This morning we reported on an XSS vulnerability in WordPress 4.2, 4.1.2, 4.1.1, and 3.9.3, which allows an attacker to compromise a site via its comments. The security team quickly patched the vulnerability and released 4.2.1 within hours of being notified.

WordPress’ official statement on the security issue:

The WordPress team was made aware of a XSS issue a few hours ago that we will release an update for shortly. It is a core issue, but the number of sites vulnerable is much smaller than you may think because the vast majority of WordPress-powered sites run Akismet, which blocks this attack. When the fix is tested and ready in the coming hours WordPress users will receive an auto-update and should be safe and protected even if they don’t use Akismet.

That auto-update is now being rolled out to sites where updates have not been disabled. If you are unsure of whether or not your site can perform automatic background updates, Gary Pendergast linked to the Background Update Tester plugin in the security release. This is a core-supported plugin that will check your site for background update compatibility and explain any issues.

Since Akismet is active on more than a million websites, the number of affected users that were not protected is much smaller than it might have been otherwise.

WordPress 4.2.1 is a critical security release for a widely publicized vulnerability that you do not want to ignore. Users are advised to update immediately. The background update may already have hit your site. If not, you can update manually by navigating to Dashboard → Updates.