The PhotoSwipe WordPress plugin through 4.1.1.1 uses the title attribute of author-supplied link...

The PhotoSwipe WordPress plugin through 4.1.1.1 uses the title attribute of author-supplied link markup as a lightbox caption that is written into the page DOM without escaping. Because the title attribute survives the post-content sanitization applied to users who lack the unfiltered_html capability, an authenticated user with Author-level access can store a JavaScript payload that executes in the browser of any visitor, including an administrator, who clicks the link.