A stored XSS vulnerability in WordPress allows an user with the posting capability to compromise the website.
The malicious script is executed when an administrator views the page. From this point on the attack could proceed to server-side code execution in the same way as with the other recently published WordPress XSS's.
The following code demonstrates the vulnerability. It should be entered in a page or posting using the HTML edit mode (instead of the default WYSIWYG):
<a href="[caption code=">]</a><a title=" onmouseover=alert('test') ">link</a>
WordPress shortcode processing manipulates this into the following form:
<a href="</a><a title=" onmouseover=alert('test') ">link</a>
A real-world exploit could use a style attribute to create a transparent tag covering the whole browser window to force execution of the onmouseover code.
Versions before 4.2.3 starting from at least 3.0 (released in 2010) are vulnerable.
The bug was corrected on July 23, 2015. It has been patched automatically for most users.
Klikki Oy discovered the bug while investigating another stored XSS vulnerability in November 2014. WordPress security team had requested us to review a patch they developed to address HTML formatting bugs. This was a "final review" four days before the planned release. However, we found a new vulnerability that affected the patched version and reported it to WordPress on the same day. The release that had been planned for the next Monday was cancelled. WordPress acknowledged that they are working to patch the new bug too.
WordPress's advisory seems to suggest that their security team independently found the same bug on the same day, within 17 hours from sending us the patches for final review.
WordPress didn't inform us about the upcoming release eight months later. We got no updates about the status of this bug after November 2014.
WordPress didn't originally credit us for the finding and quietly removed a link to this advisory after it was posted in a comment on WordPress.org. A mention about our bug report was added in their advisory afterwards.
Timeline detailing some of the key points:
Updated 25 July, 2015: added some explanation under "Vendor response".