CVE-2026-24398 affects the Hono web application framework, specifically its IP Restriction Middleware component prior to version 4.11.7. The vulnerability arises from an incorrect regular expression (CWE-185) used to validate IPv4 addresses in the `IPV4_REGEX` pattern and the `convertIPv4ToBinary` function within the `src/utils/ipaddr.ts` file. These validation mechanisms fail to ensure that each IPv4 octet is within the valid range of 0 to 255, allowing attackers to supply malformed IP addresses that can bypass IP-based access controls. This bypass undermines the intended security controls that restrict access based on client IP addresses, potentially granting unauthorized access to protected resources. The vulnerability has a CVSS v3.1 base score of 4.8 (medium severity), with the vector indicating network attack vector (AV:N), high attack complexity (AC:H), no privileges required (PR:N), no user interaction (UI:N), unchanged scope (S:U), and low impact on confidentiality and integrity (C:L/I:L) with no impact on availability (A:N). Although no known exploits are currently reported in the wild, the flaw presents a risk for applications relying on IP-based restrictions for security. The issue is resolved in Hono version 4.11.7 by correcting the regex and validation logic to properly enforce valid IPv4 octet ranges.

For European organizations, this vulnerability could lead to unauthorized access to internal or restricted web application endpoints protected by IP-based access controls implemented using Hono framework versions prior to 4.11.7. Attackers could bypass these restrictions by crafting specially malformed IP addresses, potentially exposing sensitive data or internal services. This risk is particularly relevant for organizations that rely heavily on IP whitelisting for security, such as financial institutions, government agencies, and enterprises with strict network segmentation policies. While the impact on confidentiality and integrity is rated low to medium, the ability to circumvent access controls can facilitate further attacks or data leakage. The medium CVSS score reflects the higher complexity of exploitation and the absence of known active exploits, but the threat remains significant for environments where IP restrictions are a primary security mechanism.

European organizations using the Hono framework should immediately upgrade to version 4.11.7 or later to apply the official patch that corrects the IP validation logic. Until upgrading, organizations should consider implementing additional layers of access control beyond IP-based restrictions, such as authentication tokens, VPNs, or zero-trust network access models. Security teams should audit existing IP restriction configurations to identify any reliance on vulnerable versions of Hono and monitor logs for anomalous IP address patterns that may indicate exploitation attempts. Application-level input validation can be enhanced to reject malformed IP addresses before they reach the middleware. Additionally, network-level controls such as firewall rules and intrusion detection systems should be reviewed to detect and block suspicious IP formats. Regular vulnerability scanning and penetration testing focused on IP validation mechanisms can help identify residual risks.