CVE-2023-45648 is an improper input validation vulnerability (CWE-20) in Apache Tomcat's handling of HTTP trailer headers. Tomcat versions 8.5.0 through 8.5.93, 9.0.0-M1 through 9.0.81, 10.1.0-M1 through 10.1.13, and 11.0.0-M1 through 11.0.0-M11 fail to correctly parse HTTP trailer headers, which are optional headers sent after the message body in chunked transfer encoding. A specially crafted invalid trailer header can cause Tomcat to misinterpret a single HTTP request as multiple separate requests. This parsing flaw enables HTTP request smuggling attacks when Tomcat is deployed behind a reverse proxy or load balancer that does not share the same parsing logic. Request smuggling can allow attackers to bypass security controls, poison web caches, or perform unauthorized actions by desynchronizing the front-end proxy and backend server's view of HTTP requests. The vulnerability does not require authentication or user interaction and can be exploited remotely over the network. Although no public exploits are known at this time, the flaw poses a significant risk in environments where Tomcat is exposed behind reverse proxies. The Apache Software Foundation has addressed the issue in versions 8.5.94, 9.0.81, 10.1.14, and 11.0.0-M12 and later. Users running older versions should upgrade promptly to prevent exploitation.

For European organizations, this vulnerability could lead to HTTP request smuggling attacks that undermine the integrity of web applications hosted on Apache Tomcat servers. Attackers could exploit this to bypass security controls such as web application firewalls or authentication mechanisms, potentially allowing unauthorized actions or data manipulation. While confidentiality and availability impacts are limited, the integrity of HTTP request processing is compromised, which could facilitate further attacks like session hijacking, cache poisoning, or cross-site scripting. Organizations relying on Tomcat behind reverse proxies or load balancers are particularly at risk. Given the widespread use of Apache Tomcat in European enterprises, government agencies, and critical infrastructure, exploitation could disrupt services or expose sensitive internal operations. The medium CVSS score reflects moderate risk, but the ease of remote exploitation without authentication increases urgency for patching. The absence of known exploits in the wild currently reduces immediate threat but does not eliminate future risk.

European organizations should prioritize upgrading Apache Tomcat to the fixed versions: 8.5.94 or later, 9.0.81 or later, 10.1.14 or later, or 11.0.0-M12 or later. Where immediate upgrade is not feasible, organizations should implement strict input validation and filtering at the reverse proxy or web application firewall level to detect and block malformed HTTP trailer headers. Reviewing and hardening reverse proxy configurations to ensure consistent HTTP parsing behavior between front-end and backend servers can reduce risk. Monitoring HTTP traffic for anomalies indicative of request smuggling attempts is advised. Additionally, organizations should audit their infrastructure to identify all Tomcat instances, including legacy and development environments, to ensure comprehensive patching. Employing layered security controls such as network segmentation and strict access controls around Tomcat servers can limit potential impact. Finally, maintaining up-to-date threat intelligence feeds and monitoring vendor advisories will help detect emerging exploit attempts.