Apache Tomcat, one of the most popular open-source web servers and Servlet (Java) container, has been discovered to have a security vulnerability in its latest versions. This vulnerability, tagged as CVE-2023-46589, affects the way Tomcat parses HTTP trailer headers, leading to potential request smuggling when used behind a reverse proxy. In this post, we will discuss the details of this vulnerability, its implications, and the necessary steps to mitigate it.

Description of the Vulnerability

The improper input validation vulnerability in Apache Tomcat primarily affects the way it handles HTTP trailer headers. When an HTTP trailer header exceeds the header size limit, Tomcat fails to parse it correctly, leading to treating a single request as multiple requests. This creates an opportunity for attackers to smuggle requests, which could potentially lead to bypassing security restrictions, unauthorized access, or data leakage.

Code Snippet - The Issue

In Apache Tomcat's source code, the affected section for parsing HTTP headers can be located in the org.apache.coyote.http11.Http1Parser class. The problem arises when the line continuation process does not account for the header size limit, allowing an oversized trailer header to be processed as multiple requests.

// (...)

public boolean parseHeaderMessage(ByteBuffer input)
        throws IOException {
    // (...)

    while (pos < limit) {
        // Get next byte
        chr = buf[pos];
        if (headerParseStatus == HeaderParseStatus.HEADER_START) {
            // (...)

        } else if (headerParseStatus == HeaderParseStatus.HEADER_NAME) {
            // (...)

        } else if (headerParseStatus == HeaderParseStatus.HEADER_VALUE) {
            // (...)

        } else if (headerParseStatus == HeaderParseStatus.HEADER_MULTI_LINE) {
            // (...) PROBLEMATIC SECTION

        }

        // Processed the current byte, advance position
        pos++;
    }

    return false;
}

// (...)

Exploit Details

An attacker could exploit this vulnerability by crafting malicious HTTP requests containing oversized trailer headers. These headers, when sent to a vulnerable Tomcat server behind a reverse proxy, would be processed as multiple requests. This could lead to request smuggling, allowing an attacker to bypass security restrictions, access unauthorized resources, or exfiltrate sensitive information.

Mitigation

To mitigate this vulnerability, users are advised to upgrade their Tomcat installations to the following versions or later:

Tomcat 8.5.96

These versions contain the necessary fix for the improper input validation vulnerability, ensuring that HTTP trailer headers exceeding the header size limit are parsed correctly and not allowing request smuggling.

References

1. Apache Tomcat Official Website: https://tomcat.apache.org/
2. CVE-2023-46589 Details: https://cve.mitre.org/cgi-bin/cvename.cgi?name=2023-46589
3. Apache Tomcat Changelog: https://tomcat.apache.org/tomcat-9.-doc/changelog.html

In conclusion, users must remain vigilant about updating their software and promptly addressing vulnerabilities like CVE-2023-46589. By taking the necessary precautionary measures and installing updated versions of Apache Tomcat, users can effectively mitigate the risks associated with this improper input validation vulnerability.

Timeline

Published on: 11/28/2023 16:15:06 UTC
Last modified on: 12/14/2023 10:15:08 UTC