A new proof-of-concept code that could hijack millions of WordPress websites has been released.
The cross-site scripting (XSS) vulnerability could allow a malicious attacker to take over an entire server running the blogging platform by changing passwords and creating new accounts.
The vulnerability works by injecting code into the comments section of the site, and then adding a massive amount of text -- more than 64 kilobytes worth. By default, WordPress doesn't publish a commenting user's first post until it has been approved. An attacker can thus fool the administrator with a benign first comment, which would enable further malicious comments from that user to be automatically approved.
WordPress versions 3.9.3, 4.1.1, 4.1.2, and the latest version 4.2 are affected.
Pynnonen said he disclosed the vulnerability because WordPress "refused all communication attempts" he made since November 2014.
On Monday, the company issued a "critical" security update, WordPress 4.2.1, addressing the flaw. (WordPress did not return an email for comment later in the day.)