oEmbeds are fun. They make it easy to embed third-party content on your site, like tweets, status updates, videos, images, all sorts of stuff.
Unfortunately, to do this, third-party code gets injected into your page. Don’t worry, this is by design, but it does mean that you should only oEmbed from reputable sites. WordPress Core is very picky as to the providers that it chooses to accept as oEmbed sources.
Twitter is one of these oEmbed providers. Here’s an example of an embedded tweet:
Neat, isn’t it?
Now, hover over my name.
See that little url that shows on the bottom left corner of your browser (probably)? It probably looks just like http://twitter.com/daljo628!
Now, click it. Don’t worry, I’ll wait.
Did the page you landed on have a bunch of extra cruft appended to the end of it?
After you’ve clicked on it normally once, you can come back and re-inspect it, to see that the URL on the link has now changed to the one with the referer data on it — they’re rewriting it inline and intentionally delaying it so when you first click, you wouldn’t realize that the data was being appended.
This can be a problem because some sites employ concealers for the referer http header (No, I didn’t misspell referrer) like href.li for example. By embedding this in a get parameter forcibly, it’s leaking data in a way very difficult to block, by taking advantage of the trust offered via accepting Twitter as an oEmbed provider.