library risks. Finally, conduct penetration testing focused on XML signature validation to detect potential exploitation.