According to Secunia, "A vulnerability is caused due to improper access restriction of the administration section. This can be exploited to bypass the authentication mechanism and gain administrative access by setting a specially crafted cookie. This can further be exploited to execute arbitrary PHP code."
The site also reports that "input passed to an unspecified parameter is not properly sanitized before being returned to the user. This can be exploited to execute arbitrary HTML and script code in a user's browser session in context of an affected site."
These vulnerabilities are reported in version 2.5, though prior versions may also be affected.
A solution has been made available, an upgrade to version 2.5.1 of WordPress. According to a posting on the WordPress site, the upgrade includes a number of bug fixes and performance enhancements.
"We recommend everyone update immediately, particularly if your blog has open registration. The vulnerability is not public but it will be shortly," WordPress said.
In addition to the security fix, version 2.5.1 contains over 70 other bug fixes, according to the site. These include an upgrade to TinyMCE 3.0.7, fixes for IE, Media Uploader and Widget Administration, and performance enhancements for the Dashboard, Write Post and Edit Comments pages.