The Wordfence blog has an examination of an emerging attack on the Wordpress ecosystem.
[...] In the software industry, a supply chain attack exploits a trusted relationship between software vendors or authors and their customers. For WordPress, that means figuring out how to embed malware into software updates. In one case, we saw an existing plugin author install malware on customer sites in an effort to monetize an existing plugin. In every other case we have uncovered, the attack was carried out by someone who had purchased the plugin with the express intention of attacking its users.
This is a follow-up to December's discovery of backdoor code in three mildly popular plug-ins. Those otherwise-trusted plug-ins had been purchased from the original developer by a third party, who then injected malicious code in subsequent updates.
In the last two weeks, the WordPress.org repository has closed three plugins because they contained content-injection backdoors. ... Each of them had been purchased in the previous six months as part of the same supply chain attack, with the goal of injecting SEO spam into the sites running the plugins.
Usually when this happens, the first thing I check is the good/evil toggle. Just about every time the thing has been set to evil. Fortunately they're RFC3514 compliant. If that doesn't work, find the Turtle of enormous girth and schedule a ritual of Chüd. Problem solved.