CVE-2025-31651: Apache Tomcat Auth Bypass Vulnerability

CVE-2025-31651 Overview

Improper Neutralization of Escape, Meta, or Control Sequences vulnerability in Apache Tomcat. For a subset of unlikely rewrite rule configurations, it was possible for a specially crafted request to bypass some rewrite rules. If those rewrite rules effectively enforced security constraints, those constraints could be bypassed.

Critical Impact

This vulnerability could allow attackers to bypass security constraints, potentially leading to exposure of sensitive data.

Affected Products

• Apache Tomcat 11.0.0-M1 through 11.0.5
• Apache Tomcat 10.1.0-M1 through 10.1.39
• Apache Tomcat 9.0.0.M1 through 9.0.102

Discovery Timeline

• Not Available - Vulnerability discovered by Not Available
• Not Available - Responsible disclosure to Apache
• Not Available - CVE CVE-2025-31651 assigned
• Not Available - Apache releases security patch
• 2025-04-28 - CVE CVE-2025-31651 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2025-31651

Vulnerability Analysis

The vulnerability arises due to improper neutralization of escape, meta, or control sequences. An attacker could exploit this flaw by sending specially crafted requests, which could bypass configured rewrite rules and enforce security constraints.

Root Cause

The root cause is the improper handling of escape, meta, or control sequence data in rewrite rules.

Attack Vector

The attack vector is through the Network, where crafted HTTP requests can be sent to the vulnerable Tomcat servers.

// Example exploitation code (sanitized)
import java.net.HttpURLConnection;
import java.net.URL;

public class ExploitTomcat {
    public static void main(String[] args) throws Exception {
        URL url = new URL("http://vulnerable-tomcat-server/");
        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
        conn.setRequestMethod("GET");
        int responseCode = conn.getResponseCode();
        System.out.println("Response Code: " + responseCode);
    }
}

Detection Methods for CVE-2025-31651

Indicators of Compromise

• Unusual access logs containing irregular escape sequences
• Bypassed security checks in application layer logs
• Unexpected rewrite rule bypass in audit logs

Detection Strategies

Utilize application firewall rules to detect and block unusual HTTP requests containing escape sequences. Monitoring server logs for unauthorized access patterns is crucial to detect successful exploit attempts.

Monitoring Recommendations

Implement continuous monitoring on server access logs, checking for patterns consistent with rule bypass attempts. Use automated analysis tools to flag suspicious entries.

How to Mitigate CVE-2025-31651

Immediate Actions Required

• Update Apache Tomcat to the latest fixed version immediately
• Review rewrite rule configurations for potential vulnerabilities
• Deploy Web Application Firewalls (WAF) to add an additional layer of protection

Patch Information

Please refer to the Apache Tomcat mailing list for the latest patches and upgrade instructions.

Workarounds

If immediate patching is not feasible, reviewing and correcting rewrite rule configurations will help mitigate the vulnerability temporarily.

# Configuration example
<VirtualHost *:80>
    RewriteEngine on
    # Example correction
    RewriteRule ^/path/to/resource/$ /secure/path/ [L,R=301]
</VirtualHost>