This post provides the details of a vulnerability in the WordPress plugin Flexible Captcha not discovered by us, where the discoverer hadn’t provided the details needed for us to confirm the vulnerability while we were adding it to the data set for our service, so its contents are limited to subscribers of our service. If you are not currently a subscriber, you can try out the service for free and then you can view the contents of the post. There are a lot of other reason that you will want to sign up beyond access to posts like this one, including that you would have already been warned about this vulnerability if your website was vulnerable due to it.
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One of the ways we help to improve the security of WordPress plugins, not just for our customers, but for everyone using them, is the proactive monitoring of changes made to plugins in the Plugin Directory to try to catch serious vulnerabilities. That sometimes leads to us catching a vulnerability of a more limited variant of one of those serious vulnerability types, which isn’t as much concern for the average website, but could be utilized in a targeted attack. That happened with the cross-site request forgery (CSRF)/arbitrary file upload vulnerability we found in the plugin Flexible Captcha. This vulnerability could have allowed an attacker that could get a logged in Administrator to visit a URL the attacker controls, to upload a malicious file to the website, which the hacker could then use to take additional actions on their own with the website.