CVE-2025-53506 is a vulnerability classified under CWE-400 (Uncontrolled Resource Consumption) affecting multiple versions of Apache Tomcat, a widely used Java-based web server and servlet container. The flaw arises in the HTTP/2 protocol handling within Tomcat, specifically when an HTTP/2 client does not acknowledge the initial settings frame that sets the maximum number of concurrent streams allowed. This failure prevents Tomcat from properly limiting concurrent streams, enabling a malicious client to open excessive streams and consume server resources uncontrollably. This can lead to denial-of-service (DoS) conditions by exhausting CPU, memory, or other resources, thereby degrading or halting legitimate service availability. The vulnerability affects Tomcat versions from 8.5.0 through 8.5.100, 9.0.0.M1 through 9.0.106, 10.1.0-M1 through 10.1.42, and 11.0.0-M1 through 11.0.8, including some end-of-life versions. The issue is exploitable remotely without any authentication or user interaction, increasing its risk profile. The Apache Software Foundation has addressed this vulnerability in versions 11.0.9, 10.1.43, and 9.0.107. No public exploits have been reported yet, but the nature of the flaw makes it a potential vector for DoS attacks against critical web infrastructure relying on Tomcat. The CVSS v3.1 score of 7.5 reflects a high severity, emphasizing the impact on availability with no confidentiality or integrity loss. Organizations using affected Tomcat versions should prioritize upgrading to the patched releases to mitigate this risk.

The primary impact of CVE-2025-53506 is on the availability of web services hosted on Apache Tomcat servers. An attacker can exploit this vulnerability to cause a denial-of-service by exhausting server resources through uncontrolled concurrent HTTP/2 streams. For European organizations, this can disrupt critical business applications, customer-facing portals, and internal services that rely on Tomcat, leading to operational downtime and potential financial losses. Sectors such as finance, government, healthcare, and e-commerce, which often use Tomcat for their web infrastructure, are particularly vulnerable. Additionally, prolonged service outages can damage organizational reputation and erode customer trust. Since the vulnerability requires no authentication or user interaction, it can be exploited by remote attackers with minimal effort, increasing the threat landscape. The lack of confidentiality or integrity impact means data breaches are unlikely, but service disruption alone can have severe consequences, especially for time-sensitive or critical services.

To mitigate CVE-2025-53506, organizations should immediately upgrade Apache Tomcat to the fixed versions 11.0.9, 10.1.43, or 9.0.107 depending on their current deployment. If immediate upgrade is not feasible, administrators should consider temporarily disabling HTTP/2 support to prevent exploitation, though this may impact performance. Network-level mitigations such as rate limiting and connection throttling on HTTP/2 traffic can reduce the risk of resource exhaustion. Monitoring server resource usage and HTTP/2 connection metrics can help detect anomalous activity indicative of exploitation attempts. Implementing Web Application Firewalls (WAFs) with HTTP/2 anomaly detection capabilities may provide additional protection. Regularly reviewing and updating incident response plans to include DoS scenarios related to HTTP/2 is recommended. Finally, organizations should maintain an inventory of Tomcat versions in use and ensure timely patch management processes are in place to address future vulnerabilities.