CVE-2023-46589: Apache Tomcat Request Smuggling Flaw

CVE-2023-46589 Overview

CVE-2023-46589 is an Improper Input Validation vulnerability in Apache Tomcat that affects HTTP trailer header parsing. The vulnerability exists because Tomcat does not correctly parse HTTP trailer headers when a trailer header exceeds the configured header size limit. This parsing flaw can cause Tomcat to treat a single request as multiple requests, creating conditions for HTTP Request Smuggling attacks when the affected Tomcat instance is deployed behind a reverse proxy.

Critical Impact

HTTP Request Smuggling vulnerabilities can allow attackers to bypass security controls, poison web caches, hijack other users' sessions, and gain unauthorized access to sensitive data through request manipulation.

Affected Products

• Apache Tomcat 11.0.0-M1 through 11.0.0-M10
• Apache Tomcat 10.1.0-M1 through 10.1.15
• Apache Tomcat 9.0.0-M1 through 9.0.82
• Apache Tomcat 8.5.0 through 8.5.95
• Older End-of-Life (EOL) versions may also be affected

Discovery Timeline

• 2023-11-28 - CVE-2023-46589 published to NVD
• 2025-08-07 - Last updated in NVD database

Technical Details for CVE-2023-46589

Vulnerability Analysis

This vulnerability stems from improper handling of HTTP trailer headers in Apache Tomcat's request parsing logic. HTTP trailer headers are metadata sent after the body of a chunked transfer-encoded message. When Tomcat receives a trailer header that exceeds the maximum header size limit, instead of properly rejecting or handling the oversized header, the parser incorrectly interprets portions of the request. This misinterpretation causes Tomcat to treat what was intended as a single HTTP request as multiple separate requests.

The impact is particularly severe in deployments where Tomcat operates behind a reverse proxy (such as Apache HTTP Server, nginx, or HAProxy). In such architectures, the proxy and Tomcat may interpret the same byte stream differently—the proxy sees one request while Tomcat processes multiple. This desynchronization creates the conditions for HTTP Request Smuggling attacks (CWE-444), allowing attackers to bypass security controls implemented at the proxy layer.

Root Cause

The root cause lies in Tomcat's HTTP request parsing implementation, specifically in how it handles chunked transfer encoding with trailer headers. The parser fails to properly validate and enforce header size limits for trailer headers, leading to incorrect boundary detection between HTTP requests. When an oversized trailer header is encountered, the parser's state machine enters an inconsistent state that causes request boundary misalignment.

Attack Vector

This vulnerability is exploitable over the network without requiring authentication or user interaction. An attacker can craft malicious HTTP requests with oversized trailer headers targeting a Tomcat server behind a reverse proxy. The attack exploits the parsing discrepancy between the front-end proxy and back-end Tomcat server.

The attack scenario involves:

1. Attacker sends a specially crafted HTTP request with chunked transfer encoding containing an oversized trailer header
2. The reverse proxy interprets this as a single valid request and forwards it to Tomcat
3. Tomcat's flawed parser treats the trailer overflow as the beginning of a new request
4. The "smuggled" portion of the request can bypass proxy-level security controls, poison caches, or hijack responses intended for other users

Since no verified proof-of-concept code is publicly available for this vulnerability, organizations should refer to the Apache Mailing List Thread for detailed technical information about the request smuggling mechanism.

Detection Methods for CVE-2023-46589

Indicators of Compromise

• Unusual HTTP requests with excessively large trailer headers in access logs
• Multiple rapid requests appearing to originate from a single connection where only one was expected
• Cache poisoning incidents or users receiving responses intended for other sessions
• Bypass of WAF or reverse proxy security rules without clear explanation

Detection Strategies

• Monitor reverse proxy logs for requests with abnormally large Transfer-Encoding: chunked payloads or trailer sections
• Implement intrusion detection rules that alert on HTTP requests with trailer headers exceeding normal thresholds
• Compare request counts between reverse proxy and Tomcat access logs to identify request desynchronization
• Deploy web application firewall rules specifically targeting HTTP request smuggling patterns

Monitoring Recommendations

• Enable detailed access logging on both reverse proxy and Tomcat servers to facilitate log correlation analysis
• Configure alerts for anomalous request patterns that may indicate smuggling attempts
• Monitor for unexpected cache behavior or session anomalies that could indicate successful exploitation
• Review security logs for any evidence of proxy bypass or unauthorized access attempts

How to Mitigate CVE-2023-46589

Immediate Actions Required

• Upgrade Apache Tomcat to version 11.0.0-M11 or later for the 11.x branch
• Upgrade Apache Tomcat to version 10.1.16 or later for the 10.1.x branch
• Upgrade Apache Tomcat to version 9.0.83 or later for the 9.0.x branch
• Upgrade Apache Tomcat to version 8.5.96 or later for the 8.5.x branch
• If running End-of-Life versions, migrate to a supported release immediately

Patch Information

Apache has released patched versions that correctly handle HTTP trailer headers and enforce size limits properly. The fix addresses the parsing logic to ensure proper request boundary detection even when oversized trailer headers are received.

Refer to the Apache Mailing List Thread for official vendor advisory details. Additional security information is available from Openwall OSS Security, Debian LTS Announcement, and NetApp Security Advisory.

Workarounds

• Configure reverse proxies to normalize and validate HTTP requests before forwarding to Tomcat
• Implement strict header size limits at the reverse proxy level to reject requests with oversized headers
• Consider disabling chunked transfer encoding at the proxy layer if not required for your application
• Deploy a Web Application Firewall (WAF) with HTTP request smuggling detection capabilities

# Example: Limit header size in nginx reverse proxy configuration
# Add to nginx server or location block
large_client_header_buffers 4 8k;
client_header_buffer_size 1k;

# Ensure HTTP/1.1 is used consistently between proxy and backend
proxy_http_version 1.1;