The discovery was made by researchers of security company Sucuri, and the developers of the plugin - Custom Contact Forms - have been notified, but have finally moved to patch the hole only after the WordPress Security team pushed the issue.
All versions of the plugin except the last (188.8.131.52) are affected, and users are advised to update it as soon as possible. Given the unresponsiveness of the Custom Contact Forms team, the option of switching to another form customization plugin such as Gravity Forms or an entire WP suite that offers the feature is also worth considering.
Custom Contact Form has been downloaded over 600,000 times. If only half of these users actively use it, that is still a considerable number of vulnerable WordPress sites out there.
Incidentally, WordPress and Drupal users should also patch their installations against the recently discovered vulnerability that affects the parsing of the XML-RPC Internet protocol used for cross-platform communication, and which can be misused to launch DoS attacks.
By working together, the WordPress and Drupal security teams have quickly released updated that solve the problem.
"The vulnerability exists in all WordPress and Drupal versions, affecting over 250 million websites, roughly 23% of the Internet website population today," Incapsula researchers pointed out. "Combined with the extremely large pool of targets and the high damage potential, this makes the exploit a triple threat – widespread, crippling and hard to weed out."
If, for some reason, you can't update your WP or Drupal installation, the threat can be snuffed out by deleting xmlrpc.php.