CVE-2025-40934 is a cryptographic signature verification vulnerability in the TIMLEGGE XML::Sig Perl module, affecting versions 0.27 through 0.67. XML::Sig is designed to validate digital signatures on XML documents to ensure their authenticity and integrity. However, due to improper verification logic (CWE-347), the module erroneously returns a successful validation result when the XML document contains no signature at all. This means that an attacker can remove the signature element from an XML file and still have the file accepted as valid by the module. The root cause lies in the module's failure to check for the presence of a signature before validating it, leading to a false positive validation outcome. This flaw undermines the fundamental security guarantees of XML signatures, allowing unsigned or maliciously altered XML data to be treated as trustworthy. The vulnerability affects a range of XML::Sig versions from 0.27 to 0.67, with no patch currently available or linked. No public exploits have been reported yet, but the vulnerability is publicly disclosed and documented in the CVE database. The lack of a CVSS score necessitates an independent severity assessment based on impact and exploitability factors. Because XML signatures are often used in secure communications, document signing, and configuration management, this vulnerability could have serious consequences if exploited in environments relying on XML::Sig for security validation.

For European organizations, this vulnerability poses a significant risk to the integrity and authenticity of XML-based communications and data exchanges. Systems that rely on XML::Sig for verifying signed XML documents—such as financial transaction processing, electronic invoicing (e.g., PEPPOL networks), government document workflows, and secure configuration management—may inadvertently accept unsigned or tampered XML files. This can lead to unauthorized data manipulation, fraud, or injection of malicious configurations. The impact is particularly critical in sectors with stringent data integrity requirements, including banking, healthcare, public administration, and critical infrastructure. Since the vulnerability allows bypassing signature verification without authentication or user interaction, attackers with the ability to intercept or modify XML documents in transit or at rest can exploit this flaw to compromise system trustworthiness. The absence of known exploits suggests a window for proactive mitigation, but also means organizations may be unaware of the risk. The vulnerability could also affect software vendors and developers in Europe who package or distribute affected XML::Sig versions, potentially propagating the risk downstream.

European organizations should immediately audit their use of the TIMLEGGE XML::Sig Perl module to identify affected versions (0.27 through 0.67). Until a patched version is released, implement the following mitigations: 1) Introduce explicit checks in application code to verify the presence of a signature element before invoking XML::Sig validation, rejecting any XML documents lacking signatures. 2) Employ additional cryptographic verification layers or external signature validation tools that correctly enforce signature presence and validity. 3) Monitor XML processing logs for anomalies such as unsigned documents being accepted. 4) Restrict access and modification rights to XML documents to reduce the risk of tampering. 5) Engage with the vendor or open-source maintainers to obtain or contribute patches that fix the signature presence verification logic. 6) Educate developers and system integrators about the vulnerability to avoid deploying vulnerable versions in new projects. 7) For critical systems, consider isolating or sandboxing XML processing components to limit impact if exploited. These targeted mitigations go beyond generic advice by focusing on the specific failure mode of signature absence acceptance.