Apache HTTP Server 2.4.47 Vulnerability: NULL Pointer Dereference in HTTP/2 Protocol Handler

apache2 Updated 02 Oct 2025

Summary

15/06/2021

Disclosed on

N/A

Severity

Yes i

Fix available

High

Severity

7.5

CVSS Score

ALPINE-CVE-2021-31618: Apache HTTP Server protocol handler for the HTTP/2 protocol checks received request headers against the size limitations as configured for the server and used for the HTTP/1 protocol as well. Affects apache2 versions < 2.4.48-r0. Fixed in 2.4.48-r0. How to fix ↓

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Background

Apache HTTP Server is a widely used open-source web server software that enables users to serve web content. It supports various protocols, including HTTP/2, which enhances performance and efficiency for web applications. However, a vulnerability has been identified in the HTTP/2 protocol handler that can lead to server crashes, affecting availability and reliability.

Vulnerability Detail

The vulnerability arises from the Apache HTTP Server's protocol handler for HTTP/2, which fails to properly check the size of received request headers against configured limitations. When an offending header is the first one received or appears in a footer, it leads to a NULL pointer dereference, causing the child process to crash. This can be exploited easily, resulting in a Denial of Service (DoS) attack against the server. The issue specifically affects mod_http2 version 1.15.17 and Apache HTTP Server version 2.4.47, which was never officially released.

How To Fix

Minimum fixed version

2.4.48-r0

This is the first release that contains the fix.

sudo apk add --upgrade apache2

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Workaround

As a temporary workaround, you can disable HTTP/2 support in your Apache configuration by removing or commenting out the 'Protocols h2 http/1.1' line in your Apache configuration file. Restart the server after making this change.

Conclusion

The identified vulnerability in the Apache HTTP Server's HTTP/2 protocol handler poses a significant risk, leading to potential Denial of Service attacks. It is crucial for users to update their server to the patched version to mitigate this risk and ensure the stability of their web services.

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