CVE-2025-40911 is a medium severity vulnerability affecting the Perl module Net::CIDR::Set versions 0.10 through 0.13. The vulnerability arises from improper validation of IP CIDR address strings containing leading zero characters. Specifically, the module does not correctly handle leading zeros, which can cause IP addresses to be interpreted as octal numbers rather than decimal. This misinterpretation can lead to incorrect IP address parsing and thus allow attackers to bypass IP-based access control mechanisms that rely on Net::CIDR::Set for CIDR range validation. The root cause is inherited from code originally sourced from Net::CIDR::Lite, which had a similar vulnerability (CVE-2021-47154). Since IP-based access control is a common security measure, this flaw can undermine the integrity of access restrictions by allowing unauthorized IP addresses to be treated as authorized, or vice versa. The vulnerability requires no authentication or user interaction and can be exploited remotely by supplying specially crafted IP CIDR strings with leading zeros. The CVSS v3.1 base score is 6.5, reflecting a medium severity with network attack vector, low attack complexity, no privileges required, and no user interaction needed. The impact primarily affects confidentiality and integrity, as unauthorized access could lead to data exposure or manipulation, but does not affect availability. No known exploits are currently reported in the wild, and no official patches have been linked yet. However, given the nature of the vulnerability, it is advisable to treat it seriously and apply mitigations promptly once patches are available.

For European organizations, this vulnerability poses a risk to systems that rely on Perl-based Net::CIDR::Set for IP address filtering and access control, such as web applications, network appliances, or security tools that implement IP whitelisting or blacklisting. Unauthorized bypass of IP-based restrictions could lead to data breaches, unauthorized system access, or lateral movement within networks. This is particularly concerning for sectors with strict regulatory requirements on data confidentiality and access control, such as finance, healthcare, and critical infrastructure. The vulnerability could also undermine perimeter defenses in organizations that use IP filtering as a first line of defense. While the vulnerability does not directly impact availability, the potential for unauthorized access could lead to further exploitation or data exfiltration. The lack of known exploits in the wild suggests limited immediate risk, but the ease of exploitation and network accessibility mean that attackers could develop exploits rapidly once the vulnerability becomes widely known.

European organizations should immediately audit their use of the Net::CIDR::Set Perl module, identifying all systems and applications that depend on it for IP address validation. Until an official patch is released, organizations should consider the following mitigations: 1) Implement additional validation layers for IP addresses, ensuring that leading zeros are either normalized or rejected to prevent octal interpretation. 2) Where feasible, replace Net::CIDR::Set with alternative, well-maintained libraries that correctly handle IP CIDR parsing. 3) Employ defense-in-depth by combining IP-based access control with other authentication and authorization mechanisms, such as multi-factor authentication and role-based access control. 4) Monitor network logs for suspicious access attempts that might indicate exploitation attempts involving malformed IP addresses. 5) Stay informed about vendor updates and apply patches promptly once available. 6) Conduct penetration testing focused on IP filtering bypass to validate the effectiveness of mitigations.