The Quick Page/Post Redirect Plugin WordPress plugin before 5.2.4 was distributed with a hidden update mechanism that fetched Quick Page/Post Redirect Plugin WordPress plugin before 5.2.4 updates from its author's own server instead of WordPress.org, which was used to push a backdoored build to sites, allowing the attacker controlling that server to inject arbitrary content into pages and to deliver arbitrary code to affected sites.

Version 5.2.1, as published on WordPress.org, bundled the Quick Page/Post Redirect Plugin WordPress plugin before 5.2.4 Update Checker library (updater/ directory) configured to poll https://anadnet.com/updates/?action=get_metadata&slug=quick-pagepost-redirect-Quick Page/Post Redirect Plugin WordPress plugin before 5.2.4. In March 2021 that server delivered a modified 5.2.3 build, different from the 5.2.3 later published on WordPress.org, which kept the update channel and added a the_content filter that, for logged-out visitors on single posts and pages, fetched content from https://w.anadnet.com/bro/3/ (passing the site host, request URI and visitor User-Agent) and prepended the response to the post (observed serving SEO spam links to search engine crawlers). Because the version string matched a later WordPress.org release, affected sites were not offered a clean update until 5.2.4.

Indicators of compromise: - Outbound requests to anadnet.com or w.anadnet.com - An updater/ directory (Quick Page/Post Redirect Plugin WordPress plugin before 5.2.…