The Pixelavo WordPress plugin before 1.5.4 registers an unauthenticated AJAX action, gated only by a nonce that it emits publicly on every front-end page, that forwards client-supplied event data to the configured Facebook Conversions API using the administrators stored access token. This allows an unauthenticated visitor to inject arbitrary conversion events into the administrators Facebook ads account and exhaust the configured API quota.
Weakness
The web server receives a URL or similar request from an upstream component and retrieves the contents of this URL, but it does not sufficiently ensure that the request is being sent to the expected destination.