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Static Binary or Dynamic Modules - Apache

Apache is the most popular web server on the Internet, partly because it is open source. This popularity means that security is very important. Securing the application starts with the way you configure it. This article, the first of six parts, is excerpted from chapter two of Apache Security, written by Ivan Ristic (O'Reilly; ISBN: 0596007248). Copyright © 2006 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.

  1. Apache Installation and Configuration
  2. Installation
  3. Downloading the source code
  4. Static Binary or Dynamic Modules
By: O'Reilly Media
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December 27, 2007

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The next big decision is whether to create a single static binary, or to compile Apache to use dynamically loadable modules. Again, the tradeoff is whether to spend more time in order to get more security.

  1. Static binary is reportedly faster. If you want to squeeze the last bit of performance out of your server, choose this option. But, as hardware is becoming faster and faster, the differences between the two versions will no longer make a difference.
  2. A static server binary cannot have a precompiled dynamic module backdoor added to it. (If you are unfamiliar with the concept of backdoors, see the sidebar “Apache Backdoors.”) Adding a backdoor to a dynamically compiled server is as simple as including a module into the configuration file. To add a backdoor to a statically compiled server, the attacker has to recompile the whole server from scratch.
  3. With a statically linked binary, you will have to reconfigure and recompile the server every time you want to change a single module.
  4. The static version may use more memory depending on the operating system used. One of the points of having a dynamic library is to allow the operating system to load the library once and reuse it among active processes. Code that is part of a statically compiled binary cannot be shared in this way. Some operating systems, however, have a memory usage reduction feature, which is triggered when a new process is created by duplication of an existing process (known as forking). This feature, called copy-on-write, allows the operating system to share the memory in spite of being statically compiled. The only time the memory will be duplicated is when one of the processes attempts to change it. Linux and FreeBSD support copy-on-write, while Solaris reportedly does not.  

Apache Backdoors

For many systems, a web server on port 80 is the only point of public access. So, it is no wonder black hats have come up with ideas of how to use this port as their point of entry into the system. A backdoor is malicious code that can give direct access to the heart of the system, bypassing normal access restrictions. An example of a backdoor is a program that listens on a high port of a server, giving access to anyone who knows the special password (and not to normal system users). Such backdoors are easy to detect provided the server is routinely scanned for open ports: a new open port will trigger all alarm bells.

Apache backdoors do not need to open new ports since they can reuse the open port 80. A small fragment of code will examine incoming HTTP requests, opening “the door” to the attacker when a specially crafted request is detected. This makes Apache backdoors stealthy and dangerous.

A quick search on the Internet for “apache backdoor” yields three results:

The approach in the first backdoor listed is to patch the web server itself, which requires the Apache source code and a compiler to be available on the server to allow for recompilation. A successful exploitation gives the attacker a root shell on the server (assuming the web server is started as root), with no trace of the access in the log files.

The second link is for a dynamically loadable module that appends itself to an existing server. It allows the attacker to execute a shell command (as the web server user) sent to the web server as a single, specially craftedGETrequest. This access will be logged but with a faked entry for the home page of the site, making it difficult to detect.

The third link is also for a dynamically loadable module. To gain root privileges this module creates a special process when Apache starts (Apache is still running as root at that point) and uses this process to perform actions later. The only reliable way to detect a backdoor is to use host intrusion detection techniques, discussed in Chapter 9.

Please check back next week for the continuation of this article.

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