CVE-2026-24880: Apache Tomcat HTTP Smuggling Vulnerability

CVE-2026-24880 Overview

CVE-2026-24880 is an HTTP Request/Response Smuggling vulnerability (CWE-444) affecting Apache Tomcat. The vulnerability arises from inconsistent interpretation of HTTP requests when processing invalid chunk extensions in chunked transfer encoding. This flaw can allow attackers to manipulate how HTTP requests are parsed by front-end and back-end servers, potentially leading to request smuggling attacks that bypass security controls, poison web caches, or hijack user sessions.

Critical Impact

HTTP Request Smuggling vulnerabilities can enable attackers to bypass security controls, access unauthorized resources, poison web caches, and potentially compromise user sessions across multiple requests.

Affected Products

• Apache Tomcat 11.0.0-M1 through 11.0.18
• Apache Tomcat 10.1.0-M1 through 10.1.52
• Apache Tomcat 9.0.0.M1 through 9.0.115
• Apache Tomcat 8.5.0 through 8.5.100
• Apache Tomcat 7.0.0 through 7.0.109

Discovery Timeline

• April 9, 2026 - CVE-2026-24880 published to NVD
• April 9, 2026 - Last updated in NVD database

Technical Details for CVE-2026-24880

Vulnerability Analysis

This HTTP Request Smuggling vulnerability exploits the way Apache Tomcat handles invalid chunk extensions in HTTP chunked transfer encoding. When a front-end proxy and back-end Tomcat server interpret the same malformed HTTP request differently, an attacker can craft requests that "smuggle" additional HTTP content past security controls.

The vulnerability is classified under CWE-444 (Inconsistent Interpretation of HTTP Requests), which describes scenarios where multiple HTTP entities (proxies, load balancers, web servers) interpret the same request differently. This inconsistency creates a desynchronization opportunity that attackers can exploit.

In typical exploitation scenarios, the attacker sends a specially crafted HTTP request containing invalid chunk extension data. When the front-end server and Apache Tomcat disagree on where one request ends and another begins, the attacker can inject arbitrary requests that appear to originate from other users or bypass access controls entirely.

Root Cause

The root cause lies in Apache Tomcat's handling of chunk extensions in HTTP chunked transfer encoding. When processing chunked requests, Tomcat fails to properly validate or reject certain malformed chunk extensions. This permissive parsing creates an inconsistency when Tomcat operates behind proxy servers or load balancers that enforce stricter parsing rules, enabling request smuggling attacks.

Attack Vector

The attack vector is network-based and requires no authentication. An attacker can remotely send malformed HTTP requests to exploit the parsing inconsistency between front-end proxies and Apache Tomcat. Successful exploitation typically involves:

1. Identifying a deployment where Apache Tomcat operates behind a reverse proxy or load balancer
2. Crafting HTTP requests with specially malformed chunk extensions
3. Exploiting the parsing differential to inject smuggled requests

The attack does not require user interaction and can be performed entirely through HTTP traffic. While no verified proof-of-concept code is publicly available, the technical details for constructing smuggled requests can be found in the Apache Mailing List Thread discussing this vulnerability.

Detection Methods for CVE-2026-24880

Indicators of Compromise

• Malformed HTTP requests containing unusual or invalid chunk extension characters in chunked transfer encoding
• HTTP requests with discrepancies between Content-Length and Transfer-Encoding headers
• Unexpected HTTP request sequences appearing in Tomcat access logs that do not correspond to legitimate client activity
• Cache poisoning indicators such as incorrect responses being served to legitimate users

Detection Strategies

• Deploy web application firewalls (WAF) with rules to detect and block malformed chunked transfer encoding requests
• Enable detailed access logging on both front-end proxies and Apache Tomcat to identify request parsing inconsistencies
• Monitor for HTTP requests with suspicious chunk extension patterns or mixed Content-Length/Transfer-Encoding headers
• Implement request integrity validation at the application layer to detect smuggled content

Monitoring Recommendations

• Review Apache Tomcat access logs for unusual request patterns or unexpected request boundaries
• Monitor network traffic for chunked HTTP requests containing non-standard or malformed chunk extensions
• Establish baseline metrics for request patterns and alert on anomalous deviations
• Correlate front-end proxy logs with Tomcat logs to identify parsing discrepancies

How to Mitigate CVE-2026-24880

Immediate Actions Required

• Upgrade Apache Tomcat to patched versions: 11.0.20, 10.1.52, or 9.0.116
• If immediate upgrade is not possible, implement strict HTTP parsing at the reverse proxy or load balancer level
• Review and harden reverse proxy configurations to normalize HTTP requests before forwarding to Tomcat
• Deploy WAF rules to reject requests with malformed chunked transfer encoding

Patch Information

Apache has released patched versions that address this vulnerability. Users should upgrade to the following versions:

For additional details on the fix, refer to the Apache Mailing List Thread.

Workarounds

• Configure reverse proxies (nginx, Apache HTTP Server, HAProxy) to normalize chunked transfer encoding requests before forwarding to Tomcat
• Disable chunked transfer encoding at the reverse proxy level if not required for application functionality
• Implement strict HTTP request validation rules that reject malformed chunk extensions
• Deploy network-level protections that inspect and sanitize HTTP traffic targeting Tomcat instances

# Example nginx configuration to normalize chunked requests
# Add to your nginx server block configuration
proxy_http_version 1.1;
proxy_request_buffering on;
chunked_transfer_encoding off;