Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
If Apache Tomcat 8.5.0 through 8.5.82, 9.0.0-M1 through 9.0.67, 10.0.0-M1 through 10.0.26 and 10.1.0-M1 through 10.1.0 was configured to ignore invalid HTTP headers via setting "rejectIllegalHeader" to false (the default for 8.5.x only), Tomcat did not reject a request containing an invalid Content-Length header making a request smuggling attack possible if Tomcat was located behind a reverse proxy that also failed to reject the request with the invalid header.
CWE-444 - HTTP Request Smuggling
Entities such as web servers, web caching proxies, and application firewalls could parse HTTP requests differently. When there are two or more such entities in the path of an HTTP request, an attacker can send a specially crafted HTTP request that is seen as two different sets of requests by the attacked devices, allowing the attacker to smuggle a request into one device without the other device being aware of it. Such a vulnerability can prove devastating, for it enables further attacks on the application, like web cache poisoning, session hijacking, cross-site scripting, security bypassing, and sensitive information exposure.