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WordPress is a free and open source blogging tool and a content management system (CMS) based on PHP and MySQL. Features include a plugin architecture and a template system. WordPress was used by more than 22.0% of the top 10 million websites as of August 2013. WordPress is the most popular blogging system in use on the Web, at more than 60 million websites.

It was first released on May 27, 2003, by its founders, Matt Mullenweg and Mike Little, as a fork of b2/cafelog. The license under which WordPress software is released is the GPLv2 (or later) from the Free Software Foundation.

WordPress first appeared in 2003 as a joint effort between Matt Mullenweg and Mike Little to create a fork of b2. Christine Selleck Tremoulet, a friend of Mullenweg, suggested the name WordPress.

In 2004 the licensing terms for the competing Movable Type package were changed by Six Apart and many of its most influential users migrated to WordPress. By October 2009 the 2009 Open Source content management system Market Share Report reached the conclusion that WordPress enjoyed the greatest brand strength of any open-source content-management systems.

Many security issues have been uncovered in the software, particularly in 2007 and 2008. According to Secunia, WordPress in April 2009 had 7 unpatched security advisories (out of 32 total), with a maximum rating of “Less Critical.” Secunia maintains an up-to-date list of WordPress vulnerabilities.

In January 2007, many high profile search engine optimization (SEO) blogs, as well as many low-profile commercial blogs featuring AdSense, were targeted and attacked with a WordPress exploit. A separate vulnerability on one of the project site’s web servers allowed an attacker to introduce exploitable code in the form of a back door to some downloads of WordPress 2.1.1. The 2.1.2 release addressed this issue; an advisory released at the time advised all users to upgrade immediately.

In May 2007, a study revealed that 98% of WordPress blogs being run were exploitable because they were running outdated and unsupported versions of the software. In part to mitigate this problem, WordPress made updating the software a much easier, “one click” automated process in version 2.7 (released in December 2008). However, the filesystem security settings required to enable the update process can be an additional risk.

Matt Mullenweg and Mike Little were cofounders of the project. The core contributing developers include Ryan Boren, Mark Jaquith, Matt Mullenweg, Andrew Ozz, Peter Westwood and Andrew Nacin.

WordPress is also developed by its community, including WP testers, a group of volunteers who test each release. They have early access to nightly builds, beta versions and release candidates. Errors are documented in a special mailing list, or the project’s Trac tool.

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Though largely developed by the community surrounding it, WordPress is closely associated with Automattic, the company founded by Matt Mullenweg. On September 9, 2010, Automattic handed the WordPress trademark to the newly created WordPress Foundation, which is an umbrella organization supporting WordPress.org (including the software and archives for plugins and themes), bbPress and BuddyPress.