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WordPress 4.0.1 Released to Address Critical XSS, Other Vulnerabilities

With the release of WordPress 4.0.1, the developers of the popular blogging tool and a content management system (CMS) have addressed several vulnerabilities, including a critical flaw that could have been exploited to compromise websites.

The critical security release addresses a serious cross-site scripting (XSS) bug identified and reported by Jouko Pynnonen of the Finland-based IT company Klikki Oy on September 26. The vulnerability affects WordPress 3.9.2 and earlier versions which, according to the latest statistics from WordPress, account for nearly 86% of installations. WordPress 4.0, released in early September 2014, is not affected.

"An attacker could exploit the vulnerability by entering carefully crafted comments, containing program code, on WordPress blog posts and pages. Under default settings comments can be entered by anyone without authentication (login)," Klikki Oy said in a press release. "Program code injected in comments would be inadvertently executed in the blog administrator's web browser when they view the comment. The rogue code could then perform administrative operations by covertly taking over the administered account."

A proof-of-concept published by the company shows that an attacker can exploit the vulnerability to create new administrator accounts, change the password of the current administrator, and execute arbitrary PHP code on the server.

"Exploitability without login, under default settings, and the server-side impact make this probably the most serious WordPress core vulnerability that has been reported since 2009," Klikki Oy said.

Technical details on the critical XSS vulnerability are available in an advisory published by the Finnish company on November 20.

WordPress 4.0.1 addresses an additional eight security issues, including three other XSS vulnerabilities that can be exploited by a contributor or an author, a cross-site request forgery (CSRF) that can be leveraged to trick a user into changing his/her password, and a denial-of-service (DoS) bug.

A curious vulnerability was reported by David Anderson. He discovered that an extremely unlikely hash collision can allow an attacker to hijack a user's account. The attack only works if the user hasn't logged in to the account since 2008.

WordPress has also implemented extra protections for server-side request forgery attacks when making HTTP requests. An issue related to the validity of links in password reset emails reported independently by three researchers has also been addressed.

"Version 4.0.1 also fixes 23 bugs with 4.0, and we’ve made two hardening changes, including better validation of EXIF data we are extracting from uploaded photos," WordPress said in an advisory published on Thursday.

The Web Application Attack Report released by Imperva in October shows that WordPress is the most attacked CMS. In the test period, between August 2013 and May 2014, WordPress sites were attacked 24.1% more than sites running on all other CMS platforms combined, the company said.

Numerous vulnerabilities affecting the WordPress core and plugins have been discovered over the past years. In an effort to keep a record of all the issues, the developers of WPScan have created a vulnerability database.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.