CVE-2026-23901: Apache Shiro Timing Attack Vulnerability

CVE-2026-23901 Overview

CVE-2026-23901 is an Observable Timing Discrepancy vulnerability affecting Apache Shiro, a powerful and flexible open-source security framework for Java applications. This timing-based side-channel vulnerability allows attackers to enumerate valid usernames by measuring response time differences between authentication attempts for existing versus non-existing user accounts.

The vulnerability stems from differing code execution paths when processing authentication requests. When a login attempt is made for a non-existent user, the application returns more quickly than when processing a request for an existing user with an incorrect password. An attacker capable of making precise timing measurements can exploit this discrepancy to build a list of valid usernames, which can then be used in targeted password spraying or phishing attacks.

Critical Impact

Timing attack enables username enumeration, potentially facilitating subsequent brute-force or credential stuffing attacks against valid user accounts.

Affected Products

• Apache Shiro versions 1.x (all versions)
• Apache Shiro versions 2.x before 2.0.7
• Applications using Apache Shiro for authentication services

Discovery Timeline

• 2026-02-10 - CVE CVE-2026-23901 published to NVD
• 2026-02-12 - Last updated in NVD database

Technical Details for CVE-2026-23901

Vulnerability Analysis

This vulnerability is classified as CWE-208 (Observable Timing Discrepancy), a type of side-channel attack that exploits measurable differences in processing time to infer sensitive information. In the case of Apache Shiro, the authentication subsystem exhibits timing differences that reveal whether a queried username exists in the system.

When an authentication request is processed, if the username does not exist, the system returns an authentication failure without performing the computationally expensive password verification step. Conversely, when a valid username is provided with an incorrect password, the system must retrieve the stored credentials and perform a password hash comparison before returning the failure response. This difference in execution paths creates a measurable timing discrepancy.

The attack requires local access to be effective, as network latency typically introduces too much noise for reliable timing measurements. However, in scenarios where an attacker has local access or can minimize network jitter, this vulnerability can be systematically exploited to enumerate user accounts.

Root Cause

The root cause of this vulnerability lies in the authentication flow's failure to implement constant-time comparison operations. Apache Shiro's authentication handler takes different code paths depending on whether the username lookup succeeds or fails:

1. Non-existent user path: The system performs a database/repository lookup, fails to find the user, and immediately returns an authentication failure.

2. Existing user path: The system performs the same lookup, successfully retrieves user credentials, then performs password hashing and comparison operations before returning a result.

The additional cryptographic operations in the second path create a measurable timing difference. Proper constant-time authentication implementations should perform equivalent operations regardless of whether the user exists, masking any timing signals.

Attack Vector

The attack vector for CVE-2026-23901 requires local access to the target system. An attacker would execute the following attack pattern:

1. Baseline establishment: The attacker first establishes baseline response times by sending multiple authentication requests with known non-existent usernames.

2. Username enumeration: The attacker then submits authentication requests with candidate usernames from a wordlist or generated list.

3. Timing analysis: Response times are measured with high precision. Usernames that result in longer response times indicate existing accounts.

4. Validation and exploitation: Confirmed usernames can then be used for targeted password attacks or social engineering campaigns.

The Apache Shiro Security Model documentation discusses this attack class. For additional context, refer to the Apache Mailing List Thread and the Openwall OSS-Security Post.

Detection Methods for CVE-2026-23901

Indicators of Compromise

• Unusually high volume of failed authentication attempts from a single source IP or user agent
• Sequential or pattern-based username submissions suggesting wordlist-based enumeration
• Authentication requests arriving at precise intervals, indicating automated tooling
• Statistical anomalies in authentication failure rates that deviate from normal user behavior

Detection Strategies

• Implement authentication attempt logging that captures precise timestamps and response times
• Deploy rate limiting on authentication endpoints to throttle potential enumeration attempts
• Use security information and event management (SIEM) correlation rules to detect patterns consistent with timing attacks
• Monitor for tools commonly used in timing attacks (e.g., specialized scripts with nanosecond-precision timing)

Monitoring Recommendations

• Enable detailed audit logging for all authentication events in Apache Shiro
• Configure alerting thresholds for authentication failure rates that exceed baseline metrics
• Implement network traffic analysis to identify reconnaissance patterns targeting authentication endpoints
• Review access logs periodically for evidence of systematic username enumeration attempts

How to Mitigate CVE-2026-23901

Immediate Actions Required

• Upgrade Apache Shiro to version 2.0.7 or later immediately
• Conduct an inventory of all applications using Apache Shiro to identify vulnerable deployments
• Implement infrastructure-level brute-force mitigations such as rate limiting and account lockout policies
• Review authentication logs for evidence of past exploitation attempts

Patch Information

Apache has addressed this vulnerability in Apache Shiro version 2.0.7. Users running any version of Apache Shiro 1.x or 2.x before 2.0.7 should upgrade to the patched version. The fix implements constant-time operations in the authentication flow to eliminate the observable timing discrepancy.

For detailed information about the patch, consult the Apache Mailing List Thread.

Workarounds

• Implement rate limiting at the infrastructure level (load balancer, WAF, or reverse proxy) to limit authentication attempts per source
• Deploy account lockout policies after a configurable number of failed authentication attempts
• Use CAPTCHA or similar challenge-response mechanisms after multiple failed login attempts
• Consider implementing artificial delays in authentication responses to normalize timing regardless of user existence

# Example: Rate limiting configuration for nginx reverse proxy
# Limit authentication requests to 5 per second per IP address
limit_req_zone $binary_remote_addr zone=auth_limit:10m rate=5r/s;

location /login {
    limit_req zone=auth_limit burst=10 nodelay;
    proxy_pass http://shiro_backend;
}