CVE-2025-48995 is a medium severity vulnerability affecting versions of the Python library SignXML prior to 4.0.4. SignXML is a widely used implementation of the W3C XML Signature standard, enabling XML document signing and verification. The vulnerability arises when signature verification is performed with X509 certificate validation disabled (require_x509=False) and an HMAC shared secret key is used (hmac_key parameter). Under these conditions, the verification process compares the user-supplied HMAC with the expected HMAC in a manner susceptible to a timing attack (CWE-208: Observable Timing Discrepancy). Specifically, the time taken to compare the hashes leaks information about the correct HMAC value. An attacker can exploit this timing discrepancy to iteratively reconstruct the valid HMAC for any given data, effectively bypassing the intended authentication mechanism. This vulnerability does not require authentication, user interaction, or privileges, and can be exploited remotely over the network. The CVSS 4.0 base score is 6.9, reflecting a medium severity level due to network attack vector, low complexity, and no privileges required, but limited impact on confidentiality, integrity, and availability. No known exploits are currently reported in the wild. The issue was publicly disclosed on June 2, 2025, and fixed in SignXML version 4.0.4. The root cause is the use of non-constant time comparison functions during HMAC verification when X509 validation is disabled, allowing attackers to measure response times and infer the correct HMAC byte-by-byte.

For European organizations relying on SignXML for XML signature verification, especially in scenarios where X509 certificate validation is disabled and HMAC-based authentication is used, this vulnerability poses a risk of unauthorized data manipulation or forgery. Attackers could reconstruct valid HMACs to forge signatures on XML documents, potentially bypassing integrity checks and authentication controls. This could lead to unauthorized transactions, data tampering, or privilege escalation in systems that trust XML signatures for security decisions. Sectors such as finance, government, healthcare, and critical infrastructure in Europe that use XML-based messaging or document workflows could be particularly impacted. The vulnerability's exploitation could undermine trust in signed XML documents, disrupt automated processing, and expose sensitive data or operations to attackers. However, the impact is somewhat mitigated by the requirement that X509 validation be disabled and the use of HMAC keys, which may not be the default configuration in many deployments.

European organizations should immediately upgrade SignXML to version 4.0.4 or later, where the timing attack vulnerability has been addressed by implementing constant-time comparison functions during HMAC verification. If upgrading is not immediately feasible, organizations should ensure that X509 certificate validation is enabled (require_x509=True) to avoid using the vulnerable HMAC verification path. Additionally, review and audit XML signature verification configurations to confirm that HMAC keys are not used insecurely or without proper certificate validation. Implement network-level protections such as rate limiting and anomaly detection to identify and block suspicious timing attack attempts. Where possible, employ additional layers of cryptographic validation or use alternative secure libraries that do not exhibit timing side channels. Finally, conduct security awareness and training for developers and system administrators on secure XML signature practices and the risks of disabling certificate validation.