CVE-2021-33037
MediumCVSS 5.3Exploitation Probability (EPSS)
Very high risk99th percentile - higher than 99% of all known CVEs
Summary
Apache Tomcat versions 10.0.0-M1 to 10.0.6, 9.0.0.M1 to 9.0.46, and 8.5.0 to 8.5.66 incorrectly parsed the HTTP transfer-encoding request header in some circumstances, leading to request smuggling when used with a reverse proxy. Specifically, Tomcat ignored the transfer encoding header if the client declared it would only accept an HTTP/1.0 response, honored the identity encoding, and did not ensure that chunked encoding was the final encoding.
Risk Assessment
The risk is the possibility of an attacker intercepting and manipulating HTTP requests, which could lead to bypassing security controls, data theft, or attacks on other users.
Recommendation
Update Apache Tomcat to a patched version. Also, configure reverse proxies securely to avoid issues with transfer-encoding headers.
Other vulnerabilities in Apache Tomcat
See all- CVE-2026-43513High
The vulnerability related to improper handling of case sensitivity in LockOutRealm in Apache Tomcat may lead to unauthorized access. It affects versions from 11.0.0-M1 to 11.0.21, from 10.1.0-M1 to 10.1.54, from 9.0.0.M1 to 9.0.117, from 8.5.0 to 8.5.100, and from 7.0.0 to 7.0.109.
- CVE-2026-42498High
A vulnerability in Apache Tomcat related to the exposure of the HTTP Authentication Header to unexpected hosts during WebSocket authentication. It affects versions from 11.0.0-M1 to 11.0.21, 10.1.0-M1 to 10.1.54, 9.0.2 to 9.0.117, 8.5.24 to 8.5.100, and 7.0.83 to 7.0.109.
- CVE-2026-41284High
There is a vulnerability in Apache Tomcat related to resource allocation without limits or throttling. This affects versions from 11.0.0-M1 to 11.0.21, from 10.1.0-M1 to 10.1.54, and from 9.0.0.M1 to 9.0.117.
- CVE-2025-48989High
A vulnerability in Apache Tomcat related to improper resource shutdown or release allows for a 'made you reset' attack. This affects versions from 11.0.0-M1 to 11.0.9, from 10.1.0-M1 to 10.1.43, and from 9.0.0.M1 to 9.0.107.
- CVE-2017-7675High
The HTTP/2 implementation in Apache Tomcat from version 9.0.0.M1 to 9.0.0.M21 and from 8.5.0 to 8.5.15 bypassed several security checks that prevented directory traversal attacks. It was therefore possible to bypass security constraints using a specially crafted URL.
- CVE-2016-6817High
The HTTP/2 header parser in Apache Tomcat versions 9.0.0.M1 to 9.0.0.M11 and 8.5.0 to 8.5.6 entered an infinite loop if a header larger than the available buffer was received. This made a denial of service attack possible.
- CVE-2017-5664High
The error page mechanism of the Java Servlet Specification requires that the original request and response be forwarded to the error page. For static files, the expected behavior is to serve the content of the file as if processing a GET request, which was not implemented in Apache Tomcat versions 9.0.0.M1 to 9.0.0.M20, 8.5.0 to 8.5.14, 8.0.0.RC1 to 8.0.43, and 7.0.0 to 7.0.77.
- CVE-2017-5650High
In Apache Tomcat versions 9.0.0.M1 to 9.0.0.M18 and 8.5.0 to 8.5.12, the handling of an HTTP/2 GOAWAY frame did not close streams associated with that connection waiting for a WINDOW_UPDATE, potentially leading to exhaustion of available processing threads.
- CVE-2017-5647High
A bug in the handling of pipelined requests in Apache Tomcat causes the request to be lost when send file processing of the previous request is completed. This can lead to responses being sent for the wrong requests.
- CVE-2016-6816High
The code in Apache Tomcat versions from 6.0.0 to 9.0.0.M11 allowed parsing of the HTTP request line with invalid characters. This could be exploited in conjunction with a proxy that also accepted these characters to inject data into the HTTP response.
Original NVD description (English source)
Apache Tomcat 10.0.0-M1 to 10.0.6, 9.0.0.M1 to 9.0.46 and 8.5.0 to 8.5.66 did not correctly parse the HTTP transfer-encoding request header in some circumstances leading to the possibility to request smuggling when used with a reverse proxy. Specifically: - Tomcat incorrectly ignored the transfer encoding header if the client declared it would only accept an HTTP/1.0 response; - Tomcat honoured the identify encoding; and - Tomcat did not ensure that, if present, the chunked encoding was the final encoding.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS