Apache has been the most common web server on the internet since April 1996, and is currently used by 38% of all websites. Most nefarious activity takes place on compromised servers, but just how many of these Apache servers are actually vulnerable?

The latest major release of the 2.4 stable branch is Apache 2.4.7, which was released in November 2013. However, very few websites claim to be using the stable branch of 2.4 releases, despite Apache encouraging users to upgrade from 2.2 and earlier versions.

Less than 1% of all Apache-powered websites feature an Apache/2.4.x server header, although amongst the top million websites, more than twice as many sites claim to be using Apache 2.4.x. Some of the busiest websites using the latest version of Apache (2.4.7) are associated with the Apache Software Foundation and run on the FreeBSD operating system, including httpd.apache.org, www.openoffice.org, wiki.apache.org, tomcat.apache.org and mail-archives.apache.org.

The most recent security vulnerabilities affecting Apache were addressed in version 2.4.5, which included fixes for the vulnerabilities described in CVE-2013-1896 and CVE-2013-2249. Depending which Apache modules are installed, and how they are used, earlier versions may be vulnerable to unauthorised disclosure of information and disruption of service. The previous release in the 2.4 branch (2.4.4), also addressed several cross-site scripting (XSS) vulnerabilities in various modules; such vulnerabilities can severely compromise a web application by facilitating remote session hijacking and the theft of user credentials. Nonetheless, millions of websites still appear to be using vulnerable versions of Apache, including versions which are no longer supported.

Top 15 versions of Apache in February 2014, where the full version string is announced in the Server HTTP response header.
Note that no versions of the Apache 2.4 branch appear within the top 15.
Apache 1.3.41 and 2.0.63 are both end-of-lined.

The Apache 2.0 branch was retired in July 2013 with the conclusive release of Apache 2.0.65. This release addressed a few security vulnerabilities, but no subsequent vulnerabilities will be addressed by official patches or subsequent releases in the 2.0 branch. Anyone still using this branch of releases should strongly consider updating to the latest version in the stable 2.4 or legacy 2.2 branches.

Nevertheless, 6.5 million websites claim to be using the end of life 2.0 branch of Apache, with the most common versions being 2.0.63 and 2.0.52. Only 12k sites are running the conclusive release of this branch (2.0.65). However, it is worth noting that just over half of all Apache-powered websites hide their version numbers, so it is not always possible to accurately determine which version is installed without carrying out additional tests. Hiding software version numbers is usually a deliberate act by a server administrator – Apache 2.4.7 will reveal its full version number by default when installed on Arch Linux, and installing the apache2 package on the latest version of Ubuntu Linux will also reveal "Apache 2.4.6 (Ubuntu)" as the default Server banner.

Due to hidden version numbers, the number of sites openly reporting to be running Apache 2.4.x could be regarded as a lower bound, but conversely, exhibiting a vulnerable version number does not necessarily mean that a server can be exploited by a remote attacker.

For example, the Red Hat Linux operating system uses a backporting approach to applying security fixes, which means that a vulnerability in Apache 2.2.3 can be patched without affecting the apparent version number of the software. From an external point of view, the server will still appear to be running Apache 2.2.3, but it might not be vulnerable to any security problems that would affect a fresh installation of Apache 2.2.3.

Red Hat 5 and 6 use Apache 2.2.3 and 2.2.15 respectively, which explains why these seemingly old versions remain so prominent today (2.2.3 was originally release in July 2006). Both are still supported by Red Hat, and providing the necessary backported patches have been applied, Red Hat Apache servers which exhibit these version numbers can be just as secure as the latest release of Apache. However, because the version numbers correspond to Apache versions which were released several years ago, it is not unusual for Red Hat powered websites to attract unfair criticism for appearing to run insecure versions of Apache.

Certain Apache vulnerabilities can also be eliminated by removing or simply not using the affected modules – a configuration which is also difficult to ascertain remotely. However, exhibiting an apparently-vulnerable version number can still have its downsides, even if there are no vulnerabilities to exploit – as well as attracting unwarranted criticism from observers who falsely believe that the server is insecure, it could also attract undesirable scrutiny from hackers who might stumble upon different vulnerabilities instead. These are both common reasons why server administrators sometimes opt to hide version information from a web server's headers. Sites which do this include wikipedia.org, www.bbc.co.uk, www.nytimes.com and www.paypal.com, all of which claim to be running Apache, but do not directly reveal which version.

A further 6.0 million websites are still using Apache 1.3.x, even though the final version in this branch was released four years ago. The release of Apache 1.3.42 in February 2010 marked the end of life for the 1.3 branch, although 2.4 million sites are still using the previous version, (1.3.41), which contains a denial of service and remote code execution vulnerability in in its mod_proxy module.

The busiest site still using Apache 1.3 is Weather Underground, which uses Apache 1.3.42. This currently has a Netcraft site rank of 177, which makes it even more popular than the busiest Apache 2.0.x website. It is served from a device which exhibits the characteristics of a Citrix NetScaler application delivery controller. Weather Underground also uses Apache 1.3.42 for the mobile version of its site at m.wund.com.

Amongst the million busiest websites, Linux is by far the most common operating system used to run Apache web server software. With near-ubiquitous support for PHP, such platforms make tempting targets for fraudsters. Most of the phishing sites analysed by Netcraft rely on PHP to process the content of web forms and send emails.

The Audited by Netcraft service provides a means of regularly testing internet infrastructure for similarly vulnerable web server software, faulty configurations, weak encryption and other issues which would fail to meet the PCI DSS standard. Netcraft's heuristic fingerprinting techniques can often use the behaviour of a web server to identify which version of Apache is installed, even if the server does not directly state which version is being used. These automated scans can be run as frequently as every day, and can be augmented by Netcraft's Web Application Security Testing service, which provides a much deeper manual analysis of a web application by an experienced security professional.