CVE-2025-48994 describes a vulnerability in the Python library SignXML, which implements the W3C XML Signature standard. The flaw arises in versions prior to 4.0.4 when signature verification is performed with X509 certificate validation disabled and an HMAC shared secret key is provided (via the hmac_key parameter). Under these conditions, if the user does not explicitly restrict the expected signature algorithms using the expect_config parameter, an attacker can exploit an algorithm confusion attack. Specifically, the attacker can supply a signature generated with an asymmetric key algorithm instead of the expected HMAC symmetric key algorithm. Because the verification process does not enforce strict algorithm constraints, the signature may be accepted as valid despite being signed with a different key type. This undermines the authentication guarantees of the signature verification process. Starting with SignXML version 4.0.4, the library enforces that when an hmac_key is specified, only HMAC algorithms are accepted unless the user has already restricted algorithms, thereby mitigating this attack vector. The vulnerability is classified under CWE-303 (Incorrect Implementation of Authentication Algorithm) and has a CVSS v4.0 score of 6.9 (medium severity). It requires no privileges or user interaction and can be exploited remotely over the network. No known exploits are currently reported in the wild. The vulnerability impacts any system or application using vulnerable SignXML versions for XML signature verification with the described configuration, potentially allowing attackers to bypass signature authentication and compromise data integrity and authenticity in XML-based communications or transactions.

For European organizations, the impact of this vulnerability can be significant, especially for those relying on SignXML for secure XML signature verification in critical systems such as web services, document signing, or identity federation. Exploitation could allow attackers to bypass signature validation, leading to unauthorized data manipulation, injection of malicious XML content, or impersonation of trusted entities. This could compromise confidentiality and integrity of sensitive data exchanges, disrupt business processes, or facilitate further attacks such as privilege escalation or fraud. Sectors like finance, government, healthcare, and telecommunications, which often use XML signatures for secure messaging and document workflows, are particularly at risk. Given the medium severity and ease of exploitation without authentication, attackers could target exposed APIs or services using vulnerable SignXML versions. The absence of known exploits suggests limited immediate threat but also indicates the need for proactive mitigation to prevent future attacks. Organizations that have disabled X509 validation and rely on HMAC keys without strict algorithm restrictions are the most vulnerable. Overall, the threat could undermine trust in XML-based security mechanisms and lead to data breaches or operational disruptions.

European organizations should immediately audit their use of the SignXML library to identify any deployments running versions prior to 4.0.4. If found, they should upgrade to version 4.0.4 or later, which enforces proper algorithm restrictions when using HMAC keys. Additionally, organizations should review their XML signature verification configurations to ensure that X509 certificate validation is enabled where possible, as this adds an important layer of security. Where disabling X509 validation is necessary, users must explicitly restrict accepted signature algorithms using the expect_config parameter to only allow HMAC algorithms when an hmac_key is used. Implementing strict input validation and monitoring XML signature verification logs for anomalies can also help detect attempts to exploit this vulnerability. Security teams should incorporate this vulnerability into their threat modeling and penetration testing exercises to validate defenses. Finally, organizations should maintain an inventory of software dependencies and apply security patches promptly to reduce exposure to similar algorithm confusion attacks.