Vulnerable versions include Duo WordPress plugin 1.8.1 and earlier, but the issue manifests only in multisite deployments where the plugin is enabled on a site by site basis. By exploiting this vulnerability, a WordPress user with valid credentials for one site could easily bypass the two-factor authentication procedure on the second site.
Duo Security provided the following sample scenario:
A multi-site WordPress deployment has two sites, Site1 and Site2, with the Duo WordPress plugin enabled for Site1 but disabled for Site2. Under normal circumstances, users logging into Site1 will be prompted for primary credentials and second-factor authentication; Site2 users will be prompted only for primary credentials.
A Site1 user may force browse to the login URL of Site2, which will authenticate the user (as part of the same Wordpress multi-site network), and redirect them back to Site1, without prompting for second-factor authentication.
The developers are currently working on fixing this issue and will soon release a new version of the plugin. In the mean time, it is recommended that users should enable duo_wordpress globally in a multi-site configuration.
Update - Friday, 14 February, 8:40 AM PST Duo Security have released more information about this issue as well as a new version of the plugin.