CVE-2026-24733 is a vulnerability in Apache Tomcat stemming from improper input validation related to HTTP/0.9 protocol handling. Apache Tomcat did not enforce the HTTP/0.9 specification limitation that only GET requests are valid under this protocol version. Consequently, if a security constraint was configured to allow HEAD requests but deny GET requests to a particular URI, an attacker could send a HEAD request using HTTP/0.9 to bypass the security constraint that was intended to restrict GET requests. This flaw allows unauthorized users to access resources that should be protected by the security constraints. The vulnerability affects multiple active branches of Apache Tomcat, including versions 9.0.0.M1 through 9.0.112, 10.1.0-M1 through 10.1.49, and 11.0.0-M1 through 11.0.14, as well as older end-of-life versions. The issue arises from Tomcat's failure to properly validate and restrict HTTP/0.9 requests, which are an outdated and rarely used protocol version. No public exploits have been reported yet, but the vulnerability enables bypassing access controls without requiring authentication or user interaction, increasing the risk of unauthorized data access or manipulation. The Apache Software Foundation has addressed the issue in Tomcat versions 9.0.113, 10.1.50, and 11.0.15 and later, recommending immediate upgrades to these versions to mitigate the risk.

The primary impact of CVE-2026-24733 is the potential bypass of security constraints configured in Apache Tomcat, which can lead to unauthorized access to protected web resources. For European organizations, this could mean exposure of sensitive data, unauthorized execution of restricted operations, or leakage of confidential information hosted on Tomcat-based web applications. Since Tomcat is widely used in enterprise environments for hosting Java-based web applications, the vulnerability could affect a broad range of sectors including finance, healthcare, government, and critical infrastructure. The ability to bypass access controls without authentication or user interaction increases the risk of automated exploitation by attackers scanning for vulnerable servers. This could lead to data breaches, compliance violations (e.g., GDPR), and reputational damage. The impact on availability is limited, as the vulnerability does not directly cause denial of service, but integrity and confidentiality risks are significant. Organizations relying on Tomcat for internal or external-facing applications must consider this vulnerability a serious threat to their security posture.

To mitigate CVE-2026-24733, organizations should immediately upgrade affected Apache Tomcat instances to versions 9.0.113, 10.1.50, or 11.0.15 or later, where the vulnerability has been fixed. In environments where immediate upgrade is not feasible, administrators should implement strict network-level controls to restrict access to Tomcat servers, such as firewall rules limiting HTTP traffic to trusted sources. Additionally, reviewing and tightening security constraints in Tomcat configurations can reduce risk, ensuring that access control policies are robust and do not rely solely on HTTP method restrictions. Monitoring web server logs for unusual HTTP/0.9 requests or anomalous HEAD requests can help detect attempted exploitation. Employing Web Application Firewalls (WAFs) with custom rules to block or alert on HTTP/0.9 traffic may provide temporary protection. Finally, organizations should conduct thorough security assessments of their Tomcat deployments and ensure all software components are regularly updated to minimize exposure to known vulnerabilities.