CVE-2025-53506 is a high-severity vulnerability classified under CWE-400, indicating uncontrolled resource consumption, affecting multiple versions of Apache Tomcat, a widely used open-source Java Servlet container and web server. The vulnerability arises specifically in the handling of HTTP/2 connections. When an HTTP/2 client fails to acknowledge the initial settings frame that reduces the maximum permitted concurrent streams, Apache Tomcat does not properly limit resource allocation. This flaw allows an attacker to exhaust server resources by opening numerous concurrent streams without proper flow control, leading to denial of service (DoS) conditions. The affected versions span from early milestone releases (e.g., 11.0.0-M1, 10.1.0-M1, 9.0.0.M1) through stable releases up to 11.0.8, 10.1.42, and 9.0.106, including end-of-life versions 8.5.0 through 8.5.100. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. The CVSS v3.1 base score is 7.5, reflecting high severity with network attack vector, low attack complexity, no privileges required, no user interaction, and impact limited to availability (no confidentiality or integrity impact). No known exploits are reported in the wild yet. The recommended mitigation is upgrading to patched versions 11.0.9, 10.1.43, or 9.0.107, which address the issue by enforcing proper handling of HTTP/2 stream limits and client acknowledgments to prevent resource exhaustion.

For European organizations, this vulnerability poses a significant risk to the availability of web services relying on Apache Tomcat, especially those using HTTP/2. Given Apache Tomcat's widespread adoption in enterprise applications, government portals, and critical infrastructure, exploitation could lead to denial of service, disrupting business operations, customer access, and potentially critical public services. The impact is particularly severe for organizations with high traffic volumes or those exposed directly to the internet, as attackers can remotely trigger resource exhaustion without authentication. This could result in service downtime, loss of revenue, reputational damage, and increased operational costs due to incident response and remediation. Additionally, sectors such as finance, healthcare, and public administration, which heavily depend on reliable web services, may face regulatory and compliance challenges if service availability is compromised.

1. Immediate upgrade to Apache Tomcat versions 11.0.9, 10.1.43, or 9.0.107 to apply the official patch addressing this vulnerability. 2. Implement network-level protections such as rate limiting and connection throttling on HTTP/2 traffic to mitigate potential resource exhaustion attacks. 3. Monitor HTTP/2 traffic patterns for anomalies indicating unacknowledged settings frames or excessive concurrent streams. 4. Employ Web Application Firewalls (WAFs) capable of inspecting HTTP/2 traffic and blocking suspicious behavior related to stream management. 5. For environments where immediate upgrade is not feasible, consider disabling HTTP/2 support temporarily to eliminate the attack vector, while balancing performance impacts. 6. Regularly audit and update server configurations to ensure adherence to best practices for resource management and DoS protection. 7. Maintain comprehensive logging and alerting to detect early signs of exploitation attempts.