A significant denial-of-service (DoS) vulnerability has been identified in Apache Tomcat, a widely used open-source Java servlet container and web server. This flaw, designated as CVE-2025-31650, affects Apache Tomcat versions 10.1.10 through 10.1.39. The vulnerability exploits the server’s handling of HTTP/2 priority headers, leading to memory exhaustion and potential service disruption.
Understanding the Vulnerability
Apache Tomcat serves as a critical component in many web applications, facilitating the execution of Java Servlets and JavaServer Pages (JSP). The identified vulnerability arises from the server’s processing of HTTP/2 priority headers. By sending specially crafted requests with malformed priority parameters, attackers can induce memory leaks within the Tomcat runtime environment. This method of attack can lead to significant performance degradation and, ultimately, a complete denial of service.
Technical Details of the Exploit
The exploit involves dispatching requests containing invalid priority parameters that bypass standard validation mechanisms. These requests include extreme numerical values designed to trigger edge cases in Tomcat’s HTTP/2 implementation. As a result, the server fails to properly deallocate memory resources after processing these malformed requests, leading to progressive memory exhaustion.
The attack is executed by launching concurrent asynchronous tasks that inundate the target server with thousands of such requests. A Python-based exploit tool has been developed, utilizing the httpx library with HTTP/2 support enabled. This tool, referred to as TomcatKiller, manages the attack execution and incorporates evasion techniques, such as randomized user-agent strings and dynamic header manipulation, to avoid basic detection mechanisms.
Impact and Risk Assessment
The denial-of-service attack is particularly concerning because it targets the server’s memory resources rather than network bandwidth. This approach makes it challenging to mitigate through traditional DDoS protection mechanisms. When executed successfully, the exploit can force Apache Tomcat servers to consume excessive memory, eventually leading to OutOfMemoryError exceptions and complete service disruption.
The exploit’s effectiveness is heightened by its ability to generate sustained memory pressure through relatively few concurrent connections. By default, the tool launches 300 concurrent tasks, each sending up to 100,000 requests with malformed priority headers. This method ensures that even well-provisioned servers can be overwhelmed through sustained memory exhaustion rather than connection flooding.
Mitigation Strategies
Organizations utilizing affected versions of Apache Tomcat should take immediate action to mitigate this vulnerability. The following steps are recommended:
1. Upgrade to Patched Versions: Apache has released patches addressing CVE-2025-31650. Administrators should upgrade to the latest versions of Apache Tomcat that include these fixes.
2. Disable HTTP/2 Support if Unnecessary: If HTTP/2 support is not essential for your application, consider disabling it to eliminate the attack vector associated with this vulnerability.
3. Implement Rate Limiting and Monitoring: Deploy rate limiting on incoming requests and monitor server logs for unusual patterns that may indicate exploitation attempts.
4. Conduct Regular Security Audits: Regularly review and update server configurations and software to ensure all known vulnerabilities are addressed promptly.
Conclusion
The release of a proof-of-concept exploit for CVE-2025-31650 underscores the critical need for organizations to stay vigilant and proactive in their cybersecurity practices. By understanding the nature of this vulnerability and implementing the recommended mitigation strategies, organizations can protect their Apache Tomcat servers from potential denial-of-service attacks and maintain the availability and reliability of their web applications.
