WordPress 4.5.2 is now available. This is a security release for all previous versions and we strongly encourage you to update your sites immediately.
WordPress versions 4.5.1 and earlier are affected by a SOME vulnerability through Plupload, the third-party library WordPress uses for uploading files. WordPress versions 4.2 through 4.5.1 are vulnerable to reflected XSS using specially crafted URIs through MediaElement.js, the third-party library used for media players. MediaElement.js and Plupload have also released updates fixing these issues.
Both issues were analyzed and reported by Mario Heiderich, Masato Kinugawa, and Filedescriptor from Cure53. Thanks to the team for practicing responsible disclosure, and to the Plupload and MediaElement.js teams for working closely with us to coördinate and fix these issues.
Download WordPress 4.5.2 or venture over to Dashboard → Updates and simply click “Update Now.” Sites that support automatic background updates are already beginning to update to WordPress 4.5.2.
Additionally, there are multiple widely publicized vulnerabilities in the ImageMagick image processing library, which is used by a number of hosts and is supported in WordPress. For our current response to these issues, see this post on the core development blog.