CVE-2026-23901 is a timing side-channel vulnerability classified under CWE-208 (Observable Timing Discrepancy) affecting Apache Shiro versions 1.* and 2.* before 2.0.7. Apache Shiro is a widely used Java security framework for authentication and authorization. The vulnerability arises because the code paths for handling authentication failures differ in timing depending on whether the username exists or not. Specifically, when a login attempt is made, the system responds faster or slower based on whether the username is valid, allowing an attacker to infer the existence of usernames by measuring response times. This information leakage can facilitate username enumeration, which is a common precursor to brute-force password attacks. The attack vector is primarily local, requiring the attacker to have some level of access to the system or network where Shiro is deployed. The vulnerability does not expose passwords or allow direct unauthorized access but weakens the security posture by enabling attackers to target valid accounts more efficiently. The Apache Software Foundation addressed this issue in version 2.0.7 by equalizing the timing of authentication failure responses. The CVSS 4.0 score is 1.0, reflecting low severity due to the limited impact and high attack complexity. No known exploits are currently reported in the wild. Mitigation is straightforward by upgrading to the fixed version and applying brute-force protections at the infrastructure level, such as rate limiting and account lockouts.

For European organizations, this vulnerability primarily increases the risk of username enumeration, which can be leveraged by attackers to identify valid user accounts within enterprise applications using Apache Shiro. While the direct impact on confidentiality, integrity, and availability is minimal, the information gained can facilitate targeted brute-force attacks or social engineering campaigns. Organizations in sectors with high-value targets, such as finance, government, and critical infrastructure, may face increased reconnaissance risks. The local attack vector limits remote exploitation, but insider threats or attackers with limited network access could exploit this timing discrepancy. If combined with weak password policies or lack of multi-factor authentication, the overall security posture could be degraded. Thus, the vulnerability indirectly contributes to potential account compromise and unauthorized access incidents. European entities relying on Shiro for authentication should consider this vulnerability in their risk assessments and incident response planning.

1. Upgrade Apache Shiro to version 2.0.7 or later immediately to apply the official fix that equalizes authentication response times. 2. Implement infrastructure-level brute-force protections such as rate limiting, account lockout policies, and CAPTCHA challenges to reduce the feasibility of brute-force attacks. 3. Enforce strong password policies and encourage or mandate multi-factor authentication (MFA) to mitigate risks from credential guessing. 4. Monitor authentication logs for unusual patterns indicative of enumeration or brute-force attempts, focusing on timing anomalies and repeated failed logins. 5. Restrict local access to authentication services and sensitive endpoints to trusted users and networks only, minimizing the attack surface. 6. Conduct regular security assessments and penetration tests to verify that timing side-channels are mitigated and that authentication mechanisms do not leak sensitive information. 7. Educate developers and security teams about timing side-channel risks and secure coding practices to prevent similar vulnerabilities in custom authentication logic.